[PATCH] NFSD: memory corruption due to writing beyond the stat array

[PATCH] NFSD: memory corruption due to writing beyond the stat array

[Konstantin Khorenko]

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Hi All,

i've found a bug which makes NFSD to corrupt memory due to writing beyond the stat array.

Initially this issue was found on 2.6.18-x RHEL5 kernel, but the same seems to be true for the current mainstream kernel (checked on 2.6.38-rc3).

./fs/nfsd/vfs.c:
static inline struct raparms *
nfsd_get_raparms(dev_t dev, ino_t ino)
{
...
       // here we searched our file in a readahead cache
       // but not found it
...
         depth = nfsdstats.ra_size*11/10;
...
       // some stuff, but "depth" is not changed
...
         nfsdstats.ra_depth[depth*10/nfsdstats.ra_size]++;
...
}

And here we write to the 12th array element
(nfsdstats.ra_size*11/10*10/nfsdstats.ra_size == 11), while ra_depth has only
11 elements:

struct nfsd_stats {
...
        unsigned int    ra_depth[11];   /* number of times ra entry was found that deep
                                         * in the cache (10percentiles). [10] = not found */
#ifdef CONFIG_NFSD_V4
        unsigned int    nfs4_opcount[LAST_NFS4_OP + 1]; /* count of individual
nfsv4 operations */
#endif
};

This means that each time we did not find a file in a readahead cache we
corrupt memory. Fortunately in a kernel with NFSDv4 compiled in it corrupts
(inc) NFS4 ops counter, but in a kernel with NFSDv4 disabled it corrupts (inc)
some other data, that lays in the memory beyond nfsdstats.

Proposed fix is attached.

--
Best regards,

Konstantin Khorenko

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NFSD: The patch fixes writing beyond the array.

If nfsd fails to find an exported via NFS file in the readahead cache,
it should increment corresponding nfsdstats counter (ra_depth[10]), but
due to a bug writes beside the stat array, corrupting following field.

In a kernel with NFSDv4 compiled in it corrupts (inc) NFS4 counter:
the number of individual nfsv4 operations.
In a kernel with NFSDv4 disabled it corrupts (inc) some other data,
that lays in the memory beyond nfsdstats.

Signed-off-by: Konstantin Khorenko <khorenko@openvz.org>

--- a/fs/nfsd/vfs.c.nfsd	2011-01-05 03:50:19.000000000 +0300
+++ b/fs/nfsd/vfs.c	2011-02-01 16:39:28.000000000 +0300
@@ -809,7 +809,7 @@ nfsd_get_raparms(dev_t dev, ino_t ino)
 		if (ra->p_count == 0)
 			frap = rap;
 	}
-	depth = nfsdstats.ra_size*11/10;
+	depth = nfsdstats.ra_size;
 	if (!frap) {	
 		spin_unlock(&rab->pb_lock);
 		return NULL;

Re: [PATCH] NFSD: memory corruption due to writing beyond the stat array

[J. Bruce Fields]

On Tue, Feb 01, 2011 at 05:16:29PM +0300, Konstantin Khorenko wrote:
> Hi All,
> 
> i've found a bug which makes NFSD to corrupt memory due to writing beyond the stat array.
> 
> Initially this issue was found on 2.6.18-x RHEL5 kernel, but the same seems to be true for the current mainstream kernel (checked on 2.6.38-rc3).
> 
> ./fs/nfsd/vfs.c:
> static inline struct raparms *
> nfsd_get_raparms(dev_t dev, ino_t ino)
> {
> ...
>        // here we searched our file in a readahead cache
>        // but not found it
> ...
>          depth = nfsdstats.ra_size*11/10;
> ...
>        // some stuff, but "depth" is not changed
> ...
>          nfsdstats.ra_depth[depth*10/nfsdstats.ra_size]++;
> ...
> }
> 
> And here we write to the 12th array element
> (nfsdstats.ra_size*11/10*10/nfsdstats.ra_size == 11),

Actually, I think that's true only if ra_size is divisible by 10; if
not, then depth will be something *less* than ra_size*11/10, so
depth*10/ra_size will be something less than 11 (namely 10).

> This means that each time we did not find a file in a readahead cache we
> corrupt memory.

... and ra_size is set to 16 * min(2, numthreads/8), so I think you'll
hit this only if min(2, numthreads/8) (rounded down) is a multiple of 5?
So for example someone using a number of threads that's a power of 2
shouldn't hit this case.

> Fortunately in a kernel with NFSDv4 compiled in it corrupts
> (inc) NFS4 ops counter, but in a kernel with NFSDv4 disabled it corrupts (inc)
> some other data, that lays in the memory beyond nfsdstats.
> 
> Proposed fix is attached.

So after the fix:

> -	depth = nfsdstats.ra_size*11/10;
> +	depth = nfsdstats.ra_size;

depth*10/nfsdstats will always be 10 in this case.  Looks good.

Thanks for finding this!

--b.