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

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Date:	Thu, 13 Aug 2015 09:30:26 +0100
From:	David Drysdale <drysdale@...gle.com>
To:	Kees Cook <keescook@...omium.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Drewry <wad@...omium.org>, Ingo Molnar <mingo@...nel.org>
Cc:	Alok Kataria <akataria@...are.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>, X86 ML <x86@...nel.org>
Subject: [Regression v4.2 ?] 32-bit seccomp-BPF returned errno values wrong in VM?

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'.

 - 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:

seccomp_bpf.c:533:global.ERRNO_valid:Expected 7 (7) ==
(*__errno_location ()) (1)
seccomp_bpf.c:560:global.ERRNO_zero:Expected 0 (0) == read(0, ((void
*)0), 0) (4294967295)
seccomp_bpf.c:587:global.ERRNO_capped:Expected 4095 (4095) ==
(*__errno_location ()) (1)
seccomp_bpf.c:905:precedence.errno_is_third:Expected 0 (0) ==
syscall(20) (4294967295)
seccomp_bpf.c:925:precedence.errno_is_third_in_any_order:Expected 0
(0) == syscall(20) (4294967295)
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