[Patch bpf] sock_map: fix a NULL pointer dereference in sock_map_link_update_prog()

[Cong Wang <xiyou.wangcong@xxxxxxxxx>]

From: Cong Wang <cong.wang@xxxxxxxxxxxxx>

The following race condition could trigger a NULL pointer dereference:

sock_map_link_detach():		sock_map_link_update_prog():
   mutex_lock(&sockmap_mutex);
   ...
   sockmap_link->map = NULL;
   mutex_unlock(&sockmap_mutex);
   				   mutex_lock(&sockmap_mutex);
				   ...
				   sock_map_prog_link_lookup(sockmap_link->map);
				   mutex_unlock(&sockmap_mutex);
   <continue>

Fix it by adding a NULL pointer check. In this specific case, it makes
no sense to update a link which is being released.

Reported-by: Ruan Bonan <bonan.ruan@xxxxxxxxx>
Fixes: 699c23f02c65 ("bpf: Add bpf_link support for sk_msg and sk_skb progs")
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
---
 net/core/sock_map.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 07d6aa4e39ef..9fca4db52f57 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -1760,6 +1760,10 @@ static int sock_map_link_update_prog(struct bpf_link *link,
 		ret = -EINVAL;
 		goto out;
 	}
+	if (!sockmap_link->map) {
+		ret = -EINVAL;
+		goto out;
+	}
 
 	ret = sock_map_prog_link_lookup(sockmap_link->map, &pprog, &plink,
 					sockmap_link->attach_type);
-- 
2.34.1