Re: [PATCH] async: avoid use-after-free on re-entrancy guard

[Stefan Hajnoczi]

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 
> ---
>  util/async.c | 14 --
>  1 file changed, 8 insertions(+), 6 deletions(-)

Reviewed-by: Stefan Hajnoczi 


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Re: [PATCH] async: avoid use-after-free on re-entrancy guard

[Thomas Huth]

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 
---
  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 

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


 Thomas

[PATCH] async: avoid use-after-free on re-entrancy guard

[Alexander Bulekov]

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 
---
 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