From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [PATCH v9 09/11] seccomp: introduce writer locking Date: Thu, 10 Jul 2014 17:24:18 +0200 Message-ID: <20140710152418.GB20861@redhat.com> References: <1403911380-27787-1-git-send-email-keescook@chromium.org> <1403911380-27787-10-git-send-email-keescook@chromium.org> <20140709184215.GA4866@redhat.com> <20140709185549.GB4866@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-subscribe: List-owner: List-post: List-archive: To: Kees Cook Cc: LKML , Andy Lutomirski , "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 List-Id: linux-api@vger.kernel.org On 07/10, Kees Cook wrote: > > On Wed, Jul 9, 2014 at 11:55 AM, Oleg Nesterov wrote: > > On 07/09, Oleg Nesterov wrote: > >> > >> On 06/27, Kees Cook wrote: > >> > > >> > static u32 seccomp_run_filters(int syscall) > >> > { > >> > - struct seccomp_filter *f; > >> > + struct seccomp_filter *f = ACCESS_ONCE(current->seccomp.filter); > >> > >> I am not sure... > >> > >> This is fine if this ->filter is the 1st (and only) one, in this case > >> we can rely on rmb() in the caller. > >> > >> But the new filter can be installed at any moment. Say, right after that > >> rmb() although this doesn't matter. Either we need smp_read_barrier_depends() > >> after that, or smp_load_acquire() like the previous version did? > > > > Wait... and it seems that seccomp_sync_threads() needs smp_store_release() > > when it sets thread->filter = current->filter by the same reason? > > > > OTOH. smp_store_release() in seccomp_attach_filter() can die, "current" > > doesn't need a barrier to serialize with itself. > > I have lost track of what you're suggesting to change. :) Perhaps I am just trying to confuse you and myself ;) But, > Since rmb() happens before run_filters, isn't the ACCESS_ONCE > sufficient? Yes. But see above. ACCESS_ONCE is sufficient if we read the first filter installed by another thread, in this case rmb() pairs with mb_before_atomic() before set_bit(TIF_SECCOMP). IOW, if this threads sees TIF_SECCOMP, it should also see all modifications which were done before set_bit, including the data in ->filter points to. > We only care that TIF_SECCOMP, mode, and some filter is > valid. In a tsync thread race, it's okay to use not use the deepest > filter node in the list, Yes, it is fine if we miss yet another filter which was just installed by another thread. But, unless I missed something, the problem is that we can get this new filter. Just to simplify. Suppose TIF_SECCOMP was set a long ago. This thread has a single filter F1 and it enters seccomp_run_filters(). Right before it does ACCESS_ONCE() to read the pointer, another thread does seccomp_sync_threads() and sets .filter = F2. If ACCESS_ONCE() returns F1 - everything is fine. But it can see the new pointer F2, and in this case we need a barrier to ensure that, say, LOAD(F2->prog) will see all the preceding changes in this memory. Oleg.