From: Denis Rastyogin <gerben@altlinux.org>

This error was discovered by fuzzing qemu-img.

In the QED block driver, the need_check_timer timer is freed in
bdrv_qed_detach_aio_context, but the pointer to the timer is not
set to NULL. This can lead to a use-after-free scenario
in bdrv_qed_drain_begin().

The need_check_timer pointer is set to NULL after freeing the timer.
Which helps catch this condition when checking in bdrv_qed_drain_begin().

Closes: https://gitlab.com/qemu-project/qemu/-/issues/2852
Signed-off-by: Denis Rastyogin <gerben@altlinux.org>
Message-ID: <20250304083927.37681-1-gerben@altlinux.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 2ad638a3d160923ef3dbf87c73944e6e44bdc724)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/block/qed.c b/block/qed.c
index bc2f0a61c0..b986353979 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -353,6 +353,7 @@ static void bdrv_qed_detach_aio_context(BlockDriverState *bs)
 
     qed_cancel_need_check_timer(s);
     timer_free(s->need_check_timer);
+    s->need_check_timer = NULL;
 }
 
 static void bdrv_qed_attach_aio_context(BlockDriverState *bs,
-- 
2.39.5