dm snapshot: fix data corruption

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@redhat.com>
Cc: stable@vger.kernel.org
Cc: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
(back ported from commit e9c6a182649f4259db704ae15a91ac820e63b0ca)
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 0c746420c00810f9c2e57ad346f3bd2589250821..97c3f0651c4147c752f3d75ab3e706a696f64418 100644 (file)
--- 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;