From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753317AbbHMQ3T (ORCPT ); Thu, 13 Aug 2015 12:29:19 -0400 Received: from mail-vk0-f45.google.com ([209.85.213.45]:34906 "EHLO mail-vk0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752210AbbHMQ3R (ORCPT ); Thu, 13 Aug 2015 12:29:17 -0400 MIME-Version: 1.0 In-Reply-To: <55CCB510.3060807@redhat.com> References: <55CCB510.3060807@redhat.com> From: David Drysdale Date: Thu, 13 Aug 2015 17:28:57 +0100 Message-ID: Subject: Re: [Regression v4.2 ?] 32-bit seccomp-BPF returned errno values wrong in VM? To: Denys Vlasenko Cc: Kees Cook , Andy Lutomirski , "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 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. D.