Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932214Ab0F2Q63 (ORCPT ); Tue, 29 Jun 2010 12:58:29 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:46815 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932200Ab0F2Q62 (ORCPT ); Tue, 29 Jun 2010 12:58:28 -0400 Message-ID: <4C2A2627.3080408@us.ibm.com> Date: Tue, 29 Jun 2010 09:58:15 -0700 From: Darren Hart User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100423 Thunderbird/3.0.4 MIME-Version: 1.0 To: Linus Torvalds CC: Michal Hocko , Ingo Molnar , Thomas Gleixner , LKML , Nick Piggin , Alexey Kuznetsov , Peter Zijlstra Subject: Re: [PATCH] futex: futex_find_get_task make credentials check conditional References: <20100623091307.GA11072@tiehlicka.suse.cz> <4C2417AA.4030306@us.ibm.com> <20100625082711.GA32765@tiehlicka.suse.cz> <4C24ED34.9040808@us.ibm.com> <4C253D32.6040304@us.ibm.com> <20100628144246.GA14201@tiehlicka.suse.cz> <20100628153214.GA24127@tiehlicka.suse.cz> <20100628155845.GC24127@tiehlicka.suse.cz> <20100628163952.GD24127@tiehlicka.suse.cz> <1277743748.3561.139.camel@laptop> <20100629084230.GE6215@tiehlicka.suse.cz> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2759 Lines: 70 On 06/29/2010 09:41 AM, Linus Torvalds wrote: > On Tue, Jun 29, 2010 at 1:42 AM, Michal Hocko wrote: >> >> futex_find_get_task is currently used (through lookup_pi_state) from two >> contexts, futex_requeue and futex_lock_pi_atomic. While credentials check >> makes sense in the first code path, the second one is more problematic >> because this check requires that the PI lock holder (pid parameter) has >> the same uid and euid as the process's euid which is trying to lock the >> same futex (current). > > So exactly why does it make sense to check the credentials in the > first code path then? Shouldn't the futex issue in the end depend on > whether you have a shared page or not - and not on credentials at all? > Any two processes that share a futex in the same shared page should be > able to use that without any regard for whether they are the same > user. That's kind of the point, no? I agree and haven't been able to come up with a need for the test either, but I wanted to hear back from Ingo as the he authored the original check. I was trying to see if futex_lock_pi() could somehow be abused, but if so, I don't see it: TaskUserA TaskUserB futex_lock_pi(addrA) *addrB = TID_OF(TaskUserA) futex_lock_pi(addrB) TaskUserB would lookup the pi_state, not find it as addrB and addrA don't hash to the same key, create a new pi_state and mark TaskUserA as the owner, then block. Once TaskUserA exits, the pi_list will contain the pi_state for the addrB futex. This is "bad", but the kernel cleans it up, releases the lock - but doesn't wake TaskUserB. That seems acceptable to me since TaskUserB is in the wrong here. > IOW, I personally dislike these kinds of conditional checks, > especially since the discussion (at least the part I've seen) hasn't > made it clear why it should be conditional - or exist - in the first > place. > > So I'd like the patch to include an explanation of exactly why the two > cases are different. Agreed, waiting on Ingo at the moment. > The other thing I'd like to see is to move the whole cred checking up > a level. There's no reason to check the credentials in > futex_find_get_task() that I can see - why not do it in the caller > instead? IOW, I think futex_find_get_task() should just look something > like this instead: /me beats head on desk, duh. Still, I'm hoping this isn't necessary and we can lose the credentials checking entirely. Thanks, -- Darren Hart IBM Linux Technology Center Real-Time Linux Team -- 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/