[PATCH v2 2/2] qcow2: Fix corruption bug in qcow2_detect_metadata_preallocation()

[Kevin Wolf <kwolf@redhat.com>]

qcow2_detect_metadata_preallocation() calls qcow2_get_refcount() which
requires s->lock to be taken to protect its accesses to the refcount
table and refcount blocks. However, nothing in this code path actually
took the lock. This could cause the same cache entry to be used by two
requests at the same time, for different tables at different offsets,
resulting in image corruption.

As it would be preferable to base the detection on consistent data (even
though it's just heuristics), let's take the lock not only around the
qcow2_get_refcount() calls, but around the whole function.

This patch takes the lock in qcow2_co_block_status() earlier and asserts
in qcow2_detect_metadata_preallocation() that we hold the lock.

Fixes: 69f47505ee66afaa513305de0c1895a224e52c45
Cc: qemu-stable@nongnu.org
Reported-by: Michael Weiser <michael@weiser.dinsnail.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/qcow2-refcount.c | 2 ++
 block/qcow2.c          | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index ef965d7895..0d64bf5a5e 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -3455,6 +3455,8 @@ int qcow2_detect_metadata_preallocation(BlockDriverState *bs)
     int64_t i, end_cluster, cluster_count = 0, threshold;
     int64_t file_length, real_allocation, real_clusters;
 
+    qemu_co_mutex_assert_locked(&s->lock);
+
     file_length = bdrv_getlength(bs->file->bs);
     if (file_length < 0) {
         return file_length;
diff --git a/block/qcow2.c b/block/qcow2.c
index 8b05933565..0bc69e6996 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1916,6 +1916,8 @@ static int coroutine_fn qcow2_co_block_status(BlockDriverState *bs,
     unsigned int bytes;
     int status = 0;
 
+    qemu_co_mutex_lock(&s->lock);
+
     if (!s->metadata_preallocation_checked) {
         ret = qcow2_detect_metadata_preallocation(bs);
         s->metadata_preallocation = (ret == 1);
@@ -1923,7 +1925,6 @@ static int coroutine_fn qcow2_co_block_status(BlockDriverState *bs,
     }
 
     bytes = MIN(INT_MAX, count);
-    qemu_co_mutex_lock(&s->lock);
     ret = qcow2_get_cluster_offset(bs, offset, &bytes, &cluster_offset);
     qemu_co_mutex_unlock(&s->lock);
     if (ret < 0) {
-- 
2.20.1