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Subject: Re: [syzbot] [kernel?] KASAN: slab-use-after-free Write in bus_for_each_drv
Author: kmehltretter@gmail.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git d58772d8520c7ef247c4b95c9bd76d3a25da9ff5

Please test the following patch on the known-good upstream revision from
the original report.

---
 lib/klist.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/lib/klist.c b/lib/klist.c
index 332a4fbf18ff0..e503031cd1f9d 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 +188,25 @@ static void klist_release(struct kref *kref)
 
 	WARN_ON(!knode_dead(n));
 	list_del(&n->n_node);
+	/* The woken caller may free or reuse n. */
+	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;
 
+		/* Pin the task before the waiter can return. */
+		p = waiter->process;
+		get_task_struct(p);
 		list_del(&waiter->list);
-		waiter->woken = 1;
-		mb();
-		wake_up_process(waiter->process);
+		/* Publish after the last accesses to waiter and n. */
+		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)
@@ -251,6 +259,7 @@ 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();
 	}