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

[Hillf Danton]

On Thu, 18 Apr 2024 13:00:28 -0700
> syzbot found the following issue on:
> 
> HEAD commit:    f99c5f563c17 Merge tag 'nf-24-03-21' of git://git.kernel.o..
> git tree:       net
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12f1f7cb180000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git  f99c5f563c17

--- x/kernel/bpf/queue_stack_maps.c
+++ y/kernel/bpf/queue_stack_maps.c
@@ -124,7 +124,7 @@ out:
 	return err;
 }
 
-
+static DEFINE_PER_CPU(int, map_depth);
 static long __stack_map_get(struct bpf_map *map, void *value, bool delete)
 {
 	struct bpf_queue_stack *qs = bpf_queue_stack(map);
@@ -132,12 +132,22 @@ static long __stack_map_get(struct bpf_m
 	int err = 0;
 	void *ptr;
 	u32 index;
+	int dumy;
+	int *depth = &dumy;
 
 	if (in_nmi()) {
 		if (!raw_spin_trylock_irqsave(&qs->lock, flags))
 			return -EBUSY;
 	} else {
+		preempt_disable_notrace();
+		depth = this_cpu_ptr(&map_depth);
+		if (*depth) {
+			preempt_enable_notrace();
+			return -EDEADLK;
+		}
 		raw_spin_lock_irqsave(&qs->lock, flags);
+		*depth += 1;
+		preempt_enable_notrace();
 	}
 
 	if (queue_stack_map_is_empty(qs)) {
@@ -157,6 +167,7 @@ static long __stack_map_get(struct bpf_m
 		qs->head = index;
 
 out:
+	*depth -= 1;
 	raw_spin_unlock_irqrestore(&qs->lock, flags);
 	return err;
 }
--