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

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Date:	Sat, 22 Nov 2014 15:14:35 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Manfred Spraul <manfred@...orfullife.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

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On 11/22/2014 02:14 PM, Manfred Spraul wrote:
> 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.

You're right. The reported race must have been semop vs RMID.

The kernel tree in question was missing this changeset:

commit 6e224f94597842c5eb17f1fc2208d20b6f7f7d49
Author: Manfred Spraul <manfred@...orfullife.com>
Date:   Wed Oct 16 13:46:45 2013 -0700

    ipc/sem.c: synchronize semop and semctl with IPC_RMID


- -- 
All rights reversed
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