[PATCH] klist: avoid accesses after waking klist_remove()

[Karl Mehltretter]

klist_release() publishes waiter->woken before its final accesses to the
stack waiter and node. klist_remove() can then return, allowing the waiter
to go out of scope and its caller to free or reuse the node.

Clear n_klist and take a task reference before publishing woken. Use
release/acquire accesses for that publication and wake the referenced task.

The task reference keeps the waiter task alive if it returns and exits
before wake_up_process(). wake_up_process() provides the full barrier
required by the sleep/wakeup protocol, so remove the explicit mb().

Fixes: 8b0c250be489 ("[PATCH] add klist_node_attached() to determine if a node is on a list or not.")
Fixes: 210272a28465 ("driver core: Remove completion from struct klist_node")
Reported-by: syzbot+9cb1ac7fce4944ba9165@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=9cb1ac7fce4944ba9165
Tested-by: syzbot+9cb1ac7fce4944ba9165@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
 lib/klist.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/lib/klist.c b/lib/klist.c
index 332a4fbf18ff..7242f4cc1c1b 100644
--- a/lib/klist.c
+++ b/lib/klist.c
@@ -36,6 +36,7 @@
 #include <linux/klist.h>
 #include <linux/export.h>
 #include <linux/sched.h>
+#include <linux/sched/task.h>
 
 /*
  * Use the lowest bit of n_klist to mark deleted nodes and exclude
@@ -187,18 +188,23 @@ static void klist_release(struct kref *kref)
 
 	WARN_ON(!knode_dead(n));
 	list_del(&n->n_node);
+	knode_set_klist(n, NULL);
 	spin_lock(&klist_remove_lock);
 	list_for_each_entry_safe(waiter, tmp, &klist_remove_waiters, list) {
+		struct task_struct *p;
+
 		if (waiter->node != n)
 			continue;
 
+		p = waiter->process;
+		get_task_struct(p);
 		list_del(&waiter->list);
-		waiter->woken = 1;
-		mb();
-		wake_up_process(waiter->process);
+		/* Publish only after the final waiter and n accesses */
+		smp_store_release(&waiter->woken, 1);
+		wake_up_process(p);
+		put_task_struct(p);
 	}
 	spin_unlock(&klist_remove_lock);
-	knode_set_klist(n, NULL);
 }
 
 static int klist_dec_and_del(struct klist_node *n)
@@ -250,7 +256,8 @@ void klist_remove(struct klist_node *n)
 
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (waiter.woken)
+		/* Pairs with the release store in klist_release() */
+		if (smp_load_acquire(&waiter.woken))
 			break;
 		schedule();
 	}
-- 
2.39.5 (Apple Git-154)