[qemu-xen stable-4.19] async: avoid use-after-free on re-entrancy guard

[patchbot@xxxxxxx]

commit 6a33d4b3453b46ee3fd432d80374f45919985b00
Author:     Alexander Bulekov <alxndr@xxxxxx>
AuthorDate: Mon May 1 10:19:56 2023 -0400
Commit:     Michael Tokarev <mjt@xxxxxxxxxx>
CommitDate: Sun Sep 10 19:38:01 2023 +0300

    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@xxxxxx>
    Message-Id: <20230501141956.3444868-1-alxndr@xxxxxx>
    Reviewed-by: Thomas Huth <thuth@xxxxxxxxxx>
    Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
    (cherry picked from commit 7915bd06f25e1803778081161bf6fa10c42dc7cd)
    Signed-off-by: Michael Tokarev <mjt@xxxxxxxxxx>
---
 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;
     }
 }
 
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.19