async: avoid use-after-free on re-entrancy guard

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: 9c86c97 ("async: Add an optional reentrancy guard to the BH API")
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20230501141956.3444868-1-alxndr@bu.edu>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

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;
     }
 }