linux-kernel - Re: race leading to held mutexes, inode

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Date:	Tue, 1 Apr 2008 18:38:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Sapan Bhatia" <sapan.bhatia@...il.com>
Cc:	linux-kernel@...r.kernel.org, daniel@...ac.com
Subject: Re: race leading to held mutexes, inode_cache corruption

On Tue, 1 Apr 2008 21:15:52 -0400 "Sapan Bhatia" <sapan.bhatia@...il.com> wrote:

> Hi,
> 
> We've been trying to investigate a file-system corruption issue in our
> kernel (http://svn.planet-lab.org/browser/linux-2.6/trunk) that
> manifests itself both with ext3 and ext2. It appears to be happening
> to due a contamination of the inode cache (we spent some time
> monitoring our systems to arrive at this hypothesis), and can be
> reproduced on a vanilla kernel as well.
> 
> The race that leads to this issue involves a process being terminated
> when it is waiting for a mutex in __mutex_lock_common. eg. when it is
> sent a SIGKILL, and the mutex is unlocked, causing the process to be
> woken up and sent to exit while now holding the lock.
> 
> The way it contaminates the inode_cache slab is that inode->i_mutex is
> only initialized once, and assumes that inodes coming back into the
> cache are initialized. It seems that in our case such poisoned inodes
> were leaking out of pipe.c.
> 
> This (www.cs.princeton.edu/~sapanb/mut.c) is the module we used to
> test the condition, as follows. Writing to the char device locks a
> mutex and reading from it unlocks it.
> # echo 1 > /dev/mut
> # cat /etc/passwd > /dev/mut &
> [2] 6232
> # kill -9 6232
> # cat /dev/mut
> [2]-  Killed                  cat /etc/passwd > /dev/mut
> # echo 1 > /dev/mut
> (goes to sleep)
> 
> I suppose that one could also construct an attack to proactively
> corrupt inode_cache, but I haven't tried that as yet.
> 
> Our base kernel is 2.6.22.19.

This is ... confusing.

Are you saying that some caller of mutex_lock_interruptible() is getting a
return value of -EINTR from mutex_lock_interruptible(), but this task in
fact _did_ acquire the mutex?

That's the only way in which I can interpret your second paragraph, but as
far as I can tell the code cannot do that.

Can you provide more detail?
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