[PATCH 5.15.y] io_uring/tctx: work around xa_store() allocation error issue

[PATCH 5.15.y] io_uring/tctx: work around xa_store() allocation error issue

[Robert Garcia]

From: Jens Axboe <axboe@kernel.dk>

[ Upstream commit 7eb75ce7527129d7f1fee6951566af409a37a1c4 ]

syzbot triggered the following WARN_ON:

WARNING: CPU: 0 PID: 16 at io_uring/tctx.c:51 __io_uring_free+0xfa/0x140 io_uring/tctx.c:51

which is the

WARN_ON_ONCE(!xa_empty(&tctx->xa));

sanity check in __io_uring_free() when a io_uring_task is going through
its final put. The syzbot test case includes injecting memory allocation
failures, and it very much looks like xa_store() can fail one of its
memory allocations and end up with ->head being non-NULL even though no
entries exist in the xarray.

Until this issue gets sorted out, work around it by attempting to
iterate entries in our xarray, and WARN_ON_ONCE() if one is found.

Reported-by: syzbot+cc36d44ec9f368e443d3@syzkaller.appspotmail.com
Link: https://lore.kernel.org/io-uring/673c1643.050a0220.87769.0066.GAE@google.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[ Modify the function in io_uring.c because it's located here in v5.15. ]
Signed-off-by: Robert Garcia <rob_garcia@163.com>
---
 io_uring/io_uring.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index e5889ec0273f..04e4b1e6a5b8 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -8699,8 +8699,19 @@ static int io_uring_alloc_task_context(struct task_struct *task,
 void __io_uring_free(struct task_struct *tsk)
 {
 	struct io_uring_task *tctx = tsk->io_uring;
+	struct io_tctx_node *node;
+	unsigned long index;
 
-	WARN_ON_ONCE(!xa_empty(&tctx->xa));
+	/*
+	 * Fault injection forcing allocation errors in the xa_store() path
+	 * can lead to xa_empty() returning false, even though no actual
+	 * node is stored in the xarray. Until that gets sorted out, attempt
+	 * an iteration here and warn if any entries are found.
+	 */
+	xa_for_each(&tctx->xa, index, node) {
+		WARN_ON_ONCE(1);
+		break;
+	}
 	WARN_ON_ONCE(tctx->io_wq);
 	WARN_ON_ONCE(tctx->cached_refs);
 
-- 
2.34.1

Re: [PATCH 5.15.y] io_uring/tctx: work around xa_store() allocation error issue

[Jens Axboe]

On 3/23/26 2:19 AM, Robert Garcia wrote:
> From: Jens Axboe <axboe@kernel.dk>
> 
> [ Upstream commit 7eb75ce7527129d7f1fee6951566af409a37a1c4 ]
> 
> syzbot triggered the following WARN_ON:
> 
> WARNING: CPU: 0 PID: 16 at io_uring/tctx.c:51 __io_uring_free+0xfa/0x140 io_uring/tctx.c:51
> 
> which is the
> 
> WARN_ON_ONCE(!xa_empty(&tctx->xa));
> 
> sanity check in __io_uring_free() when a io_uring_task is going through
> its final put. The syzbot test case includes injecting memory allocation
> failures, and it very much looks like xa_store() can fail one of its
> memory allocations and end up with ->head being non-NULL even though no
> entries exist in the xarray.
> 
> Until this issue gets sorted out, work around it by attempting to
> iterate entries in our xarray, and WARN_ON_ONCE() if one is found.
> 
> Reported-by: syzbot+cc36d44ec9f368e443d3@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/io-uring/673c1643.050a0220.87769.0066.GAE@google.com/
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> [ Modify the function in io_uring.c because it's located here in v5.15. ]
> Signed-off-by: Robert Garcia <rob_garcia@163.com>

I'm find adding this to 5.15 stable. However, this also need to go to
5.10-stable then as the io_uring bases are identical. Greg, when you
queue this up, please add to both. Thanks!

-- 
Jens Axboe