bpf: Allow bpf_map_lookup_elem for SOCKMAP and SOCKHASH

White-list map lookup for SOCKMAP/SOCKHASH from BPF. Lookup returns a
pointer to a full socket and acquires a reference if necessary.

To support it we need to extend the verifier to know that:

 (1) register storing the lookup result holds a pointer to socket, if
     lookup was done on SOCKMAP/SOCKHASH, and that

 (2) map lookup on SOCKMAP/SOCKHASH is a reference acquiring operation,
     which needs a corresponding reference release with bpf_sk_release.

On sock_map side, lookup handlers exposed via bpf_map_ops now bump
sk_refcnt if socket is reference counted. In turn, bpf_sk_select_reuseport,
the only in-kernel user of SOCKMAP/SOCKHASH ops->map_lookup_elem, was
updated to release the reference.

Sockets fetched from a map can be used in the same way as ones returned by
BPF socket lookup helpers, such as bpf_sk_lookup_tcp. In particular, they
can be used with bpf_sk_assign to direct packets toward a socket on TC
ingress path.

Suggested-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200429181154.479310-2-jakub@cloudflare.com

1 files changed, 16 insertions, 2 deletions

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index b08dfae10f88..00a26cf2cfe9 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -343,7 +343,14 @@ static struct sock *__sock_map_lookup_elem(struct bpf_map *map, u32 key)
 
 static void *sock_map_lookup(struct bpf_map *map, void *key)
 {
-	return __sock_map_lookup_elem(map, *(u32 *)key);
+	struct sock *sk;
+
+	sk = __sock_map_lookup_elem(map, *(u32 *)key);
+	if (!sk || !sk_fullsock(sk))
+		return NULL;
+	if (sk_is_refcounted(sk) && !refcount_inc_not_zero(&sk->sk_refcnt))
+		return NULL;
+	return sk;
 }
 
 static void *sock_map_lookup_sys(struct bpf_map *map, void *key)
@@ -1051,7 +1058,14 @@ static void *sock_hash_lookup_sys(struct bpf_map *map, void *key)
 
 static void *sock_hash_lookup(struct bpf_map *map, void *key)
 {
-	return __sock_hash_lookup_elem(map, key);
+	struct sock *sk;
+
+	sk = __sock_hash_lookup_elem(map, key);
+	if (!sk || !sk_fullsock(sk))
+		return NULL;
+	if (sk_is_refcounted(sk) && !refcount_inc_not_zero(&sk->sk_refcnt))
+		return NULL;
+	return sk;
 }
 
 static void sock_hash_release_progs(struct bpf_map *map)