block: Zero block driver state before reopening

Block drivers assume in their .bdrv_open() implementation that their
state in bs->opaque has been zeroed; it is initially allocated with
g_malloc0() in bdrv_open_driver().

bdrv_snapshot_goto() needs to make sure that it is zeroed again before
calling drv->bdrv_open() to avoid that block drivers use stale values.

One symptom of this bug is VMDK running into a double free when the user
tries to apply an internal snapshot like 'qemu-img snapshot -a test
test.vmdk'. This should be a graceful error because VMDK doesn't support
internal snapshots.

==25507== Invalid free() / delete / delete[] / realloc()
==25507==    at 0x484B347: realloc (vg_replace_malloc.c:1801)
==25507==    by 0x54B592A: g_realloc (gmem.c:171)
==25507==    by 0x1B221D: vmdk_add_extent (../block/vmdk.c:570)
==25507==    by 0x1B1084: vmdk_open_sparse (../block/vmdk.c:1059)
==25507==    by 0x1AF3D8: vmdk_open (../block/vmdk.c:1371)
==25507==    by 0x1A2AE0: bdrv_snapshot_goto (../block/snapshot.c:299)
==25507==    by 0x205C77: img_snapshot (../qemu-img.c:3500)
==25507==    by 0x58FA087: (below main) (libc_start_call_main.h:58)
==25507==  Address 0x832f3e0 is 0 bytes inside a block of size 272 free'd
==25507==    at 0x4846B83: free (vg_replace_malloc.c:989)
==25507==    by 0x54AEAC4: g_free (gmem.c:208)
==25507==    by 0x1AF629: vmdk_close (../block/vmdk.c:2889)
==25507==    by 0x1A2A9C: bdrv_snapshot_goto (../block/snapshot.c:290)
==25507==    by 0x205C77: img_snapshot (../qemu-img.c:3500)
==25507==    by 0x58FA087: (below main) (libc_start_call_main.h:58)

This error was discovered by fuzzing qemu-img.

Cc: qemu-stable@nongnu.org
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2853
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2851
Reported-by: Denis Rastyogin <gerben@altlinux.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250310104858.28221-1-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit b75c5f9879166b86ed7c48b772fdcd0693e8a9a3)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/block/snapshot.c b/block/snapshot.c
index 86e29ca59f9783ab9764c8689984f47270002bb7..2f6c35f7ebcab45eb261a49d92d6c76b997ba106 100644 (file)
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -286,6 +286,7 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
         bdrv_unref_child(bs, fallback);
 
         ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp);
+        memset(bs->opaque, 0, drv->instance_size);
         open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err);
         qobject_unref(options);
         if (open_ret < 0) {