linux-kernel - Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma initialization

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Date:	Sat, 22 Nov 2014 20:14:52 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Davidlohr Bueso <davidlohr@...com>,
	Rafael Aquini <aquini@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma initialization

On 11/21/2014 09:29 PM, Rik van Riel wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 11/21/2014 03:09 PM, Andrew Morton wrote:
>> On Fri, 21 Nov 2014 14:52:26 -0500 Rik van Riel <riel@...hat.com>
>> wrote:
>>
>>> When manipulating just one semaphore with semop, sem_lock only
>>> takes that single semaphore's lock. This creates a problem during
>>> initialization of the semaphore array, when the data structures
>>> used by sem_lock have not been set up yet. The sma->lock is
>>> already held by newary, and we just have to make sure everything
>>> else waits on that lock during initialization.
>>>
>>> Luckily it is easy to make sem_lock wait on the sma->lock, by
>>> pretending there is a complex operation in progress while the sma
>>> is being initialized.
>>>
>>> The newary function already zeroes sma->complex_count before
>>> unlocking the sma->lock.
>> What are the runtime effects of the bug?
>>
> NULL pointer dereference in spin_lock from sem_lock,
> if it is called before sma->sem_base has been pointed
> somewhere valid.
No, this can't happen:
- sma is initialized to 0 with memset()
- sma->sem_nsems is set last.
- semtimedop() contains a "max >= sma->sem_nsems".

with sma->sem_nsems==0, this will always fail and therefore sem_lock() 
can't be reached.

The only misbehavior (apart from returning -EFBIG) is that 
find_alloc_undo() could allocate a wrong-sized undo structure.
Would cause random memory corruptions - but not NULL pointer dereference.

Which which kernel version have you seen the NULL pointer dereference?

--
     Manfred
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