Re: [Patch bpf] bpf: fix a potential use-after-free in bpf_link_free()

[Jiri Olsa <olsajiri@gmail.com>]

On Sun, Jun 02, 2024 at 11:27:03AM -0700, Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> 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.
> 
> Reported-by: syzbot+1989ee16d94720836244@syzkaller.appspotmail.com
> Fixes: 1a80dbcb2dba ("bpf: support deferring bpf_link dealloc to after RCU grace period")
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> ---
>  kernel/bpf/syscall.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 2222c3ff88e7..d8f244069495 100644
> --- 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;
> @@ -3074,8 +3075,7 @@ 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)
> +	} else if (link->ops->dealloc)
>  		link->ops->dealloc(link);

nice catch

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka