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

After commit 1a80dbcb2dba, 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: 1a80dbcb2dba ("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

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 2222c3ff88e7fd639390112de66581b934584457..f45ed6adc092af680ae01b01027b1d572e5c418f 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/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)