Date: Mon, 13 Sep 2004 14:01:01 -0700
From: Chris Wright <chrisw@osdl.org>
To: Neil Horman <nhorman@redhat.com>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] close race condition in shared memory mapping/unmapping
Message-ID: <20040913140101.S1973@build.pdx.osdl.net>
References: <4146041F.2040106@redhat.com>
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In-Reply-To: <4146041F.2040106@redhat.com>; from nhorman@redhat.com on Mon, Sep 13, 2004 at 04:33:35PM -0400
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* Neil Horman (nhorman@redhat.com) wrote:
> Hey all-
> 	Found this the other day poking through the ipc code.  There appears to 
> be a race condition in the counter that records how many processes are 
> accessing a given shared memory segment.  In most places the shm_nattch 
> variable is protected by the shm_ids.sem semaphore, but there are a few 
> openings which appear to be able to allow a corruption of this variable 
> when run on SMP systems.  I've attached a patch to 2.6.9-rc2 for review. 
>   The locking may be a little over-aggressive (I was following examples 
> from other points in this file), but I figure better safe than sorry :).

Are you sure you've got this right?  I thought that the shmid_kernel
struct protects shm_nattch with a local (per structure) lock which is
embedded in kern_ipc_perm.  Did you find shm_nattch changes w/out
shm_lock/shm_unlock around it?  I believe shm_ids.sem is protecting the
id allocation, not per object data such as shm_nattch.

thanks,
-chris
-- 
Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net
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