From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757990AbaFYR5b (ORCPT ); Wed, 25 Jun 2014 13:57:31 -0400 Received: from mail-ob0-f180.google.com ([209.85.214.180]:60449 "EHLO mail-ob0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757670AbaFYR5Q (ORCPT ); Wed, 25 Jun 2014 13:57:16 -0400 MIME-Version: 1.0 In-Reply-To: <20140625172410.GA17133@redhat.com> References: <1403642893-23107-1-git-send-email-keescook@chromium.org> <1403642893-23107-10-git-send-email-keescook@chromium.org> <20140625142121.GD7892@redhat.com> <20140625165209.GA14720@redhat.com> <20140625172410.GA17133@redhat.com> Date: Wed, 25 Jun 2014 10:57:15 -0700 X-Google-Sender-Auth: Y_RnnhWMDXw3HCi12hhr1c66dok Message-ID: Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC From: Kees Cook To: Oleg Nesterov 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 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 10:24 AM, Oleg Nesterov wrote: > On 06/25, Kees Cook wrote: >> >> On Wed, Jun 25, 2014 at 9:52 AM, Oleg Nesterov wrote: >> > >> > Yes, at least this should close the race with suid-exec. And there are no >> > other users. Except apparmor, and I hope you will check it because I simply >> > do not know what it does ;) >> > >> >> I wonder if changes to nnp need to "flushed" during syscall entry >> >> instead of getting updated externally/asynchronously? That way it >> >> won't be out of sync with the seccomp mode/filters. >> >> >> >> Perhaps secure computing needs to check some (maybe seccomp-only) >> >> atomic flags and flip on the "real" nnp if found? >> > >> > Not sure I understand you, could you clarify? >> >> Instead of having TSYNC change the nnp bit, it can set a new flag, say: >> >> task->seccomp.flags |= SECCOMP_NEEDS_NNP; >> >> This would be set along with seccomp.mode, seccomp.filter, and >> TIF_SECCOMP. Then, during the next secure_computing() call that thread >> makes, it would check the flag: >> >> if (task->seccomp.flags & SECCOMP_NEEDS_NNP) >> task->nnp = 1; >> >> This means that nnp couldn't change in the middle of a running syscall. > > Aha, so you were worried about the same thing. Not sure we need this, > but at least I understand you and... > >> Hmmm. Perhaps this doesn't solve anything, though? Perhaps my proposal >> above would actually make things worse, since now we'd have a thread >> with seccomp set up, and no nnp. If it was in the middle of exec, >> we're still causing a problem. > > Yes ;) > >> I think we'd also need a way to either delay the seccomp changes, or >> to notice this condition during exec. Bleh. > > Hmm. confused again, I mean to suggest that the tsync changes would be stored in each thread, but somewhere other than the true seccomp struct, but with TIF_SECCOMP set. When entering secure_computing(), current would check for the "changes to sync", and apply them, then start the syscall. In this way, we can never race a syscall (like exec). >> What actually happens with a multi-threaded process calls exec? I >> assume all the other threads are destroyed? > > Yes. But this is the point-of-no-return, de_thread() is called after the execing > thared has already passed (say) check_unsafe_exec(). > > However, do_execve() takes cred_guard_mutex at the start in prepare_bprm_creds() > and drops it in install_exec_creds(), so it should solve the problem? I can't tell yet. I'm still trying to understand the order of operations here. It looks like de_thread() takes the sighand lock. do_execve_common does: prepare_bprm_creds (takes cred_guard_mutex) check_unsafe_exec (checks nnp to set LSM_UNSAFE_NO_NEW_PRIVS) prepare_binprm (handles suid escalation, checks nnp separately) security_bprm_set_creds (checks LSM_UNSAFE_NO_NEW_PRIVS) exec_binprm load_elf_binary flush_old_exec de_thread (takes and releases sighand->lock) install_exec_creds (releases cred_guard_mutex) I don't see a way to use cred_guard_mutex during tsync (which holds sighand->lock) without dead-locking. What were you considering here? -Kees -- Kees Cook Chrome OS Security