FAILED: patch "[PATCH] futex/requeue: Prevent NULL pointer dereference in" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

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 <stable@vger.kernel.org>.

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 74e144274af39935b0f410c0ee4d2b91c3730414
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026072141-ravage-display-0696@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

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

From 74e144274af39935b0f410c0ee4d2b91c3730414 Mon Sep 17 00:00:00 2001
From: Ji'an Zhou <eilaimemedsnaimel@gmail.com>
Date: Tue, 2 Jun 2026 09:12:04 +0000
Subject: [PATCH] futex/requeue: Prevent NULL pointer dereference in
 remove_waiter() on self-deadlock

When FUTEX_CMP_REQUEUE_PI requeues a non-top waiter that already owns the
target PI futex, task_blocks_on_rt_mutex() returns -EDEADLK before setting
waiter->task.

The subsequent remove_waiter() in rt_mutex_start_proxy_lock() dereferences
the NULL waiter->task, causing a kernel crash.

Add a self-deadlock check for non-top waiters before calling
rt_mutex_start_proxy_lock(), analogous to the top-waiter check in
futex_lock_pi_atomic().

Fixes: 3bfdc63936dd4773109b7b8c280c0f3b5ae7d349 ("rtmutex: Use waiter::task instead of current in remove_waiter()")
Signed-off-by: Ji'an Zhou <eilaimemedsnaimel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org

diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index b597cb3d17fc..1d99a84dc9ad 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -643,6 +643,12 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 				continue;
 			}
 
+			/* Self-deadlock: non-top waiter already owns the PI futex. */
+			if (rt_mutex_owner(&pi_state->pi_mutex) == this->task) {
+				ret = -EDEADLK;
+				break;
+			}
+
 			ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
 							this->rt_waiter,
 							this->task);