Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758067AbaFYQyI (ORCPT ); Wed, 25 Jun 2014 12:54:08 -0400 Received: from mail-ob0-f170.google.com ([209.85.214.170]:63750 "EHLO mail-ob0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757788AbaFYQyD (ORCPT ); Wed, 25 Jun 2014 12:54:03 -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> Date: Wed, 25 Jun 2014 09:54:01 -0700 X-Google-Sender-Auth: C4SwRTLOti7eX0n26vQa2PKUUj8 Message-ID: Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines From: Kees Cook To: Andy Lutomirski 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 9:10 AM, Andy Lutomirski wrote: > 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. The TIF_SECCOMP is needed for the syscall entry path. The check in secure_computing() is just because the "I am being traced" trigger includes a call to secure_computing, which filters out tracing reasons. Your fast path work would clean a lot of that up, as you say. But it still doesn't change the ordering check here. TIF_SECCOMP indicates seccomp.mode must be checked, so that ordering will remain, and if mode == FILTER, seccomp.filter must be valid. Isn't there a way we can force the assignment ordering in seccomp_assign_mode()? -Kees -- 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/