[PATCH] ocfs2/cluster: hold a reference on the heartbeat thread

[Karl Mehltretter <kmehltretter@gmail.com>]

Since commit 688bc88e2046 ("ocfs2/cluster: keep heartbeat local node
stable"), o2hb_thread() leaves its loop and returns when the local node
changes, for example after "echo 0 > node/<name>/local". The thread
was started with kthread_run() and nothing holds a reference to its
task_struct, so the task is freed once it exits, while reg->hr_task
still points to it.

Reading the region's pid attribute then reads the freed task, and
removing the region calls kthread_stop() on it:

  BUG: KASAN: slab-use-after-free in o2hb_region_pid_show+0xb3/0xc0
  refcount_t: addition on 0; use-after-free.
  Oops: Oops: 0000 [#1] SMP KASAN NOPTI
  RIP: 0010:kthread_stop+0xb1/0x390

The thread could already return by itself before, when heartbeat start
was aborted or on an unclean stop, but the local node change makes it
reachable from userspace at any time.

Create the thread parked, take a reference on it and only then wake it,
so the reference cannot race with the thread exiting. Drop it with
kthread_stop_put().

Fixes: 688bc88e2046 ("ocfs2/cluster: keep heartbeat local node stable")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260616074931.3774929-1-zzzccc427%40gmail.com
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Reproduced under QEMU x86_64 with KASAN, using one node over configfs
and a loop device as the heartbeat region:

  echo 0 > cluster/c1/node/n0/local     # heartbeat thread returns
  cat cluster/c1/heartbeat/<uuid>/pid   # reads the freed task_struct
  rmdir cluster/c1/heartbeat/<uuid>     # kthread_stop() on it

With the patch, that sequence, a normal region removal and a start
interrupted by a signal ran clean on two CPUs, five rounds each, and
every heartbeat task_struct was freed.

 fs/ocfs2/cluster/heartbeat.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/ocfs2/cluster/heartbeat.c b/fs/ocfs2/cluster/heartbeat.c
index 1c3def99bb076..a4c8ea695f5cf 100644
--- a/fs/ocfs2/cluster/heartbeat.c
+++ b/fs/ocfs2/cluster/heartbeat.c
@@ -1966,18 +1966,22 @@ static ssize_t o2hb_region_dev_store(struct config_item *item,
 	atomic_set(&reg->hr_unsteady_iterations, (live_threshold * 3));
 	o2hb_set_region_stopping(reg, false);
 
-	hb_task = kthread_run(o2hb_thread, reg, "o2hb-%s",
-			      reg->hr_item.ci_name);
+	hb_task = kthread_create(o2hb_thread, reg, "o2hb-%s",
+				 reg->hr_item.ci_name);
 	if (IS_ERR(hb_task)) {
 		ret = PTR_ERR(hb_task);
 		mlog_errno(ret);
 		goto out;
 	}
+	/* The thread may exit on its own, so pin it before it can run. */
+	get_task_struct(hb_task);
 
 	spin_lock(&o2hb_live_lock);
 	reg->hr_task = hb_task;
 	spin_unlock(&o2hb_live_lock);
 
+	wake_up_process(hb_task);
+
 	ret = wait_event_interruptible(o2hb_steady_queue,
 				atomic_read(&reg->hr_steady_iterations) == 0 ||
 				reg->hr_node_deleted);
@@ -2022,7 +2026,7 @@ static ssize_t o2hb_region_dev_store(struct config_item *item,
 		spin_unlock(&o2hb_live_lock);
 
 		if (hb_task)
-			kthread_stop(hb_task);
+			kthread_stop_put(hb_task);
 
 		o2hb_unmap_slot_data(reg);
 
@@ -2208,7 +2212,7 @@ static void o2hb_heartbeat_group_drop_item(struct config_group *group,
 	spin_unlock(&o2hb_live_lock);
 
 	if (hb_task)
-		kthread_stop(hb_task);
+		kthread_stop_put(hb_task);
 
 	if (o2hb_global_heartbeat_active()) {
 		spin_lock(&o2hb_live_lock);
-- 
2.53.0