[PATCH] block/snapshot: fix *errp handling in bdrv_snapshot_goto

[gerben@altlinux.org]

From: Denis Rastyogin <gerben@altlinux.org>

This error was discovered by fuzzing qemu-img.

If bdrv_snapshot_goto() returns an error, it is not handled immediately,
allowing *errp to be reassigned when qcow_open() fails, which triggers
assert(*errp == NULL) in util/error.c: void error_setv().

This patch ensures that errors from bdrv_snapshot_goto() are handled
immediately after the call, preventing *errp from being modified twice
and avoiding unnecessary assertion failures.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/2851
Signed-off-by: Denis Rastyogin <gerben@altlinux.org>
---
 block/snapshot.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/block/snapshot.c b/block/snapshot.c
index 9c44780e96..d1b5a8d33d 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -296,14 +296,20 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
         bdrv_graph_wrunlock();
 
         ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp);
+        if (ret < 0) {
+            bdrv_unref(fallback_bs);
+            bs->drv = NULL;
+            /* A bdrv_snapshot_goto() error takes precedence */
+            error_propagate(errp, local_err);
+            return ret;
+        }
         open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err);
         qobject_unref(options);
         if (open_ret < 0) {
             bdrv_unref(fallback_bs);
             bs->drv = NULL;
-            /* A bdrv_snapshot_goto() error takes precedence */
             error_propagate(errp, local_err);
-            return ret < 0 ? ret : open_ret;
+            return open_ret;
         }
 
         /*
-- 
2.42.2