From ea76ea3f2c6c4fe41bec6d97678a94bf8e5363a2 Mon Sep 17 00:00:00 2001
From: Alexander Bulekov <alxndr@bu.edu>
Date: Mon, 1 May 2023 10:19:56 -0400
Subject: [PATCH] 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: 9c86c97f12 ("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 | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/util/async.c b/util/async.c
index 0627f1c201..a6b51c40c1 100644
--- a/util/async.c
+++ b/util/async.c
@@ -95,18 +95,20 @@ 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;
     }
 }
 
-- 
2.27.0

