[PATCH net-next] net: bpf: fix request_sock leak in filter.c

[Jon Maxwell]

A customer reported a request_socket leak in a Calico cloud environment. We 
found that a BPF program was doing a socket lookup with takes a refcnt on 
the socket and that it was finding the request_socket but returning the parent 
LISTEN socket via sk_to_full_sk() without decrementing the child request socket 
1st, resulting in request_sock slab object leak. This patch retains the 
existing behaviour of returning full socks to the caller but it also decrements
the child request_socket if one is present before doing so to prevent the leak.

Thanks to Curtis Taylor for all the help in diagnosing and testing this. And 
thanks to Antoine Tenart for the reproducer and patch input.

Signed-off-by: Jon Maxwell <jmaxwell37@xxxxxxxxx>
Tested-by: Curtis Taylor <cjebpub@xxxxxxxxx>
Co-developed-by: Antoine Tenart <atenart@xxxxxxxxxx>
---
 net/core/filter.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 5af58eb48587..f7d74acfef5a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6514,13 +6514,14 @@ __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
 {
 	struct sock *sk = __bpf_skc_lookup(skb, tuple, len, caller_net,
 					   ifindex, proto, netns_id, flags);
+	struct sock *sk1 = sk;
 
 	if (sk) {
 		sk = sk_to_full_sk(sk);
-		if (!sk_fullsock(sk)) {
-			sock_gen_put(sk);
+		if (!sk_fullsock(sk1)) 
+			sock_gen_put(sk1);
+		if (!sk_fullsock(sk))
 			return NULL;
-		}
 	}
 
 	return sk;
@@ -6551,13 +6552,14 @@ bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
 {
 	struct sock *sk = bpf_skc_lookup(skb, tuple, len, proto, netns_id,
 					 flags);
+	struct sock *sk1 = sk;
 
 	if (sk) {
 		sk = sk_to_full_sk(sk);
-		if (!sk_fullsock(sk)) {
-			sock_gen_put(sk);
+		if (!sk_fullsock(sk1))
+			sock_gen_put(sk1);
+		if (!sk_fullsock(sk))
 			return NULL;
-		}
 	}
 
 	return sk;
-- 
2.31.1