From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758002AbaFYSIR (ORCPT ); Wed, 25 Jun 2014 14:08:17 -0400 Received: from mail-lb0-f169.google.com ([209.85.217.169]:63837 "EHLO mail-lb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757657AbaFYSIN (ORCPT ); Wed, 25 Jun 2014 14:08:13 -0400 MIME-Version: 1.0 In-Reply-To: References: <1403642893-23107-1-git-send-email-keescook@chromium.org> <1403642893-23107-6-git-send-email-keescook@chromium.org> <20140625135121.GB7892@redhat.com> <20140625173245.GA17695@redhat.com> <20140625175136.GA18185@redhat.com> From: Andy Lutomirski Date: Wed, 25 Jun 2014 11:07:51 -0700 Message-ID: Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines To: Kees Cook Cc: Oleg Nesterov , LKML , "Michael Kerrisk (man-pages)" , Alexei Starovoitov , Andrew Morton , Daniel Borkmann , Will Drewry , Julien Tinnes , David Drysdale , Linux API , "x86@kernel.org" , "linux-arm-kernel@lists.infradead.org" , linux-mips@linux-mips.org, linux-arch , linux-security-module 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 Wed, Jun 25, 2014 at 11:00 AM, Kees Cook wrote: > On Wed, Jun 25, 2014 at 10:51 AM, Oleg Nesterov wrote: >> On 06/25, Andy Lutomirski wrote: >>> >>> On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov wrote: >>> > On 06/25, Andy Lutomirski wrote: >>> >> >>> >> Write the filter, then smp_mb (or maybe a weaker barrier is okay), >>> >> then set the bit. >>> > >>> > Yes, exactly, this is what I meant. Plas rmb() in __secure_computing(). >>> > >>> > But I still can't understand the rest of your discussion about the >>> > ordering we need ;) >>> >>> Let me try again from scratch. >>> >>> Currently there are three relevant variables: TIF_SECCOMP, >>> seccomp.mode, and seccomp.filter. __secure_computing needs >>> seccomp.mode and seccomp.filter to be in sync, and it wants (but >>> doesn't really need) TIF_SECCOMP to be in sync as well. >>> >>> My suggestion is to rearrange it a bit. Move mode into seccomp.filter >>> (so that filter == NULL implies no seccomp) and don't check > > This would require that we reimplement mode 1 seccomp via mode 2 > filters. Which isn't too hard, but may add complexity. > >>> TIF_SECCOMP in secure_computing. Then turning on seccomp is entirely >>> atomic except for the fact that the seccomp hooks won't be called if >>> filter != NULL but !TIF_SECCOMP. This removes all ordering >>> requirements. >> >> Ah, got it, thanks. Perhaps I missed somehing, but to me this looks like >> unnecessary complication at first glance. >> >> We alredy have TIF_SECCOMP, we need it anyway, and we should only care >> about the case when this bit is actually set, so that we can race with >> the 1st call of __secure_computing(). >> >> Otherwise we are fine: we can miss the new filter anyway, ->mode can't >> be changed it is already nonzero. >> >>> Alternatively, __secure_computing could still BUG_ON(!seccomp.filter). >>> In that case, filter needs to be set before TIF_SECCOMP is set, but >>> that's straightforward. >> >> Yep. And this is how seccomp_assign_mode() already works? It is called >> after we change ->filter chain, it changes ->mode before set(TIF_SECCOMP) >> just it lacks a barrier. > > Right, I think the best solution is to add the barrier. I was > concerned that adding the read barrier in secure_computing would have > a performance impact, though. > I can't speak for ARM, but I think that all of the read barriers are essentially free on x86. (smp_mb is a very different story, but that shouldn't be needed here.) --Andy