FAILED: patch "[PATCH] io-wq: check that the predecessor is hashed in" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x d6a2d7b04b5a093021a7a0e2e69e9d5237dfa8cc
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026052036-arbitrate-wasp-6200@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d6a2d7b04b5a093021a7a0e2e69e9d5237dfa8cc Mon Sep 17 00:00:00 2001
From: Nicholas Carlini <[email protected]>
Date: Mon, 11 May 2026 18:02:16 +0000
Subject: [PATCH] io-wq: check that the predecessor is hashed in
io_wq_remove_pending()

io_wq_remove_pending() needs to fix up wq->hash_tail[] if the cancelled work was the tail of its hash bucket. When doing this, it checks whether the preceding entry in acct->work_list has the same hash value, but never checks that the predecessor is hashed at all. io_get_work_hash() is simply atomic_read(&work->flags) >> IO_WQ_HASH_SHIFT, and the hash bits are never set for non-hashed work, so it returns 0. Thus, when a hashed bucket-0 work is cancelled while a non-hashed work is its list predecessor, the check spuriously passes and a pointer to the non-hashed io_kiocb is stored in wq->hash_tail[0].

Because non-hashed work is dequeued via the fast path in io_get_next_work(), which never touches hash_tail[], the stale pointer is never cleared. Therefore, after the non-hashed io_kiocb completes and is freed back to req_cachep, wq->hash_tail[0] is a dangling pointer. The io_wq is per-task (tctx->io_wq) and survives ring open/close, so the dangling pointer persists for the lifetime of the task; the next hashed bucket-0 enqueue dereferences it in io_wq_insert_work() and wq_list_add_after() writes through freed memory.

Add the missing io_wq_is_hashed() check so a non-hashed predecessor never inherits a hash_tail[] slot.

index 7a9f94a0ce6f..8cc7b47d3089 100644
@@ -1124,7 +1124,8 @@ static inline void io_wq_remove_pending(struct io_wq *wq,
 	if (io_wq_is_hashed(work) && work == wq->hash_tail[hash]) {
 		if (prev)
 			prev_work = container_of(prev, struct io_wq_work, list);
-		if (prev_work && io_get_work_hash(prev_work) == hash)
+		if (prev_work && io_wq_is_hashed(prev_work) &&
+		    io_get_work_hash(prev_work) == hash)
 			wq->hash_tail[hash] = prev_work;
 		else
 			wq->hash_tail[hash] = NULL;

Here's one for 5.15-stable AND 5.10-stable.

Jens Axboe

From fb133351bed5c5aefc20129dcf3b3a62d44e16ea Mon Sep 17 00:00:00 2001
From: Nicholas Carlini <[email protected]>
Date: Mon, 11 May 2026 18:02:16 +0000
Subject: [PATCH] io-wq: check that the predecessor is hashed in
io_wq_remove_pending()

io_wq_remove_pending() needs to fix up wq->hash_tail[] if the cancelled work was the tail of its hash bucket. When doing this, it checks whether the preceding entry in acct->work_list has the same hash value, but never checks that the predecessor is hashed at all. io_get_work_hash() is simply atomic_read(&work->flags) >> IO_WQ_HASH_SHIFT, and the hash bits are never set for non-hashed work, so it returns 0. Thus, when a hashed bucket-0 work is cancelled while a non-hashed work is its list predecessor, the check spuriously passes and a pointer to the non-hashed io_kiocb is stored in wq->hash_tail[0].

Because non-hashed work is dequeued via the fast path in io_get_next_work(), which never touches hash_tail[], the stale pointer is never cleared. Therefore, after the non-hashed io_kiocb completes and is freed back to req_cachep, wq->hash_tail[0] is a dangling pointer. The io_wq is per-task (tctx->io_wq) and survives ring open/close, so the dangling pointer persists for the lifetime of the task; the next hashed bucket-0 enqueue dereferences it in io_wq_insert_work() and wq_list_add_after() writes through freed memory.

Add the missing io_wq_is_hashed() check so a non-hashed predecessor never inherits a hash_tail[] slot.

---
 io_uring/io-wq.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index 926890f5086e..5d95e604b7e6 100644
@@ -1014,7 +1014,8 @@ static inline void io_wqe_remove_pending(struct io_wqe *wqe,
 	if (io_wq_is_hashed(work) && work == wqe->hash_tail[hash]) {
 		if (prev)
 			prev_work = container_of(prev, struct io_wq_work, list);
-		if (prev_work && io_get_work_hash(prev_work) == hash)
+		if (prev_work && io_wq_is_hashed(prev_work) &&
+		    io_get_work_hash(prev_work) == hash)
 			wqe->hash_tail[hash] = prev_work;
 		else
 			wqe->hash_tail[hash] = NULL;

2.53.0

All of these now queued up, thanks!

greg k-h