[bpf-next PATCH 2/4] bpf: sockmap, duplicates release calls may NULL sk_prot

[John Fastabend <john.fastabend@gmail.com>]

It is possible to have multiple ULP tcp_release call paths in flight
if a sock is closed and simultaneously being removed from the sockmap
control path. The result would be setting the sk_prot to the saved
values on the first iteration and then on the second iteration setting
the value to NULL.

This patch resolves this by ensuring we only reset the sk_prot pointer
if we have a valid saved state to set.

Fixes: 4f738adba30a7 ("bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 kernel/bpf/sockmap.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/sockmap.c b/kernel/bpf/sockmap.c
index 8ddf326..8dd9210 100644
--- a/kernel/bpf/sockmap.c
+++ b/kernel/bpf/sockmap.c
@@ -182,8 +182,10 @@ static void bpf_tcp_release(struct sock *sk)
 		psock->cork = NULL;
 	}
 
-	sk->sk_prot = psock->sk_proto;
-	psock->sk_proto = NULL;
+	if (psock->sk_proto) {
+		sk->sk_prot = psock->sk_proto;
+		psock->sk_proto = NULL;
+	}
 out:
 	rcu_read_unlock();
 }