linux-kernel - Re: [RFC, PATCH] fix SEM

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Date:	Mon, 31 Mar 2008 18:14:47 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Pavel Emelyanov <xemul@...nvz.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC, PATCH] fix SEM_UNDO with namespaces

Pavel Emelyanov wrote:
> Manfred Spraul wrote:
>   
>> Hi,
>>
>> the attached patch should fix the combination of CLONE_NEWIPC with 
>> shared sysv undo structures (the common case, just 
>> sys_unshare(CLONE_NEWIPC)):
>> lookup_undo() now locates the undo array based on both semid and the 
>> namespace pointer.
>>     
>
> If you start using any IPC object and then call unshare with CLONE_NEWIPC,
> then it's your problem, but not the kernel.
>   
The result is a kernel memory corruption, and kernel memory corruptions 
are always the kernel's problem.

The code assumed that a semaphore id is globally unique. With 
namespaces,  this is not true anymore.
If two semaphore arrays exist with the same id, but different sizes, 
then semops will cause memory corruptions: The undo structure contains 
one element for each semaphore, thus the semop will write behind the end 
of the memory allocation.

> I agree, that we should probably destroy this one when the task calls 
> unshare, but trying to keep this list relevant is useless.
>   
A very tricky question: Let's assume we have a process with two threads.
The undo structure is shared, as per opengroup standard.
Now one thread calls unshare(CLONE_NEWIPC). What should happen? We 
cannot destroy the undo structure, the other thread might be still 
interested in it.
If we allow sys_unshare() for multithreaded processes with CLONE_NEWIPC 
and without CLONE_SYSVSEM, then we must handle this case.

--
    Manfred
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