Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757599AbaFYQK4 (ORCPT ); Wed, 25 Jun 2014 12:10:56 -0400 Received: from mail-lb0-f173.google.com ([209.85.217.173]:35380 "EHLO mail-lb0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757067AbaFYQKy (ORCPT ); Wed, 25 Jun 2014 12:10:54 -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> From: Andy Lutomirski Date: Wed, 25 Jun 2014 09:10:32 -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 7:51 AM, Kees Cook wrote: > On Wed, Jun 25, 2014 at 6:51 AM, Oleg Nesterov wrote: >> On 06/24, Kees Cook wrote: >>> >>> +static inline void seccomp_assign_mode(struct task_struct *task, >>> + unsigned long seccomp_mode) >>> +{ >>> + BUG_ON(!spin_is_locked(&task->sighand->siglock)); >>> + >>> + task->seccomp.mode = seccomp_mode; >>> + set_tsk_thread_flag(task, TIF_SECCOMP); >>> +} >> >> OK, but unless task == current this can race with secure_computing(). >> I think this needs smp_mb__before_atomic() and secure_computing() needs >> rmb() after test_bit(TIF_SECCOMP). >> >> Otherwise, can't __secure_computing() hit BUG() if it sees the old >> mode == SECCOMP_MODE_DISABLED ? >> >> Or seccomp_run_filters() can see ->filters == NULL and WARN(), >> smp_load_acquire() only serializes that LOAD with the subsequent memory >> operations. > > Hm, actually, now I'm worried about smp_load_acquire() being too slow > in run_filters(). > > The ordering must be: > - task->seccomp.filter must be valid before > - task->seccomp.mode is set, which must be valid before > - TIF_SECCOMP is set > > But I don't want to impact secure_computing(). What's the best way to > make sure this ordering is respected? Remove the ordering requirement, perhaps? What if you moved mode into seccomp.filter? Then there would be little reason to check TIF_SECCOMP from secure_computing; instead, you could smp_load_acquire (or read_barrier_depends, maybe) seccomp.filter from secure_computing and pass the result as a parameter to __secure_computing. Or you could even remove the distinction between secure_computing and __secure_computing -- it's essentially useless anyway to split entry hook approaches like my x86 fastpath prototype. --Andy -- 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/