Code: static int persistent_prepare_exception(struct dm_exception_store *store,
 					struct dm_exception *e)
 {
 	struct pstore *ps = get_info(store);
	uint32_t stride;
	chunk_t next_free;
 	sector_t size = get_dev_size(dm_snap_cow(store->snap)->bdev);
 
 	/* Is there enough room ? */
	if (size < ((ps->next_free + 1) * store->chunk_size))
		return -ENOSPC;

	e->new_chunk = ps->next_free;

	/*
 	 * Move onto the next free pending, making sure to take
 	 * into account the location of the metadata chunks.
 	 */
	stride = (ps->exceptions_per_area + 1);
	next_free = ++ps->next_free;
	if (sector_div(next_free, stride) == 1)
		ps->next_free++;
 
 	atomic_inc(&ps->pending_count);
 	return 0;
}

Vulnerability Type: +Info 
CWE ID: CWE-264
Summary: Interpretation conflict in drivers/md/dm-snap-persistent.c in the Linux kernel through 3.11.6 allows remote authenticated users to obtain sensitive information or modify data via a crafted mapping to a snapshot block device.
Commit Message: dm snapshot: fix data corruption

This patch fixes a particular type of data corruption that has been
encountered when loading a snapshot's metadata from disk.

When we allocate a new chunk in persistent_prepare, we increment
ps->next_free and we make sure that it doesn't point to a metadata area
by further incrementing it if necessary.

When we load metadata from disk on device activation, ps->next_free is
positioned after the last used data chunk. However, if this last used
data chunk is followed by a metadata area, ps->next_free is positioned
erroneously to the metadata area. A newly-allocated chunk is placed at
the same location as the metadata area, resulting in data or metadata
corruption.

This patch changes the code so that ps->next_free skips the metadata
area when metadata are loaded in function read_exceptions.

The patch also moves a piece of code from persistent_prepare_exception
to a separate function skip_metadata to avoid code duplication.

CVE-2013-4299

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Cc: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>