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Posted 19 Aug 2005


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Entering the kernel without a driver and getting interrupt information from APIC

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19 Aug 2005CPOL32 min read
Tips and tricks of Windows masters.


Although making user-mode application enter the kernel is definitely an exciting exercise, it is far from being something unheard of. It has been first done by Matt Pietrek (he did it on Windows 95 many years ago). His technique was later adjusted to Windows NT by Prasad Dabak, Sandeep Phadke and Milind Borate. In order to enter the kernel right from an application, one has to set up call gate descriptor in Global Descriptor Table (GDT), so that an application can enter the kernel via the call gate. However, once user-mode code is not allowed to access GDT, above mentioned authors used a kernel-mode driver in order to set up call gate descriptor. Certainly, quite logical question arises - what is the point of entering the kernel without a driver if you still need a driver in order to make it work??? After all, it just defeats the purpose, don't you think?

This article describes how user-mode application can access the kernel address space and set up call gate descriptor in GDT without using a driver. It explains how virtual-to-physical address translation works on 32-bit processors, and describes how the user-mode application can find out which physical address some given virtual address represents. "Methodology" of solving this task is 100% of my own design - you will be unable to find anything similar anywhere. This article also thoroughly explains how protection of kernel address space is implemented by Windows NT, how the transition from non-privileged to privileged mode can be made on x86-based system, and how applications can enter the kernel without a driver.

In addition to the above, this article introduces the reader to Advanced Programmable Interrupt Controller (APIC), and explains how interrupt information can be obtained from it. This topic seems to be barely known to the Windows community, although APIC is briefly mentioned by Mark Russinovich and David Solomon in Windows Internals, fourth edition. However, this book does not explain how to actually program APIC. I never came across any explanation of APIC programming in any Windows-focused article either - I had to figure out everything myself from Intel manuals. Therefore, I believe this information must be of great interest to Windows developers.

To summarize, if you want to learn more about the system internals, this article is right for you.

Accessing the kernel address space

Let's presume we want to access some address in the kernel address space, and to do it while running in user mode. Is it possible? Yes and no. If we try to access it directly as a virtual address, we will get Access Violation exception. However, there is a workaround - physical RAM can be opened as a section named "\\Device\\PhysicalMemory" with NtOpenSection() , and then mapped with NtMapViewOfSection() native API functions. By using this technique we can access any page in RAM. If we know which physical address our target address represents, our task is really easy - all we have to do is to map this physical address with NtMapViewOfSection() pointer, returned by this function, refers to the same physical address as our target address in the kernel address space, but it is numerically below 0x80000000, i.e. we are able to access it from the user mode. If we need a write access to it, we will have to do few extra things before mapping. The problem is that non-system processes, i.e. the ones that have not been started by the user SYSTEM, have no write access to "\\Device\\PhysicalMemory". Therefore, you have to grant yourself a write access to "\\Device\\PhysicalMemory". The code below does this:

                  NewDacl=NULL; PVOID security;
HANDLE Section; 
INIT_UNICODE_STRING(name, L"\\Device\\PhysicalMemory");
OBJECT_ATTRIBUTES oa ={sizeof(oa),0,&name,0,0,0};  

memset(&Access, 0, sizeof(EXPLICIT_ACCESS));
NtOpenSection(&Section, WRITE_DAC | READ_CONTROL, &oa);
GetSecurityInfo(Section, SE_KERNEL_OBJECT, 
                         NULL, NULL, &OldDacl,
                         NULL, &security);
Access.grfAccessPermissions = SECTION_ALL_ACCESS; 
Access.grfAccessMode        = GRANT_ACCESS;
Access.grfInheritance       = NO_INHERITANCE;
Access.Trustee.MultipleTrusteeOperation = 
Access.Trustee.TrusteeForm  = TRUSTEE_IS_NAME;
Access.Trustee.TrusteeType  = TRUSTEE_IS_USER;
Access.Trustee.ptstrName = "CURRENT_USER";

SetEntriesInAcl(1, &Access, OldDacl, &NewDacl);
SetSecurityInfo(Section, SE_KERNEL_OBJECT,
                         NULL, NULL, NewDacl, 


In order to run this code, you have to be logged on as a user with Administrator privileges. In my experience, restricted users cannot open "\\Device\\PhysicalMemory" for any access - NtOpenSection() will always return an error code, even if you have requested read-only access to "\\Device\\PhysicalMemory".

Therefore, as long as we have Admin privileges, we can gain access to any page in RAM, and do it while running in user mode - even if the page in question corresponds to some address in the kernel address space. In other words, we can gain indirect access to the kernel address space. Don't you find it exciting? However, in order to make any practical use of such possibility, we have to find out which physical address our target address in the kernel address space represents. Kernel-mode drivers can call MmGetPhysicalAddress(), but user-mode code has no chance of calling this function. Furthermore, there is no user-mode API function that may be of help. Therefore, we have to figure out everything ourselves. This is why, first of all, we have to learn how virtual - to - physical address translation works on 32-bit processors.

On x86 system page size can be either 4 KB or 4 MB. If page size is 4KB, 32-bit virtual address contains three pieces of information that CPU needs in order to get to the physical location this address represents. Low-order 12 bits of a virtual address represent an offset to the physical memory page. Bits 12-21 of a virtual address represent an index in a Page Table, which holds 1024 entries, describing physical pages. Every process may theoretically have up to 1024 page tables. Therefore,1024*1024*4096 gives us 4 GB of addressable space. Addresses of all page tables of a process are stored in a Page Directory. Bits 31-22 of a virtual address are used for locating the appropriate page table in a page directory. Every process maintains its own page directory. Physical address of the page directory of currently running process is stored in CR3 register. This register is supposed to be modified only upon the task switch, so it is not supposed to be accessed by anyone, apart from the system. Upon the task switch, the system loads CR3 with a different page directory. As a result, virtual address that previously referred to some physical page X may now be referring either to the same page X, or to some other page Y, or to no page at all. This is why any virtual address, valid in the address space of the process A, may be meaningless for the process B, unless they refer to the same physical address. For example, drivers are loaded into RAM only once, and are mapped to the same virtual addresses in all processes, so that change of CR3 does not affect them.

Binary layout of every Page Directory and Page Table entry is described by the following 32-bit structure:

struct PageDirectoryOrTableEntry
    DWORD Present:1;
    DWORD ReadWrite:1;
    DWORD UserSupervisor:1;
    DWORD WriteThrough:1;
    DWORD CacheDisabled:;
    DWORD Accessed:1;
    DWORD Reserved:1;
    DWORD Size:1;
    DWORD Global:1;
    DWORD Unused:3;
    DWORD PhysicalAddress:20

If page size is 4 KB, address translation works the following way:

  1. CPU gets the page directory of currently running process from CR3 register. High-order 10 bits of the virtual address represent the index (i), which CPU is going to use in order to locate the address of the page table, corresponding to the given virtual address, in this directory. If Present bit of directory's ith entry is not set, CPU raises Page Fault exception (INT 0xE). Page Fault exception may be raised for various reasons, such as invalid address, write access to read-only memory, etc. Therefore, first of all the system checks the reason for Page Fault exception. If the exception has been raised only because of the state of Present bit , the system comes to the conclusion that page table in question has been swapped to the disk. Therefore, it loads page table in question into RAM, sets Present bit of corresponding page directory entry, and CPU tries to access the virtual address again. All the above activities are transparent to the client code.
  2. After having located the page table, CPU proceeds to locating the page, corresponding to the given virtual address, in this table. Bits 12-21 of the virtual address represent the index (i), which CPU is going to use in order to locate the address of the target page in the page table. If Present bit of page table's ith entry is not set, CPU raises Page Fault exception, which is handled by the system in a way we have already seen.
  3. Finally, after having located the target page, CPU uses low-order 12 bits of the virtual address as an offset into the page.

This is how address translation works if page size is 4 KB. If page size is 4 MB, 10 high-order bits of the address represent the index that CPU needs to locate the page in the page directory, and remaining 22 bits are used as an offset into this page (1024*4 MB gives us, again, 4 GB of addressable space).

64-bit processors use more advanced address translation schemes. For example, Itanium allows so-called Data Execution Prevention (DEP) at the hardware level. It is mistakenly believed by many that DEP is a feature of Windows XP SP2. It is not - DEP is a CPU feature that Windows XP SP2 may take advantage of. If Windows XP SP2 runs on the CPU without support for DEP, DEP is not going to work - there is no way to prevent executing handcrafted machine instructions on processors without DEP feature. We are going to stick to 32- bit processors and 4 KB pages in our discussion.

Under Windows NT, page directory of currently running process is mapped to the virtual address 0xC0300000. This information, combined with our knowledge of virtual-to-physical address translation, leads us to two conclusions:

  1. Under Windows NT, 0x300th entry of a page directory holds the physical address of the page directory itself.
  2. Page table, corresponding to some virtual address, is accessible as 0xC0000000+((address>>10)&0x3FF000). With such translation page table, corresponding to the address 0xC0300000, is 0xC0300000 itself. In other words, page directory is also a page table that corresponds to the virtual address of a page directory itself.

Now let's try some practical exercise. Imagine the following kernel-mode code:

    mov ebx,0xfec00000
    or ebx, 0x13
    mov eax,0xc0000000
    mov dword ptr[eax],ebx

//what are we doing??? Are we insane???

What is going to happen if we run this code? The answer seems to be obvious, but it is wrong - we are not going to crash. We wrote 0xfec00013 ( 20 upper bits indicate the physical page 0xfec00, 12 lower bits indicate Present, ReadWrite and CacheDisabled flags that are set) as a very first entry of the page table, address of which is stored in the very first entry of our page directory. Now think of how CPU is going to translate the virtual address 0, and you will understand that, due to our modification, it will translate 0 to the physical address 0xfec00000. From now on virtual range 0 - 0x1000 becomes a perfectly valid virtual range in the address space of the process that runs this code!!! Null pointer becomes usable, and it refers to the physical address 0xfec00000!!! Not so boring, don't you think??? Later you will see that such tricks can be extremely useful in some situations.

However, let's get back to our task - as you may remember, our current task is to discover which physical address our target virtual address represents. Furthermore, this job has to be done by user-mode code. If we know the address of the page directory of our process in physical RAM, our task is trivial. Let's presume our target virtual address is V. We will map page directory to the virtual address D by NtMapViewOfSection(), and think of D as of an array of 1024 DWORDs. The physical page, described by 20 upper bits of (V>>22)th entry of D, is a page table, corresponding to the virtual address V. Therefore, we will map this page to the virtual address T, and, again, think of it as an array of 1024 DWORDs - 20 upper bits of ((V>>12)&0x3FF)th entry of T is a physical page that the virtual address V represents. Pure and simple. There is only one problem - CR3 register, from which the physical address of our process's page directory is available, cannot be accessed by the user-mode code. Therefore, first of all we have to find the physical address of the page directory of our process.

What we are going to do is run a memory scan, mapping every page in RAM into the address space of our process, so that sooner or later we will come across the page directory of our process. How are we going to recognize it? Let's presume the physical page we examine is P, and it is mapped to the virtual address V by NtMapViewOfSection(). We will think of V as of an array of 1024 DWORDs. If P is a physical page that holds the page directory of our process, then:

  1. 20 upper bits of 0x300th entry of V must be equal to P, because 0x300th entry of a page directory must hold the physical address of the page directory itself
  2. Lowest bit of 0x300th entry of V must be set, because it indicates the presence of page in RAM
  3. If P is a page directory, then (V>>22)th entry of V describes a page table, corresponding to the virtual address V itself, and this page table is definitely loaded into RAM. Therefore, lowest bit of (V>>22)th entry of V must be set.

If any of the above is not true, we can conclude that P is definitely not our page directory, so we can proceed to the next page. Otherwise, we will map the page, described by the 20 upper bits of (V>>22)th entry of V (let's call the resulting virtual address T), and assume that T is a page table, corresponding to the virtual address V. If our assumption is correct, then:

  1. 20 upper bits of ((V>>12)&0x3FF)th entry of T must be equal to P.
  2. Lower bit of ((V>>12)&0x3FF)th entry of T is set.

If both the above conditions are met, we can conclude that P is really our page directory, so it can be accessed from the virtual address V . What makes us believe so? Read again about the virtual-to-physical address translation, think about the translation of V to its physical address P, and, I hope, everything will become clear to you. Look at the code below:

//check how much RAM we've got
MEMORYSTATUS meminfo;GlobalMemoryStatus(&meminfo);

//get handle to RAM

status = NtOpenSection(&Section,

DWORD found=0,MappedSize,x;
PVOID DirectoryMappedAddress,TableMappedAddress;
DWORD DirectoryOffset,TableOffset; 
     //map current page in RAM
     phys.QuadPart=x; DirectoryMappedAddress=0;
     status = NtMapViewOfSection(Section, 
                  (HANDLE) -1, 
                  0L,MappedSize, &phys, 
                  &MappedSize, ViewShare,0, 

     //get offsets

     //let's check if this page can be a page 
     //directory - 20 upper bits of 0x300-th entry
     //must be //equal to P, and Present bit must be 
     //set in 0x300-th and V>>22-th entries.
     //If not,proceed to next page
        || (entry[DirectoryOffset]&1)!=1)
          {NtUnmapViewOfSection((HANDLE) -1, 

     //seems to be OK for the time being. Now let's 
     //try to map a possible page table
     status = NtMapViewOfSection(Section, (HANDLE) -1, 
                   &TableMappedAddress, 0L,MappedSize,
                   &phys, &MappedSize, ViewShare,0, 

     if(status){NtUnmapViewOfSection((HANDLE) -1, 

     //now let's check if this is really a page table If 
     //yes, 20 upper bits of (V>>12)&0x3ff-th
     //entry must be equal to P, and Present 
     //bit must be set in this entry.
     //If the above is true, P is really a page directory
     if((entry[TableOffset]&1)==1 && 

     NtUnmapViewOfSection((HANDLE) -1, TableMappedAddress);

     //directory is found - no need to proceed further
     NtUnmapViewOfSection((HANDLE) -1, 

How reliable is this code? Can we somehow mistake some page in RAM for our page directory? Random coincidence of 42 bits is needed for such a mistake. Therefore, such mistakes may happen only in 1 out of 2 ^42 cases, i.e. probability of it is negligible, for the practical purposes. Can we somehow miss the page directory of our process? Our sequential scan is guaranteed to bump into it, unless it gets moved around in physical RAM while our code runs. Theoretically Memory Manager may move any page, including the page directory, in physical RAM, but this may happen only if the page in question had, at some point, been swapped to the disk, and later got reloaded to some other location in RAM. Once swapping frequently accessed pages to the disk degrades the performance dramatically, the system would not swap frequently accessed pages. A process has to stay inactive for quite a while before its page directory gets swapped to the disk - as long as the code runs, the page directory of its process gets accessed upon every instruction's execution, so it cannot become a candidate for paging. Therefore, we can safely assume that our page directory will remain at some fixed address in RAM throughout our code's execution. In other words, our approach is, in practical terms, quite reliable - I used it very, very, very many times without a single(!!!) failure.

Now, once we know the physical address of our page directory, we can easily obtain the physical address, corresponding to any virtual one we are interested in - "methodology" of solving this task has already been described above. Let's get the physical address of the page that holds Global Descriptor Table (GDT)

//get base address of gdt

BYTE gdtr[8]; DWORD gdtbase,physgdtbase;
    sgdt gdtr
    lea eax,gdtr
    mov ebx,dword ptr[eax+2]
    mov gdtbase,ebx

//get directory and table offsets


//map page table - phys. address of it is 20 
//upper bits of (V-22)-th entry of page directory
status = NtMapViewOfSection(Section, (HANDLE) -1, 
              &TableMappedAddress, 0L,MappedSize,
              &phys, &MappedSize, ViewShare,0, 

//phys page is in 20 upper bits of (V>>12)&0x3ff-th 
// entry of page table
// this is what we need
//unmap everything
NtUnmapViewOfSection((HANDLE) -1, 

NtUnmapViewOfSection((HANDLE) -1, 

GDT is not accessible to the user-mode code, but, as it has been explained above, this limitation does not apply to us any more - we have already found the physical address of the page where it resides, haven't we? In order to access GDT, first we have to map this page to some virtual address V with NtMapViewOfSection().

There is no guarantee that GDT starts right at the beginning of the page, so we have to use the 12 lower bits of its virtual address as an offset into the page. Therefore, we have to add (gdtbase&0xFFF) to V. From now on we are able to read and write GDT from the virtual address V. Therefore, we can get the read and write access to the kernel address space right from the user-mode application. Would it not be great if we could get the execute access as well? This is what we are up to, this is why everything above has been said, and this is why we got the physical address of GDT, rather than some other address in the kernel address space - GDT will help us to enter the kernel mode right from our program, without a driver.

Code privilege level and protection

GDT can hold Segment descriptors, Local Descriptor Table (LDT) descriptors, Call Gate descriptors and Task State Segment (TSS) descriptors. All the above descriptors are all 8 bytes in size, although each of them has its own binary layout . Binary layout of Segment descriptors and Call Gate descriptors is described by the following 8-byte structures:

struct  SegmentDescriptor
    WORD LimitLow;
    WORD BaseLow;
    DWORD BaseMid : 8;
    DWORD Type : 5;
    DWORD Dpl : 2;
    DWORD Pres : 1;
    DWORD LimitHi : 4;
    DWORD Sys : 1;
    DWORD Reserved_0 : 1;
    DWORD Default_Big : 1;
    DWORD Granularity : 1;
    DWORD BaseHi : 8;

struct CallGateDescriptor

   WORD offset_low;
   WORD selector;
   BYTE param_count :5;
   BYTE   unused   :3;
   BYTE  type        :5;
   BYTE  dpl         :2;
   BYTE present     :1;
   WORD offset_high;

LDTs and call gates are not used by Windows NT. Although, for performance reasons, all user processes run in the context of a single task under Windows NT, there are few TSS descriptors in GDT. They are mainly reserved for the "exceptional circumstances", i.e. system crash - their task is to make sure that the system is able to operate long enough to throw a blue screen before the CPU resets itself. They are of no interest to us anyway. What about segment descriptors? One may think that, once Windows implements the flat memory model, we should not be bothered about segment descriptors.

In actuality, things are not that simple. Windows flat memory model is implemented by setting BaseLow, BaseMid and BaseHi fields of code, data, stack and extra segment descriptors in GDT to 0. As a result, code, data, stack and extra segments are mapped to the same virtual address 0, so we have no need to specify segment and offset when we address memory. However, segments are still there behind the scenes, because there is no way to implement protected operating system on x86-based machine without using segmentation. The problem is that, strictly speaking, there is no such thing as kernel or user operating mode on x86-based system. Instead, the ability to execute privileged instructions and access supervisor-only pages is controlled by Descriptor Privilege Level (DPL) field of the code segment descriptor. Therefore, in order to separate privileged code from non-privileged one, two code segments are needed - privileged code segment must have DPL 0, and non-privileged one must have DPL 3. Although they may be mapped to the same virtual address 0, they will be treated as separate code segments by the processor. CS register is interpreted as the offset in bytes from the beginning of the table to the descriptor of the currently running code segment, ORed with the DPL of this code segment. This is how CPU knows the privilege level of the currently running code. Under Windows NT, CS register value can be either 0X8 (when privileged code executes), or 0X1B (when non-privileged code executes).

Therefore, privilege level is defined by the CS register, rather than the address of the code itself. If some function executes at the time when CS equals 0X8, it is treated as privileged code, but if exactly the same function executes at the time when CS equals 0X1B, it is treated as non-privileged one. I predict your questions - after all, non-privileged code has no chance to access the addresses above 0x80000000. There must be some contradiction then? Not at all. Look at the PageDirectoryOrTableEntry structure once again, pay special attention to UserSupervisor bit, and everything will become clear to you - Windows just marks those pages that map to the addresses above 0x80000000 as supervisor-only in their page tables. If non-privileged code tries to access such page, access violation exception gets raised. Any function that resides in such page just cannot run if CS equals 0X1B - access violation exception will get raised straightaway. Therefore, kernel address space protection is achieved by combining segmentation with page-level protection.

Transition from non-privileged code segment to privileged one can be made in one of the following ways:

  1. Via INT n instruction - This instruction pushes the value of user-mode SS register, the value of user-mode ESP, EFLAGS register, user-mode CS register and the return address (all pushes occur in the above described order) on the kernel stack, and transfers execution to INT n handler. Windows NT sets the DPL of Interrupt Descriptor Table entries in such way that the user-mode code is allowed to execute only the interrupts 0X3, 0X4, 0X2A, 0X2B, 0X2C, 0X2D and 0X2E.
  2. SYSENTER instruction - This instruction sets the ESP of the calling thread to the value, specified by SYSENTER_ESP_MSR model-specific register, CS to the value, specified by SYSENTER_CS_MSR model-specific register, and transfers execution to the address, specified by SYSENTER_EIP_MSR model-specific register (these registers cannot be accessed by user-mode code). Flags and return address are not preserved. Windows NT/2000 does not use SYSENTER instruction, and under Windows XP this instruction transfers execution to the system service dispatcher.
  3. Far call via the call gate - When the kernel entry is made via the call gate, CPU copies up to 32 DWORDs (the exact number is specified by param_count field of call gate descriptor) from the user stack to the kernel one, then pushes the user-mode CS and the return address to the kernel stack, then sets CS to the value, specified by selector field of call gate descriptor, and then transfers execution to the address, specified by offset_low and offset_high fields of call gate descriptor. Windows does not use call gates.

First 2 options are used by Windows, and they transfer the execution to some system-defined address. Therefore, we cannot use them in order to transfer execution to some arbitrary address of our own choice. Call gates have quite a different story - once Windows does not use them, we are free to use call gates the way we like. If we set up a call gate in GDT, we can transfer the execution to any address that we have specified in the call gate descriptor. Certainly, this address has to be valid in the address space of our process, but it does not have to reside in the kernel address space. Privileged code can access any address that is valid in the address space of the calling process, and, as you must have understood, the privilege level of running code is defined by the value of CS, rather than EIP, register. Therefore, if we specify the address of some function, implemented by our application, in the call gate descriptor, and then call it via the call gate, this function will be treated as a privileged code by CPU, although it resides in the user address space of our process. This function will be able to access any memory address, IO ports, raise interrupts, call ntoskrnl.exe exports, i.e. do almost everything that kernel-mode drivers can do. At the same time, this function should not try calling API functions that are implemented by the user-mode DLLs. Why? Because these API functions may call native API, and native API functions invoke system services, i.e. enter the kernel via the system service dispatcher - at the time when we are already in privileged mode!!!. This is not going to do us any good. The general rule is that the less Windows knows about our tricks, the better it is for us. How are we going to return back to the user mode? It can be done by either IRETD, SYSEXIT or RETF instructions. Although the way the return is made is supposed to match the way the call has been made, this is not an absolute necessity. For example, under Window XP, the return from INT 0x2B handler is made by SYSEXIT, rather than IRETD, instruction - as long as both the parties adjust the kernel and user stacks to the way they handle entry and return, everything works fine. In our particular case RETF instruction is, apparently, the most logical way of leaving the kernel mode, so we are going to return back to the user mode with REFT instruction.

Setting up call gate in GDT

Now we know everything we need to know in order to set up a call gate in GDT and enter the kernel right from our application. Look at the code below:

//now let's map gdt
PBYTE GdtMappedAddress=0;
NtMapViewOfSection(Section, (HANDLE) -1, 
              0L,MappedSize, &phys, 
              &MappedSize, ViewShare,0, 

CallGateDescriptor * gate=
   (CallGateDescriptor * )GdtMappedAddress;

//now let's find free entry in GDT. Type of 
//current gdt entry does not matter - Present
// bit is 48-th bit for all type of 
//descriptors, so we interpret all descriptors
//as call gates

// now let's set up a call gate
gate[selector].offset_low  = 
   (WORD)((DWORD)kernelfunction & 0xFFFF);
gate[selector].selector     = 8;
//we will pass a parameter
gate[selector].param_count  = 1; 
gate[selector].unused    = 0;
// 32-bit callgate
gate[selector].type       = 0xc;     
// must be 3
gate[selector].dpl          = 3;      
gate[selector].present      = 1;
gate[selector].offset_high = 
   (WORD)((DWORD)kernelfunction >> 16);

//we don't need physical memory any more
NtUnmapViewOfSection((HANDLE) -1, 

First of all, we map GDT in a way we have already described above, and look for some unused entry in it. All descriptors in GDT are 8 bytes in size, with presence indicated by the 48th bit. This is the same for all types of descriptors. Therefore, when we look for a free entry in GDT, we can treat all GDT entries as call gate descriptors, regardless of their actual type. After having found the free entry, we set it up as a call gate descriptor. We set its type to 0XC in order to indicate 32-bit call gate, its DPL to 3, so that it is accessible to user-mode code, its selector field to 0X8, and its offset_low and offset_high fields to respectively 16 low-order and 16 high-order bits of the function we are about to call. What about a parameter, specified by the param_count field? I believe that, this article is all about doing unconventional things, it would be a good idea to make our kernel code do something that is not so widely known to the general public. One of the articles on the CodeProject explains to you how the interrupt information can be obtained from the registry. I am going to show you, apparently, more exciting way of getting interrupt information, without either translating interrupt resources or digging in the registry. Therefore, we are going to pass IRQ as parameter to our function, and our function will return interrupt vector that corresponds to this particular IRQ.

Advanced Programmable Interrupt Controller

How does the system map IRQs to interrupt vectors and define their priority? It depends on whether your machine supports Advanced Programmable Interrupt Controller (APIC). This can be discovered by CPUID instruction and read from APIC_BASE_MSR model-specific register. If APIC is present and you make CPUID instruction with 1 in EAX, bit 9 of EDX register will be set by this instruction. In order to find out whether APIC is enabled, you have to read the APIC_BASE_MSR model-specific register - bit 11 of it must be set if APIC is enabled. Unless your computer is completely outdated, I am 99.9% sure that APIC is present and enabled on your machine. If it is not, then the interrupt vector, corresponding to some given IRQ, equals 0x30+IRQ, so that timer (IRQ0) interrupt vector is 0x30, keyboard (IRQ1) interrupt vector is 0x31, etc. This how how Windows NT maps hardware interrupts if APIC is not present or disabled. In such cases interrupt priority is implied by IRQ - there is nothing than can be done here.

If APIC is present and enabled, things become much more interesting to program. Every CPU in the system has its own local APIC, physical address of which is specified by APIC_BASE_MSR model-specific register. Local APIC can be programmed by reading from and writing to its registers. For example, processor's IRQL can be manipulated via Task Priority register, which is located at the offset of 0x80 from the local APIC's base address - this is what KeRaiseIrql() and KeLowerIrql() do. If you want to raise an interrupt, you can do it via Interrupt Command register, which is located at the offset of 0x300 from the local APIC's base address - this is what HalRequestSoftwareInterrupt() does. You can also specify whether you want the CPU to interrupt itself or whether you want interrupt to be dispensed to all CPUs in the system. Local APIC programming is quite an extensive topic, so it is well beyond the scope of this article. If you need more information, I would strongly advise you to read Volume 3 of Intel Developer's Manual.

All local APICs communicate with IO APIC, which is located on the motherboard, via APIC bus. IO APIC maps IRQs to interrupt vectors, and it is able to map up to 24 interrupts. IO APIC can be programmed by reading from and writing to its registers. These are 32-bit ID Register (located at the offset of 0), 32-bit Version Register (located at the offset of 0X1), 32-bit Arbitration Register (located at the offset of 0X2), and 24 64-bit Redirection Table Registers, with every Redirection Table Register corresponding to some given IRQ. The location of Redirection Table Register, corresponding to any given IRQ, can be calculated as 0X10+2*IRQ. If you want to know the binary layout of Redirection Table, I suggest you should read Intel IOAPIC manual - we are interested only in 8 low-order bits of Redirection Table, because they indicate interrupt vector that corresponds to the given IRQ. Interrupt priority can be calculated as vector/16, and, once operating system designers can map IRQs to interrupt vectors in any way they wish, they can assign any interrupt priority level to any given IRQ.

IO APIC uses indirect addressing scheme, which means all the above mentioned registers cannot be accessed directly. How can they be accessed then??? IO APIC provides 2 direct access registers for this purpose. These are IOREGSEL and IOWIN registers, located at the offsets of respectively 0 and 0X10 from IO APIC's base address. IO APIC is mapped to the physical memory at the address 0XFEC00000. Although Intel allows operating system designers to relocate IO APIC to some other physical address, Windows NT does not relocate it. Therefore, we will make a bold assumption that IO APIC is located at the physical address 0XFEC00000 on your machine, so that physical addresses of IOREGSEL and IOWIN registers are respectively 0XFEC00000 and 0XFEC00010. In order to access these registers, you have to map them to non-cached memory. In order to read any indirect access register, you have to write its offset to IOREGSEL register - subsequent read of IOWIN register will return the value of the target indirect access register. All reads are 32-bit. If you want to read 32 low-order or 32 high-order bits of Redirection Table Register that corresponds to some given IRQ, you have to write respectively 0X10+2*IRQ or 0X10+2*IRQ+1 to IOREGSEL register, and then read IOWIN register in order to get the sought information.

How are we going to map IO APIC to the virtual memory? If we used a regular driver, we would call MmMapIoSpace(). However, in our case things are slightly different. If CPU treats our code as a privileged one, it does not necessarily imply that Windows always shares its opinion on the subject - everything depends on what you want to do. Some ntoskrnl.exe's exports ( for example, ExAllocatePool()) can be called by our code without a slightest problem, but MmMapIoSpace() is not among them - if our code calls MmMapIoSpace(), we will get a blue screen with IRQL_NOT_LESS_OR_EQUAL error code. What are we going to do then? This is when our trick with mapping some page to the virtual address 0 comes handy, so we are going to use it.

The code below maps IO APIC to the virtual address 0, and obtains interrupt vector that corresponds to some given IRQ:

 //map ioapic - make sure that we map 
 //it to non-cached memory.


    mov ebx,0xfec00000
    or ebx,0x13
    mov eax,0xc0000000
    mov dword ptr[eax],ebx

//now we are about to get 
//interrupt vector

//write 0x10+2*irq to IOREGSEL

// subsequent read from IOWIN returns 32 
// low-order bits of Redirection Table
//that corresponds to our IRQ.
// 8 low-order bits are interrupt vector, 
// corresponding to our IRQ
DWORD vector=(array[4]&0xff);

As you can see, IO APIC programming is among those things that are easily done than explained - so much explanation and only few simple lines of code. But why did we choose to map IO APIC to 0, rather than to some more conventional address? Just because the address 0 is guaranteed to be unused, so mapping IO APIC to this address is the very first thing that gets into the head.

Putting it all together

Now let's put it all together. Look at the code below - it calls the kernel function:

// now we will get interrupt vectors
DWORD res; 
DWORD resultarray[24];

for (x=0;x<25;x++)

    //let's call the function via the 
    //call gate. Are you ready???
    WORD   farcall[3];
    farcall[2] = (selector<<3);
        mov ebx,x
        push ebx
        call fword ptr [farcall]
        mov res,eax
    //if the return value is 500 and this 
    //was not the final invocation,
    //apic is not present. Inform the user 
    //about it, and that't it
              "APIC is not supported",

There is no way to make a far call via the call gate in C, so we have no option other than calling the kernel function from ASM block. The client code in itself is straightforward - it pushes the value of IRQ on the stack, calls the kernel function via the call gate, and saves the result in the array. It does so for IRQs 0 to 23, plus makes a final invocation with non-existent IRQ24. Upon the receipt of 24 as a parameter, in order to make sure that no traces of our experiments are left anywhere, the kernel function cleans up the call gate in GDT. After having obtained all the information about all IRQs, we will inform the user about each IRQ with MessageBox(). I hope there is no need to list this code here.

Now let's look at our kernel function:

void kernelfunction(DWORD usercs,DWORD irq)
    DWORD absent =0; 
    BYTE gdtr[8];
    //check if ioapic is 
    //present and enabled
            mov eax,1
            and edx, 0x00000200
            cmp edx,0
            jne skip1
            mov absent,1
            skip1: mov ecx,0x1b
            and eax,0x00000800
            cmp eax,0
            jne skip2
            mov absent,1
       //if APIC is enabled, get vector 
       //from it and return
       skip2: if(!absent)
                //map ioapic - make sure that we 
                //map it to non-cached memory.
                //Certainly,we have /to do it only upon the
                //function's very first invocation, 
                //i.e. when irq is 0         
                    mov ebx,0xfec00000
                    or ebx,0x13
                    mov eax,0xc0000000
                    mov dword ptr[eax],ebx
                //now we are about to get 
                //interrupt vector
                PULONG array=NULL;
                //write 0x10+2*irq to IOREGSEL
                // subsequent read from IOWIN returns 
                // 32 low-order bits of Redirection Table
                //that corresponds to our IRQ.
                // 8 low-order bits are interrupt vector, 
                // corresponding to our IRQ
                DWORD vector=(array[4]&0xff);
                // return interrupt vector. Dont forget 
                // that we must return with RETF,
                // and pop 4 bytes off the stack
                    mov eax,vector
                    mov esp,ebp
                    pop ebp
                    retf 4
    //either apic is not supported, or irq is 
    //above 23,i.e. this is the last invocation
    //therefore, clean up gdt and return 500
        //clean up gdt
        sgdt gdtr
        lea eax,gdtr
        mov ebx,dword ptr[eax+2]
        mov eax,0
        mov ax,selector
        shl eax,3
        add ebx,eax
        mov dword ptr[ebx],0
        mov dword ptr[ebx+4],0
        // adjust stack and return
        mov eax,500
        mov esp,ebp
        pop ebp
        retf 4

Once our kernel function declares the local variables, and, hence, needs a standard function prolog, it does not make sense to write it as a naked routine . Our function is supposed to take only 1 parameter, but, once it is going to get invoked via the call gate, CPU will push the value of user-mode CS on the stack below the return address. Do you know a way of explaining it to the compiler? Me neither. Therefore, to make sure that the compiler generates the code properly, we present this extra value on the stack as a function parameter - we are going to ignore it anyway. If IRQ parameter is 24, i.e. this is the function's final invocation, or if APIC is disabled, kernelfunction() cleans up GDT and returns with the error code 500. If everything is OK, it maps IO APIC to the virtual address 0, obtains interrupt vector, corresponding to IRQ parameter, and returns this vector. There is nothing special here. The only thing worth mentioning is that we have to restore EBP and ESP registers before we return - this is very important. It is understandable that we have to return with RETF instruction, and to pop 4 bytes off the stack.

There is one more thing left to deal with - we have to make sure that our code is suitable for running on both uni-processor and SMP machines. With the advent of hyperthreading technology (HT), we should always make an assumption that our code may run on SMP machine - CPU that supports HT is treated as two independent processors by Windows, and, as far as I am concerned, Intel does not produce CPUs without support for HT any more. Under Windows running on SMP machine, every CPU has its own GDT, and any thread may run on any CPU in the system by default. I hope you can imagine the mess we are guaranteed to create by allowing our code to be executed by different processors - we may set up a call gate while running on CPU A and try to enter the kernel while running on CPU B. Therefore, we have to prevent our code from running on more than one processor. This can be done in the following way (go() is the function that runs all the user-mode code that you've seen in this article):

HANDLE thread=CreateThread(0,0,

SetThreadAffinityMask (thread,1);

If the target machine has more that one processor, calling SetThreadAffinityMask() makes sure that our code is allowed to run only on the processor that we have specified. Calling SetThreadAffinityMask() on uni-processor machine does not result in error - this call just has no effect. Therefore, the above adjustment is suitable for both uni-processor and SMP machines.


In conclusion I want to say a few words of warning. First of all, the functionality of our privileged code will always be limited, compared to that of conventional kernel-mode driver. You already know that not all ntoskrnl.exe's exports may be safely called by our code (MmMapIoSpace() is just one example). Therefore, you should use this approach sparingly. Second, I would not advise you to use any of these tricks in production applications - they are intended to be used only in the development of analysis tools, intrusion detection systems and other "unsupported" software. The problem with all unsupported tricks is that they may be system-specific. To make things even worse, they may be hardware-specific - the code that does not pose even a slightest problem on machine A can crash machine B, even if they both run the same version of Windows. Therefore, if your code works perfectly well on your development machine, it is too early to celebrate victory - you never know how it may behave on some other platform.

The sample application has been thoroughly tested on my machine, which runs Windows XP SP2 - it works perfectly well and does not seem to pose even a slightest problem. However, I really don't know how it is going to behave on your system - it is your task to find it out. If something goes wrong, don't hesitate to inform me about it. In such cases, it would be great if you could provide me with some info about your machine (CPU, motherboard, OS version, etc.), as well as the description of the problem - who knows, maybe there are some more hidden bugs that are to be fixed. In order to run the sample, the only thing you have to do is to click on its bitmap, and then wait for message boxes - it may take a few seconds before they pop up, so you have to be patient.

I would highly appreciate if you send me an e-mail with your comments and suggestions.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Comments and Discussions

QuestionTo my opinion a flip should be made Pin
duratio17-Sep-12 23:36
Memberduratio17-Sep-12 23:36 
QuestionWhy we have to look for the address of our process page directory Pin
duratio15-Sep-12 8:18
Memberduratio15-Sep-12 8:18 
Questionprocess address space Pin
nadia_tarik5-Jan-12 10:09
Membernadia_tarik5-Jan-12 10:09 
GeneralAccess violation reading location 0x00000008 Pin
Lutosław3-May-11 2:36
MemberLutosław3-May-11 2:36 
General2 questions regarding some early conclusions Pin
rusty99023-Jul-10 9:28
Memberrusty99023-Jul-10 9:28 
GeneralHelp, NtOpenSection is denied Pin
hustsolo11-Jun-10 22:22
Memberhustsolo11-Jun-10 22:22 
GeneralI got solution for access violation. [modified] Pin
Pakhanbum6-Apr-10 17:46
MemberPakhanbum6-Apr-10 17:46 
GeneralMy vote of 1 Pin
Michael Chourdakis30-Jan-10 7:19
MemberMichael Chourdakis30-Jan-10 7:19 
GeneralRe: My vote of 1 Pin
Kerem Guemruekcue15-Sep-15 21:52
MemberKerem Guemruekcue15-Sep-15 21:52 
GeneralI tried to apply this to Vista and... Pin
webteca28-Aug-09 0:47
Memberwebteca28-Aug-09 0:47 
Generalrun on PAE Pin
huntrsky2-Mar-09 20:31
Memberhuntrsky2-Mar-09 20:31 
GeneralRe: run on PAE Pin
Member 125704512-Sep-09 4:30
MemberMember 125704512-Sep-09 4:30 
Questionget/set interrupt polarity bit Pin
benchi18-Feb-09 22:26
Memberbenchi18-Feb-09 22:26 
GeneralMay be you could help me, Anton Pin
luixilva10-Oct-07 12:52
Memberluixilva10-Oct-07 12:52 
i need to replace a printer for another computer to send the "output" of the lpt1 to a file.

I have some problems to read the data, status and control ports, I mean, i can't manage the busy and acknowledge in the second computer, in fact, i can't read what the first computer is sending to the printer. Could you please help me? or telling me what can i do for solve my problem?

I have a code to make this in VC++ 6.0, but... it doesn't work.

Thank you in advance for your attention.


QuestionWhat about other rings? Pin
Michael Chourdakis8-Oct-07 11:21
MemberMichael Chourdakis8-Oct-07 11:21 
GeneralRe: Tried to count the # of resident pages and failed :-( Pin
Anton Bassov18-Aug-07 8:25
MemberAnton Bassov18-Aug-07 8:25 
GeneralThat article isn&#8217;t applicable to WinServer2003 and x64 Pin
Vitaliy Jungle25-Mar-07 2:54
MemberVitaliy Jungle25-Mar-07 2:54 
GeneralSecurity descriptor stuff simplified Pin
FMShadow5-Nov-06 4:30
MemberFMShadow5-Nov-06 4:30 
GeneralThe best Pin
macescandell27-Oct-06 9:57
Membermacescandell27-Oct-06 9:57 
Questionvery good but... [modified] Pin
konma27-Aug-06 11:14
Memberkonma27-Aug-06 11:14 
GeneralCrashes on XP Pin
D.Dilber9-May-06 11:37
MemberD.Dilber9-May-06 11:37 
GeneralRe: Crashes on XP Pin
Anton Bassov9-May-06 20:21
MemberAnton Bassov9-May-06 20:21 
GeneralRe: Crashes on XP Pin
D.Dilber10-May-06 10:40
MemberD.Dilber10-May-06 10:40 
AnswerRe: Crashes on XP Pin
Spy Hunter31-Aug-06 20:56
MemberSpy Hunter31-Aug-06 20:56 
GeneralRe: Crashes on XP Pin
Anton Bassov1-Sep-06 1:49
MemberAnton Bassov1-Sep-06 1:49 

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