A BH callback can free the BH, causing a use-after-free in aio_bh_call.
Fix that by keeping a local copy of the re-entrancy guard pointer.

Buglink: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58513
Fixes: 9c86c97f12 ("async: Add an optional reentrancy guard to the BH API")
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 util/async.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/util/async.c b/util/async.c
index 9df7674b4e..055070ffbd 100644
--- a/util/async.c
+++ b/util/async.c
@@ -156,18 +156,20 @@ void aio_bh_call(QEMUBH *bh)
 {
     bool last_engaged_in_io = false;
 
-    if (bh->reentrancy_guard) {
-        last_engaged_in_io = bh->reentrancy_guard->engaged_in_io;
-        if (bh->reentrancy_guard->engaged_in_io) {
+    /* Make a copy of the guard-pointer as cb may free the bh */
+    MemReentrancyGuard *reentrancy_guard = bh->reentrancy_guard;
+    if (reentrancy_guard) {
+        last_engaged_in_io = reentrancy_guard->engaged_in_io;
+        if (reentrancy_guard->engaged_in_io) {
             trace_reentrant_aio(bh->ctx, bh->name);
         }
-        bh->reentrancy_guard->engaged_in_io = true;
+        reentrancy_guard->engaged_in_io = true;
     }
 
     bh->cb(bh->opaque);
 
-    if (bh->reentrancy_guard) {
-        bh->reentrancy_guard->engaged_in_io = last_engaged_in_io;
+    if (reentrancy_guard) {
+        reentrancy_guard->engaged_in_io = last_engaged_in_io;
     }
 }
 
-- 
2.39.0

On Mon, May 01, 2023 at 10:19:56AM -0400, Alexander Bulekov wrote:
> A BH callback can free the BH, causing a use-after-free in aio_bh_call.
> Fix that by keeping a local copy of the re-entrancy guard pointer.
> 
> Buglink: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58513
> Fixes: 9c86c97f12 ("async: Add an optional reentrancy guard to the BH API")
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  util/async.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

On 01/05/2023 16.19, Alexander Bulekov wrote:
> A BH callback can free the BH, causing a use-after-free in aio_bh_call.
> Fix that by keeping a local copy of the re-entrancy guard pointer.
> 
> Buglink: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58513
> Fixes: 9c86c97f12 ("async: Add an optional reentrancy guard to the BH API")
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>   util/async.c | 14 ++++++++------
>   1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/util/async.c b/util/async.c
> index 9df7674b4e..055070ffbd 100644
> --- a/util/async.c
> +++ b/util/async.c
> @@ -156,18 +156,20 @@ void aio_bh_call(QEMUBH *bh)
>   {
>       bool last_engaged_in_io = false;
>   
> -    if (bh->reentrancy_guard) {
> -        last_engaged_in_io = bh->reentrancy_guard->engaged_in_io;
> -        if (bh->reentrancy_guard->engaged_in_io) {
> +    /* Make a copy of the guard-pointer as cb may free the bh */
> +    MemReentrancyGuard *reentrancy_guard = bh->reentrancy_guard;
> +    if (reentrancy_guard) {
> +        last_engaged_in_io = reentrancy_guard->engaged_in_io;
> +        if (reentrancy_guard->engaged_in_io) {
>               trace_reentrant_aio(bh->ctx, bh->name);
>           }
> -        bh->reentrancy_guard->engaged_in_io = true;
> +        reentrancy_guard->engaged_in_io = true;
>       }
>   
>       bh->cb(bh->opaque);
>   
> -    if (bh->reentrancy_guard) {
> -        bh->reentrancy_guard->engaged_in_io = last_engaged_in_io;
> +    if (reentrancy_guard) {
> +        reentrancy_guard->engaged_in_io = last_engaged_in_io;
>       }
>   }

Reviewed-by: Thomas Huth <thuth@redhat.com>

I'll assemble a pull request with this later today, to avoid that people run 
into this regression.

  Thomas