linux-kernel - Re: [PATCH] futex: futex_find_get_task make credentials check conditional

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Date:	Tue, 29 Jun 2010 09:58:15 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Michal Hocko <mhocko@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	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>
Subject: Re: [PATCH] futex: futex_find_get_task make credentials check conditional

On 06/29/2010 09:41 AM, 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? 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
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