[PATCH] io_uring: fix task_work add use-after-free with SQPOLL

[Jens Axboe <axboe@kernel.dk>]

For SQPOLL, the sqpoll thread runs task_work off its own list and
doesn't need to be told about it, so it can pop and complete a request
as soon as mpscq_push() has published it. If that was the last request
holding the ring alive, io_ring_exit_work() then frees the ctx, stops
the thread, and with it the tctx, while io_req_normal_work_add() is
still looking at both after the push:

  BUG: KASAN: slab-use-after-free in io_req_normal_work_add+0x439/0x510
  Read of size 4 at addr ff11000000ca0000 by task repro/55
  (...)
  BUG: KASAN: slab-use-after-free in queue_work_on+0x25/0x70
  Write of size 8 at addr ff11000000c3bd00 by task repro/55

Hold an RCU read lock across the add, like io_req_local_work_add() already
does, and have io_ring_exit_work() wait for a grace period for SQPOLL rings
as well before freeing the ctx.

Reported-by: J?r?my Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Link: https://lore.kernel.org/io-uring/20260924205056.3759980-2-Jeremy.Jean@oss.cyber.gouv.fr/
Fixes: af5d68f8892f ("io_uring/sqpoll: manage task_work privately")
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 2cb9b58d04dd..c2ce83c7c1f1 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2405,8 +2405,8 @@ static __cold void io_ring_exit_work(struct work_struct *work)
 	spin_lock(&ctx->completion_lock);
 	spin_unlock(&ctx->completion_lock);
 
-	/* pairs with RCU read section in io_req_local_work_add() */
-	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
+	/* pairs with the RCU read sections in the task_work add paths */
+	if (ctx->flags & (IORING_SETUP_DEFER_TASKRUN | IORING_SETUP_SQPOLL))
 		synchronize_rcu();
 
 	io_ring_ctx_free(ctx);
diff --git a/io_uring/tw.c b/io_uring/tw.c
index f573bcc3af6a..f9affd68dd26 100644
--- a/io_uring/tw.c
+++ b/io_uring/tw.c
@@ -210,6 +210,9 @@ void io_req_normal_work_add(struct io_kiocb *req)
 	struct io_uring_task *tctx = req->tctx;
 	struct io_ring_ctx *ctx = req->ctx;
 
+	/* SQPOLL can retire the request on push, see io_ring_exit_work() */
+	guard(rcu)();
+
 	/* tw run already pending, nothing else to do */
 	if (!mpscq_push(&tctx->task_list, &req->io_task_work.node))
 		return;

-- 
Jens Axboe