[Stable-8.2.10,40/42] block: Zero block driver state before reopening

Commit Message

Michael Tokarev March 15, 2025, 7:42 a.m. UTC

From: Kevin Wolf <[e-mail]>

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: [e-mail]
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2853
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2851
Reported-by: Denis Rastyogin <[e-mail]>
Signed-off-by: Kevin Wolf <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Kevin Wolf <[e-mail]>
(cherry picked from commit b75c5f9879166b86ed7c48b772fdcd0693e8a9a3)
Signed-off-by: Michael Tokarev <[e-mail]>

Patch

diff --git a/block/snapshot.c b/block/snapshot.c
index c4d40e80dd..7da0588712 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -297,6 +297,7 @@  int bdrv_snapshot_goto(BlockDriverState *bs,
         bdrv_graph_wrunlock(NULL);
 
         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) {