bpf: sockmap missing NULL psock check

Add psock NULL check to handle a racing sock event that can get the
sk_callback_lock before this case but after xchg happens causing the
refcnt to hit zero and sock user data (psock) to be null and queued
for garbage collection.

Also add a comment in the code because this is a bit subtle and
not obvious in my opinion.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 9 insertions, 2 deletions

diff --git a/kernel/bpf/sockmap.c b/kernel/bpf/sockmap.c
index 5ee2e41893d9..1712d319c2d8 100644
--- a/kernel/bpf/sockmap.c
+++ b/kernel/bpf/sockmap.c
@@ -591,8 +591,15 @@ static void sock_map_free(struct bpf_map *map)
 
                 write_lock_bh(&sock->sk_callback_lock);
                 psock = smap_psock_sk(sock);
-                smap_list_remove(psock, &stab->sock_map[i]);
-                smap_release_sock(psock, sock);
+                /* This check handles a racing sock event that can get the
+                 * sk_callback_lock before this case but after xchg happens
+                 * causing the refcnt to hit zero and sock user data (psock)
+                 * to be null and queued for garbage collection.
+                 */
+                if (likely(psock)) {
+                        smap_list_remove(psock, &stab->sock_map[i]);
+                        smap_release_sock(psock, sock);
+                }
                 write_unlock_bh(&sock->sk_callback_lock);
         }
         rcu_read_unlock();