Re: [syzbot] [bpf?] possible deadlock in kvfree_call_rcu

[Uladzislau Rezki]

On Tue, Mar 26, 2024 at 12:00:22PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    fe46a7dd189e Merge tag 'sound-6.9-rc1' of git://git.kernel..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11547a65180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=4d90a36f0cab495a
> dashboard link: https://syzkaller.appspot.com/bug?extid=1fa663a2100308ab6eab
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> 

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 7033999ecd7b

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3467,19 +3467,19 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
         */
        kmemleak_ignore(ptr);

-       // Set timer to drain after KFREE_DRAIN_JIFFIES.
-       if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
-               schedule_delayed_monitor_work(krcp);
-
 unlock_return:
        krc_this_cpu_unlock(krcp, flags);

-       /*
-        * Inline kvfree() after synchronize_rcu(). We can do
-        * it from might_sleep() context only, so the current
-        * CPU can pass the QS state.
-        */
-       if (!success) {
+       if (success) {
+               // Set timer to drain after KFREE_DRAIN_JIFFIES.
+               if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
+                       schedule_delayed_monitor_work(krcp);
+       } else {
+               /*
+                * Inline kvfree() after synchronize_rcu(). We can do
+                * it from might_sleep() context only, so the current
+                * CPU can pass the QS state.
+                */
                debug_rcu_head_unqueue((struct rcu_head *) ptr);
                synchronize_rcu();
                kvfree(ptr);