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

[David Drysdale]

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="secccomp_bpf.ke"
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="secccomp_bpf.ke"
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.

D.
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