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

From: Andy Lutomirski
Date: Thu Aug 13 2015 - 13:16:15 EST

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.


> D.

Andy Lutomirski
AMA Capital Management, LLC
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