bpf: Fix a potential use-after-free in bpf_link_free()

After commit 1a80dbcb, bpf_link can be freed by
link->ops->dealloc_deferred, but the code still tests and uses
link->ops->dealloc afterward, which leads to a use-after-free as
reported by syzbot. Actually, one of them should be sufficient, so
just call one of them instead of both. Also add a WARN_ON() in case
of any problematic implementation.

Fixes: 1a80dbcb ("bpf: support deferring bpf_link dealloc to after RCU grace period")
Reported-by: syzbot+1989ee16d94720836244@syzkaller.appspotmail.com
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240602182703.207276-1-xiyou.wangcong@gmail.com

kernel/bpf/syscall.c

@@ -2998,6 +2998,7 @@ static int bpf_obj_get(const union bpf_attr *attr)
 void bpf_link_init(struct bpf_link *link, enum bpf_link_type type,
 		   const struct bpf_link_ops *ops, struct bpf_prog *prog)
 {
+	WARN_ON(ops->dealloc && ops->dealloc_deferred);
 	atomic64_set(&link->refcnt, 1);
 	link->type = type;
 	link->id = 0;
@@ -3056,16 +3057,17 @@ static void bpf_link_defer_dealloc_mult_rcu_gp(struct rcu_head *rcu)
 /* bpf_link_free is guaranteed to be called from process context */
 static void bpf_link_free(struct bpf_link *link)
 {
+	const struct bpf_link_ops *ops = link->ops;
 	bool sleepable = false;
 
 	bpf_link_free_id(link->id);
 	if (link->prog) {
 		sleepable = link->prog->sleepable;
 		/* detach BPF program, clean up used resources */
-		link->ops->release(link);
+		ops->release(link);
 		bpf_prog_put(link->prog);
 	}
-	if (link->ops->dealloc_deferred) {
+	if (ops->dealloc_deferred) {
 		/* schedule BPF link deallocation; if underlying BPF program
 		 * is sleepable, we need to first wait for RCU tasks trace
 		 * sync, then go through "classic" RCU grace period
@@ -3074,9 +3076,8 @@ static void bpf_link_free(struct bpf_link *link)
 			call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
 		else
 			call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
-	}
-	if (link->ops->dealloc)
-		link->ops->dealloc(link);
+	} else if (ops->dealloc)
+		ops->dealloc(link);
 }
 
 static void bpf_link_put_deferred(struct work_struct *work)