Vulnerability: 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.
Vulnerability ID: CVE-2013-4299
CWE: CWE-264
Severity score: 6.4
Project: https://github.com/torvalds/linux
Published: 2013-10-24
File: drivers/md/dm-snap-persistent.c
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>
Commit author: Mikulas Patocka

Vulnerable version (before fix):
 static chunk_t area_location(struct pstore *ps, chunk_t area) 	return NUM_SNAPSHOT_HDR_CHUNKS + ((ps->exceptions_per_area + 1) * area); }  /*  * Read or write a metadata area.  Remembering to skip the first  * chunk which holds the header. static int read_exceptions(struct pstore *ps,  	ps->current_area--;  	return 0; }  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 ? */ 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++;  	atomic_inc(&ps->pending_count); 	return 0;

Fixed version (after fix):
 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. static int read_exceptions(struct pstore *ps,  	ps->current_area--;  	skip_metadata(ps);  	return 0; }  static int persistent_prepare_exception(struct dm_exception_store *store, 					struct dm_exception *e) { 	struct pstore *ps = get_info(store); 	sector_t size = get_dev_size(dm_snap_cow(store->snap)->bdev);  	/* Is there enough room ? */ 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. 	 */ 	ps->next_free++; 	skip_metadata(ps);  	atomic_inc(&ps->pending_count); 	return 0;

Patch:
patch: @@ -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;
path: drivers/md/dm-snap-persistent.c