linux-kernel - Re: [PATCH] futex: futex_find_get

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Date:	Thu, 8 Jul 2010 11:32:41 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <dvhltc@...ibm.com>
Subject: Re: [PATCH] futex: futex_find_get_task remove credentails check


* Michal Hocko <mhocko@...e.cz> wrote:

> On Wed 30-06-10 09:43:27, Darren Hart wrote:
> > On 06/30/2010 02:55 AM, Michal Hocko wrote:
> > >On Wed 30-06-10 09:01:15, Michal Hocko wrote:
> > >>On Tue 29-06-10 09:41:02, Linus Torvalds wrote:
> > >>>On Tue, Jun 29, 2010 at 1:42 AM, Michal Hocko<mhocko@...e.cz>  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?
> > >>
> > >>I though that requeue needs this for security reasons (don't let requeue
> > >>process for other user), but when I thought about that again you are
> > >>right and the only what matters should be accessibility of the shared
> > >>memory.
> > >
> > >And here is the patch which does the thing.
> > >
> > >--
> > >
> > > From 082c5ad2c482a8e78b61b17e213e750b006176aa Mon Sep 17 00:00:00 2001
> > >From: Michal Hocko<mhocko@...e.cz>
> > >Date: Wed, 30 Jun 2010 09:51:19 +0200
> > >Subject: [PATCH] futex: futex_find_get_task remove credentails check
> > >
> > >futex_find_get_task is currently used (through lookup_pi_state) from two
> > >contexts, futex_requeue and futex_lock_pi_atomic. None of the paths
> > >looks it needs the credentials check, though. Different (e)uids
> > >shouldn't matter at all because the only thing that is important for
> > >shared futex is the accessibility of the shared memory.
> > >
> > >The credentail check results in glibc assert failure or process hang (if
> > >glibc is compiled without assert support) for shared robust pthread
> > >mutex with priority inheritance if a process tries to lock already held
> > >lock owned by a process with a different euid:
> > >
> > >pthread_mutex_lock.c:312: __pthread_mutex_lock_full: Assertion `(-(e)) != 3 || !robust' failed.
> > >
> > >The problem is that futex_lock_pi_atomic which is called when we try to
> > >lock already held lock checks the current holder (tid is stored in the
> > >futex value) to get the PI state. It uses lookup_pi_state which in turn
> > >gets task struct from futex_find_get_task. ESRCH is returned either when
> > >the task is not found or if credentials check fails.
> > >futex_lock_pi_atomic simply returns if it gets ESRCH. glibc code,
> > >however,  doesn't expect that robust lock returns with ESRCH because it
> > >should get either success or owner died.
> > >
> > >Signed-off-by: Michal Hocko<mhocko@...e.cz>
> > 
> > Without hearing back from Ingo on the original intent of the
> > credentials check, this looks right to me.
> 
> Could you comment on that Ingo, please?

I think that's more of a question to Thomas :-)

My memories are hazy and nothing springs out as some credible original intent. 
So please assume it doesnt exist :-)

	Ingo
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