[Lucid][CVE 1/1] dm snapshot: fix data corruption

[Luis Henriques]

From: Mikulas Patocka <mpatocka at redhat.com>

CVE-2013-4299

BugLink: http://bugs.launchpad.net/bugs/1241769

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 at redhat.com>
Cc: stable at vger.kernel.org
Cc: Mike Snitzer <snitzer at redhat.com>
Signed-off-by: Alasdair G Kergon <agk at redhat.com>
(back ported from commit e9c6a182649f4259db704ae15a91ac820e63b0ca)
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/md/dm-snap-persistent.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 0c74642..97c3f06 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -252,6 +252,14 @@ static chunk_t area_location(struct pstore *ps, chunk_t area)
 	return 1 + ((ps->exceptions_per_area + 1) * area);
 }
 
+static void skip_metadata(struct pstore *ps)
+{
+	uint32_t stride = ps->exceptions_per_area + 1;
+	chunk_t next_free = ps->next_free;
+	if (sector_div(next_free, stride) == 1)
+		ps->next_free++;
+}
+
 /*
  * Read or write a metadata area.  Remembering to skip the first
  * chunk which holds the header.
@@ -481,6 +489,8 @@ static int read_exceptions(struct pstore *ps,
 
 	ps->current_area--;
 
+	skip_metadata(ps);
+
 	return 0;
 }
 
@@ -587,8 +597,6 @@ static int persistent_prepare_exception(struct dm_exception_store *store,
 					struct dm_snap_exception *e)
 {
 	struct pstore *ps = get_info(store);
-	uint32_t stride;
-	chunk_t next_free;
 	sector_t size = get_dev_size(store->cow->bdev);
 
 	/* Is there enough room ? */
@@ -601,10 +609,8 @@ static int persistent_prepare_exception(struct dm_exception_store *store,
 	 * 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++;
+	ps->next_free++;
+	skip_metadata(ps);
 
 	atomic_inc(&ps->pending_count);
 	return 0;
-- 
1.8.3.2