From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753591AbbHMRj6 (ORCPT ); Thu, 13 Aug 2015 13:39:58 -0400 Received: from mail-vk0-f46.google.com ([209.85.213.46]:35431 "EHLO mail-vk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751993AbbHMRj5 (ORCPT ); Thu, 13 Aug 2015 13:39:57 -0400 MIME-Version: 1.0 In-Reply-To: References: <55CCB510.3060807@redhat.com> From: David Drysdale Date: Thu, 13 Aug 2015 18:39:36 +0100 Message-ID: Subject: Re: [Regression v4.2 ?] 32-bit seccomp-BPF returned errno values wrong in VM? To: Andy Lutomirski Cc: Denys Vlasenko , Kees Cook , "linux-kernel@vger.kernel.org" , Will Drewry , Ingo Molnar , Alok Kataria , Linus Torvalds , Borislav Petkov , Alexei Starovoitov , Frederic Weisbecker , "H. Peter Anvin" , Oleg Nesterov , Steven Rostedt , X86 ML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 13, 2015 at 6:15 PM, Andy Lutomirski wrote: > On Thu, Aug 13, 2015 at 9:28 AM, David Drysdale wrote: >> On Thu, Aug 13, 2015 at 4:17 PM, Denys Vlasenko 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. > > 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