Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758117AbaFYSAz (ORCPT ); Wed, 25 Jun 2014 14:00:55 -0400 Received: from mail-ob0-f174.google.com ([209.85.214.174]:39298 "EHLO mail-ob0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752600AbaFYSAw (ORCPT ); Wed, 25 Jun 2014 14:00:52 -0400 MIME-Version: 1.0 In-Reply-To: <20140625175136.GA18185@redhat.com> 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> Date: Wed, 25 Jun 2014 11:00:51 -0700 X-Google-Sender-Auth: OqbIQ1uTXUI-aEPiNFDo8JxIExk Message-ID: Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines From: Kees Cook To: Oleg Nesterov Cc: Andy Lutomirski , 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 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. -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/