Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755197Ab1EDSDV (ORCPT ); Wed, 4 May 2011 14:03:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:22616 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755110Ab1EDSDU (ORCPT ); Wed, 4 May 2011 14:03:20 -0400 Subject: Re: [PATCH 5/7] seccomp_filter: Document what seccomp_filter is and how it works. From: Eric Paris To: Steven Rostedt Cc: Frederic Weisbecker , Will Drewry , Ingo Molnar , linux-kernel@vger.kernel.org, kees.cook@canonical.com, agl@chromium.org, jmorris@namei.org, Randy Dunlap , Linus Torvalds , Andrew Morton , Tom Zanussi , Arnaldo Carvalho de Melo , Peter Zijlstra , Thomas Gleixner Date: Wed, 04 May 2011 14:02:20 -0400 In-Reply-To: <1304527161.25414.2438.camel@gandalf.stny.rr.com> References: <1303960136-14298-1-git-send-email-wad@chromium.org> <1303960136-14298-4-git-send-email-wad@chromium.org> <20110428070636.GC952@elte.hu> <1304002571.2101.38.camel@localhost.localdomain> <20110429131845.GA1768@nowhere> <20110503012857.GA8399@nowhere> <1304511396.25414.2422.camel@gandalf.stny.rr.com> <1304524465.10692.21.camel@localhost.localdomain> <1304525169.25414.2427.camel@gandalf.stny.rr.com> <1304526163.10692.33.camel@localhost.localdomain> <1304527161.25414.2438.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Message-ID: <1304532144.10692.64.camel@localhost.localdomain> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4778 Lines: 155 On Wed, 2011-05-04 at 12:39 -0400, Steven Rostedt wrote: > On Wed, 2011-05-04 at 12:22 -0400, Eric Paris wrote: [rewriting history] > > Although logically the same, it's not just one huge rule. I don't see > > any need for any operation other than an &&. Before the first "apply" you > > can add new syscalls. After the first apply you can only && onto existing > > syscalls. So the following set of operations: > > > > SECCOMP_FILTER_SET, __NR_foo, "a=0" > > SECCOMP_FILTER_SET, __NR_read, "1" > > SECCOPM_FILTER_APPLY > > > > SECCOMP_FILTER_SET, __NR_foo, "b=0" > > SECCOMP_FILTER_APPLY > > > > SECCOMP_FILTER_SET, __NR_write, "1" > > SECCOMP_FILTER_APPLY > > > > Would return EPERM for the __NR_write entry since it was a new syscall > > after an apply. I think we agree on all this. > > Do you mean "after a apply"? As the second line above is a new syscall > after the first set. Clearly. Especially since given my revisionist history it's what I said! > > > > I do have a question on some syntax proposed a while back. Given: > > SECCOMP_FILTER_SET, __NR_foo, "a=0" > > SECCOMP_FILTER_SET, __NR_foo, "b=0" > > SECCOMP_FILTER_APPLY > > > > I would think to keep the interface consistent that should result in > > foo: (a=0) && (b=0) > > I agree. > > > > > But I think the proposal was that we should instead have just > > foo: (b=0) > > Yeah, that's what it looked like Frederic showed. I rather have the > first instance. > > Perhaps we could have a "unset"? that would only work on things that > haven't been applied yet. > > > > > What's the logic behind having a second call overwrite uncommitted > > changes? I sorta feel like if I put it in there, I must have wanted it > > in there :) > > Perhaps for making the user code simpler? > > SET a=1 > SET b=2 > > [ some nasty if logic ] > > UNSET b=2 > > APPLY > > > Thus a default setting can be made and then we can selectively remove > settings before we do the apply based on information about the process > that we will exec. We can start out with "limit the hell out of it" and > then selectively remove things. I think this is a simply way to > understand what is being done. Kind of like iptables, where you can set > up default rules, but then selectively override them. > > One thing I know about security, the easier it is to set up, the more > secure it becomes. I'm ok with an explicit unset/remove/delete for rules since the last apply if anyone thinks it will be useful. But I don't like an implicit 'overwrite.' I'm starting to debate if I think between rules should be an implicit && or should be an implicit ||. After an 'apply' I believe the next block definitely needs an &&. I feel like the code in userspace becomes simpler with || but maybe it would be more confusing for the human coder to have to know the distinctions. The example in my head, which I think will be common, involves handling multiple fd's for read. We could either do: int handle_read_fd(int fd) { char buf[4096]; snprintf(buf, 4096, "a0=%d", fd) return prctl(SECCOMP_FILTER_SET, __NR_read, buf); } main() { .... fd1 = open("readfile1", O_RDONLY); if (fd1 < 0) return if (handle_read_fd(fd1)) return .... fd2 = open("readfile2, O_RDONLY); if (fd2 < 0) return if (handle_read_fd(fd2)) return .... prctl(SECCOMP_FILTER_APPLY); } Or case2 with the implicit && like we talked about to handle an arbitrary number of read fd's you need (pseudocode): int handle_read_fds(int *fds, int len) { char buf[4096]; size_t saved; while(len--) { saved = snprintf(buf, 4096, "a0=%d ||"); buf += saved; } return prctl(SECCOMP_FILTER_SET, __NR_read, buf) } main() { int fd[2]; .... fd[0] = open("readfile1", O_RDONLY); if (fd[0] < 0) return .... fd[1] = open("readfile2, O_RDONLY); if (fd[1] < 0) return .... handle_read_fds(fd, 2) prctl(SECCOMP_FILTER_APPLY); } The latter of the examples requires what I think to be the common case of complex rules to be required to hold state whereas the other is done in the kernel. It's a lot easier to code the first one but it might be harder on the coder to decide "now is than an && or an ||"? Like I said, I'm ok with just declaring everything &&, especially if people think complex filters are likely to be anything more than rule1 || rule2 || rule3 || rule4 where ruleX is an independent clause, but I figured I'd throw it out there.... No matter what after the APPLY I think that: SECCOMP_FILTER, __NR_read, "0" should result in: (rule1 || rule2 || rule3 || rule4) && 0 -Eric -- 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/