Re: [Regression v4.2 ?] 32-bit seccomp-BPF returned errno values wrong in VM?

From: Kees Cook
Date: Thu Aug 13 2015 - 14:47:31 EST

On Thu, Aug 13, 2015 at 10:39 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
> On Thu, Aug 13, 2015 at 6:15 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Thu, Aug 13, 2015 at 9:28 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
>>> On Thu, Aug 13, 2015 at 4:17 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
>>>> On 08/13/2015 10:30 AM, David Drysdale wrote:
>>>>> Hi folks,
>>>>> I've got an odd regression with the v4.2 rc kernel, and I wondered if anyone
>>>>> else could reproduce it.
>>>>> The problem occurs with a seccomp-bpf filter program that's set up to return
>>>>> an errno value -- an errno of 1 is always returned instead of what's in the
>>>>> filter, plus other oddities (selftest output below).
>>>>> The problem seems to need a combination of circumstances to occur:
>>>>> - The seccomp-bpf userspace program needs to be 32-bit, running against a
>>>>> 64-bit kernel -- I'm testing with seccomp_bpf from
>>>>> tools/testing/selftests/seccomp/, built via 'CFLAGS=-m32 make'.
>>>> Does it work correctly when built as 64-bit program?
>>> Yep, 64-bit works fine (both at v4.2-rc6 and at commit 3f5159).
>>>>> - The kernel needs to be running as a VM guest -- it occurs inside my
>>>>> VMware Fusion host, but not if I run on bare metal. Kees tells me he
>>>>> cannot repro with a kvm guest though.
>>>>> Bisecting indicates that the commit that induces the problem is
>>>>> 3f5159a9221f19b0, "x86/asm/entry/32: Update -ENOSYS handling to match the
>>>>> 64-bit logic", included in all the v4.2-rc* candidates.
>>>>> Apologies if I've just got something odd with my local setup, but the
>>>>> bisection was unequivocal enough that I thought it worth reporting...
>>>>> Thanks,
>>>>> David
>>>>> seccomp_bpf failure outputs:
>>> [snip]
>>>> End result should be:
>>>> pt_regs->ax = -E2BIG (via syscall_set_return_value())
>>>> pt_regs->orig_ax = -1 ("skip syscall")
>>>> and syscall_trace_enter_phase1() usually returns with 0,
>>>> meaning "re-execute syscall at once, no phase2 needed".
>>>> This, in turn, is called from .S files, and when it returns there,
>>>> execution loops back to syscall dispatch.
>>>> Because of orig_ax = -1, syscall dispatch should skip calling syscall.
>>>> So -E2BIG should survive and be returned...
>>> So I was just about to send:
>>> That makes sense, and given that exactly the same 32-bit binary
>>> runs fine on a different machine, there's presumably something up
>>> with my local setup. The failing machine is a VMware guest, but
>>> maybe that's not the relevant interaction -- particularly if no-one
>>> else can repro.
>>> But then I noticed some odd audit entries in the main log:
>>> Aug 13 16:52:56 ubuntu kernel: [ 20.687249] audit: type=1326
>>> audit(1439481176.034:62): auid=4294967295 uid=1000 gid=1000
>>> ses=4294967295 pid=2621 comm=""
>>> exe="/home/dmd/secccomp_bpf.kees.m32" sig=9 arch=40000003 syscall=172
>>> compat=1 ip=0xf773cc90 code=0x0
>>> Aug 13 16:52:56 ubuntu kernel: [ 20.691157] audit: type=1326
>>> audit(1439481176.038:63): auid=4294967295 uid=1000 gid=1000
>>> ses=4294967295 pid=2631 comm=""
>>> exe="/home/dmd/secccomp_bpf.kees.m32" sig=31 arch=40000003 syscall=20
>>> compat=1 ip=0xf773cc90 code=0x10000000
>>> ...
>>> I didn't think I had any audit stuff turned on, and indeed:
>>> # auditctl -l
>>> No rules
>>> But as soon as I'd run that auditctl command, the 32-bit
>>> seccomp_bpf binary started running fine!
>>> So now I'm confused, and I can no longer reproduce the
>>> problem. Which probably means this was a false alarm, in
>>> which case, my apologies.
>> You might have triggered TIF_AUDIT or whatever it's called, which
>> causes a whole different path through the asm tangle, so you might
>> really have a problem.
>> Try auditctl -a task,never. If that doesn't change anything, try
>> rebooting the guest.
> Aha, that seems to re-instate the problem -- with that auditctl setup
> I get the 32-bit seccomp failures on two different machines (one VM,
> one bare). So can anyone else repro?
> I guess the relevant steps are thus:
> - sudo auditctl -a task,never
> - cd tools/testing/selftests/seccomp
> - CFLAGS=-m32 make clean run_tests

That was it! I can reproduce this now on kvm (after adding the auditctl rule).


Kees Cook
Chrome OS Security
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