Re: [PATCH 4.4 00/37] 4.4.110-stable review

[Kees Cook]

On Thu, Jan 4, 2018 at 9:33 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, Jan 4, 2018 at 12:43 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>
>>> On Jan 4, 2018, at 12:29 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>>> On Thu, Jan 4, 2018 at 12:16 PM, Thomas Voegtle <tv@xxxxxxxx> wrote:
>>>>
>>>> Attached a screenshot.
>>>> Is that useful? Are there some debug options I can add?
>>>
>>> Not much of an oops, because the SIGSEGV happens in user space. The
>>> only reason you get any kernel stack printout at all is because 'init'
>>> dying will make the kernel print that out.
>>>
>>> The segfault address for init looks like the fixmap area to me (first
>>> byte in the last page of the fixmap?). "Error 5" means that it's a
>>> user-space read that got a protection fault. So it's not a LDT of GDT
>>> update or anything like that, it's a normal access from user space (or
>>> a qemu emulation bug, but that sounds unlikely).
>>>
>>> Is that the vsyscall page?
>>>
>>> Adding Luto to the participants. I think he noticed one of the
>>> vsyscall patches missing earlier in the 4.9 series. Maybe the 4.4
>>> series had something similar..
>>>
>>
>> That's almost certainly it.
>>
>> I'll try to find some time today or tomorrow to add a proper selftest.
>>
>
> Give this a shot:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=17c5ebeb2e00879b0af1a9c32bf37ecdd9b9b31b
>
> Boot with each of vsyscall=none, vsyscall=native, and vsyscall=emulate
> and run both the 32-bit and 64-bit variants of that test.  All six
> combinations should pass.  But I bet they don't on 4.4.

With my 4.4.110-rc1 under QEMU -cpu=host (Xeon E5-2690 v3)

vsyscall=emulate:

# ./test_vsyscall_64
...
[RUN]   Checking read access to the vsyscall page
[FAIL]  We don't have read access, but we should

vsyscall=native:

# ./test_vsyscall_64
...
[RUN]   Checking read access to the vsyscall page
[FAIL]  We don't have read access, but we should

Everything else passes.

Note that test_vsyscall_32 warns:

# ./test_vsyscall_32
Warning: failed to find getcpu in vDSO
...

-Kees

-- 
Kees Cook
Pixel Security