Wednesday, December 28, 2016

ExInterlockedPopEntrySList processing by scheduler.

I believe this topic on ExInterlockedPopEntrySList might be interesting for Windows drivers developers.

Safety of using ExInterlockedPopEntrySList

The question was

To my knowledge, pre-Windows 8 x64 implementations of SList use 9-bit sequence numbers in the SLIST_HEADER. This means that 512 operations can complete concurrently (without progress from particular thread) until an ABA problem potentially manifests. I wonder whether, depending on the number of threads and physical cores, this couldn't plausibly occur. To further complicate, the kernel could run on a vcpu, creating time discontinuities. I would like to ask: 1. Does the Windows scheduler protect against ABA by, e.g., restarting interlocked operation upon preemption? 2. Is there some protection against hypervisor interference? 3. In the light of the above concerns, is SList on a pre-Windows 8 x64 deployment really safe for all workloads? I would have speculated that per-thread kernel allocator behavior was factored in for the ABA avoidance, but the primitives are in the Win32 API as well and any driver can employ custom pool allocator.
My answer was

I looked at the code again and found that interrupt processing code has a fixup for SList . There is a routine KiCheckForSListAddress. This routine is called at DISPATCH_LEVEL before returning from an interrupt and it fixes the EIP(RIP for x64) of a trap frame to restart SList pop operation if interrupt happened inside ExInterlockedPopEntrySList. So when an interrupt processing code returns execution to an interrupted code the code resumes at the beginning of ExInterlockedPopEntrySList ( namely ExpInterlockedPopEntrySListResume ). kd> uf KiCheckForSListAddress nt!KiCheckForSListAddress: 82acbdf1 0fb7416c movzx eax,word ptr [ecx+6Ch] 82acbdf5 8b5168 mov edx,dword ptr [ecx+68h] 82acbdf8 6683f808 cmp ax,8 82acbdfc 7511 jne nt!KiCheckForSListAddress+0x1e (82acbe0f) Branch nt!KiCheckForSListAddress+0xd: 82acbdfe b8f4dda882 mov eax,offset nt!ExpInterlockedPopEntrySListResume (82a8ddf4) 82acbe03 3bd0 cmp edx,eax 82acbe05 7222 jb nt!KiCheckForSListAddress+0x38 (82acbe29) Branch nt!KiCheckForSListAddress+0x16: 82acbe07 81fa1fdea882 cmp edx,offset nt!ExpInterlockedPopEntrySListEnd (82a8de1f) 82acbe0d eb15 jmp nt!KiCheckForSListAddress+0x33 (82acbe24) Branch nt!KiCheckForSListAddress+0x1e: 82acbe0f 6683f81b cmp ax,1Bh 82acbe13 7514 jne nt!KiCheckForSListAddress+0x38 (82acbe29) Branch nt!KiCheckForSListAddress+0x24: 82acbe15 a1ac69bb82 mov eax,dword ptr [nt!KeUserPopEntrySListResume (82bb69ac)] 82acbe1a 3bd0 cmp edx,eax 82acbe1c 720b jb nt!KiCheckForSListAddress+0x38 (82acbe29) Branch nt!KiCheckForSListAddress+0x2d: 82acbe1e 3b15a469bb82 cmp edx,dword ptr [nt!KeUserPopEntrySListEnd (82bb69a4)] nt!KiCheckForSListAddress+0x33: 82acbe24 7703 ja nt!KiCheckForSListAddress+0x38 (82acbe29) Branch nt!KiCheckForSListAddress+0x35: 82acbe26 894168 mov dword ptr [ecx+68h],eax nt!KiCheckForSListAddress+0x38: 82acbe29 c3 ret Branch

Sunday, December 25, 2016

MacOS network filter

I have added a MacOS network sockets filter to my GitHub repository - MacOSX-Network-Sockets-Filter . The filter allows to inspect and modify network data in a user mode application.

Thursday, December 22, 2016

MacOS VFS file system isolation filter.

I have committed a MacOS VFS isolation filter project to my GitHub repository - MacOSX-VFS-Isolation-Filter The filter allows to isolate files I/O operation. The possible applications for a filter are content analyzing, encryption or any advanced data flow modification.

Thursday, December 15, 2016

Linux process. The beginning.

Do you ever wonder how a Linux process address space looks like when the first user mode instruction is executed? The answer is below ( the executable file is /bin/grep )

00400000-0042d000 r-xp 00000000 08:11 27316285             /bin/grep
0062d000-0062f000 rw-p 0002d000 08:11 27316285             /bin/grep
0062f000-00630000 rw-p 00000000 00:00 0                    [heap]
7ffff7dda000-7ffff7dfd000 r-xp 00000000 08:11 11172260     /lib/x86_64-linux-gnu/
7ffff7ffa000-7ffff7ffc000 r-xp 00000000 00:00 0            [vdso]
7ffff7ffc000-7ffff7ffe000 rw-p 00022000 08:11 11172260     /lib/x86_64-linux-gnu/
7ffff7ffe000-7ffff7fff000 rw-p 00000000 00:00 0 
7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0            [stack]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0    [vsyscall]

The first user mode instruction is

0x00007ffff7ddb2d0 in _start () from /lib64/