qcow2: Don't rely on free_cluster_index in alloc_refcount_block() (CVE-2014-0147)

free_cluster_index is only correct if update_refcount() was called from
an allocation function, and even there it's brittle because it's used to
protect unfinished allocations which still have a refcount of 0 - if it
moves in the wrong place, the unfinished allocation can be corrupted.

So not using it any more seems to be a good idea. Instead, use the
first requested cluster to do the calculations. Return -EAGAIN if
unfinished allocations could become invalid and let the caller restart
its search for some free clusters.

The context of creating a snapsnot is one situation where
update_refcount() is called outside of a cluster allocation. For this
case, the change fixes a buffer overflow if a cluster is referenced in
an L2 table that cannot be represented by an existing refcount block.
(new_table[refcount_table_index] was out of bounds)

[Bump the qemu-iotests 026 refblock_alloc.write leak count from 10 to
11.
--Stefan]

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index cc1bfebf29..d4d991cbb3 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1602,7 +1602,7 @@ static int qcow2_create2(const char *filename, int64_t total_size,
      */
     BlockDriverState* bs;
     QCowHeader *header;
-    uint8_t* refcount_table;
+    uint64_t* refcount_table;
     Error *local_err = NULL;
     int ret;
 
@@ -1654,9 +1654,10 @@ static int qcow2_create2(const char *filename, int64_t total_size,
         goto out;
     }
 
-    /* Write an empty refcount table */
-    refcount_table = g_malloc0(cluster_size);
-    ret = bdrv_pwrite(bs, cluster_size, refcount_table, cluster_size);
+    /* Write a refcount table with one refcount block */
+    refcount_table = g_malloc0(2 * cluster_size);
+    refcount_table[0] = cpu_to_be64(2 * cluster_size);
+    ret = bdrv_pwrite(bs, cluster_size, refcount_table, 2 * cluster_size);
     g_free(refcount_table);
 
     if (ret < 0) {
@@ -1681,7 +1682,7 @@ static int qcow2_create2(const char *filename, int64_t total_size,
         goto out;
     }
 
-    ret = qcow2_alloc_clusters(bs, 2 * cluster_size);
+    ret = qcow2_alloc_clusters(bs, 3 * cluster_size);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not allocate clusters for qcow2 "
                          "header and refcount table");