Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758054AbaFYQx5 (ORCPT ); Wed, 25 Jun 2014 12:53:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48957 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758036AbaFYQxx (ORCPT ); Wed, 25 Jun 2014 12:53:53 -0400 Date: Wed, 25 Jun 2014 18:52:09 +0200 From: Oleg Nesterov 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 Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC Message-ID: <20140625165209.GA14720@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/25, Kees Cook wrote: > > On Wed, Jun 25, 2014 at 7:21 AM, Oleg Nesterov wrote: > > > > But. Doesn't this change add a new security hole? > > > > Obviously, we should not allow to install a filter and then (say) exec > > a suid binary, that is why we have no_new_privs/LSM_UNSAFE_NO_NEW_PRIVS. > > > > But what if "thread->seccomp.filter = caller->seccomp.filter" races with > > any user of task_no_new_privs() ? Say, suppose this thread has already > > passed check_unsafe_exec/etc and it is going to exec the suid binary? > > Oh, ew. Yeah. It looks like there's a cred lock to be held to combat this? Yes, cred_guard_mutex looks like an obvious choice... Hmm, but somehow initially I thought that the fix won't be simple. Not sure why. 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? But I was also worried that task_no_new_privs(current) is no longer stable inside the syscall paths, perhaps this is what you meant? However I do not see something bad here... And this has nothing to do with the race above. Also. Even ignoring no_new_privs, SECCOMP_FILTER_FLAG_TSYNC is not atomic and we can do nothing with this fact (unless it try to freeze the thread group somehow), perhaps it makes sense to document this somehow. I mean, suppose you want to ensure write-to-file is not possible, so you do seccomp(SECCOMP_FILTER_FLAG_TSYNC, nack_write_to_file_filter). You can't assume that this has effect right after seccomp() returns, this can obviously race with a sub-thread which has already entered sys_write(). Once again, I am not arguing, just I think it makes sense to at least mention the limitations during the discussion. Oleg. -- 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/