FAILED: patch "[PATCH] coredump: hold RCU while releasing parked threads" 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 61d58591aa0188bf2463e263962256316ff8b576
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026102801-afford-shock-9e90@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

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

From 61d58591aa0188bf2463e263962256316ff8b576 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Mon, 21 Sep 2026 15:44:50 +0200
Subject: [PATCH] coredump: hold RCU while releasing parked threads

coredump_finish() releases the parked threads by clearing ->task in
their core_thread entry and calling wake_up_process() on each of them.
It does that without holding a reference on the task and without being
inside rcu.

But calling wake_up_process(task) without rcu here isn't safe. A parked
thread doesn't need that wakeup to leave. A spurious wakeup or a
preemption after the store is enough for the task to go away.

So if the coredump client is preempted between the store and
wake_up_process() the thread can exit and be freed in the meantime and
try_to_wake_up() takes pi_lock in freed memory.

Hold rcu across the loop.

Fixes: a94e2d408eae ("coredump: kill mm->core_done")
Cc: stable@vger.kernel.org
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-1-8e4adb1619e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

diff --git a/fs/coredump.c b/fs/coredump.c
index 7c0f276c3e0d..d88a65e37ee1 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -562,6 +562,8 @@ static void coredump_finish(bool core_dumped)
 	current->signal->core_state = NULL;
 	spin_unlock_irq(&current->sighand->siglock);
 
+	/* A released thread may exit and be freed before it is woken. */
+	guard(rcu)();
 	while ((curr = next) != NULL) {
 		next = curr->next;
 		task = curr->task;
@@ -570,6 +572,7 @@ static void coredump_finish(bool core_dumped)
 		 * ->task == NULL before we read ->next.
 		 */
 		smp_mb();
+		/* Any wakeup now lets the thread exit, rcu keeps it alive. */
 		curr->task = NULL;
 		wake_up_process(task);
 	}