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>

1 files changed, 12 insertions, 6 deletions

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 4caa8e6d59d7..2d2b1b7588d7 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -269,6 +269,14 @@ static chunk_t area_location(struct pstore *ps, chunk_t area)
         return NUM_SNAPSHOT_HDR_CHUNKS + ((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) == NUM_SNAPSHOT_HDR_CHUNKS)
+                ps->next_free++;
+}
+
 /*
  * Read or write a metadata area.  Remembering to skip the first
  * chunk which holds the header.
@@ -502,6 +510,8 @@ static int read_exceptions(struct pstore *ps,
 
         ps->current_area--;
 
+        skip_metadata(ps);
+
         return 0;
 }
 
@@ -616,8 +626,6 @@ 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 ? */
@@ -630,10 +638,8 @@ 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;