[PATCH 12/13] bpf: Don't refcount LISTEN sockets in sk_assign()

[Khalid Elmously]

From: Joe Stringer <joe at wand.net.nz>

[ upstream commit 7ae215d23c12a939005f35d1848ca55b6109b9c0 ]

Avoid taking a reference on listen sockets by checking the socket type
in the sk_assign and in the corresponding skb_steal_sock() code in the
the transport layer, and by ensuring that the prefetch free (sock_pfree)
function uses the same logic to check whether the socket is refcounted.

Suggested-by: Martin KaFai Lau <kafai at fb.com>
Signed-off-by: Joe Stringer <joe at wand.net.nz>
Signed-off-by: Alexei Starovoitov <ast at kernel.org>
Acked-by: Martin KaFai Lau <kafai at fb.com>
Link: https://lore.kernel.org/bpf/20200329225342.16317-4-joe@wand.net.nz
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
---
 include/net/sock.h | 25 +++++++++++++++++--------
 net/core/filter.c  |  6 +++---
 net/core/sock.c    |  3 ++-
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 6cb1f0efa01b..e13e9b7c34d1 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2492,6 +2492,21 @@ skb_sk_is_prefetched(struct sk_buff *skb)
 #endif /* CONFIG_INET */
 }
 
+/* This helper checks if a socket is a full socket,
+ * ie _not_ a timewait or request socket.
+ */
+static inline bool sk_fullsock(const struct sock *sk)
+{
+	return (1 << sk->sk_state) & ~(TCPF_TIME_WAIT | TCPF_NEW_SYN_RECV);
+}
+
+static inline bool
+sk_is_refcounted(struct sock *sk)
+{
+	/* Only full sockets have sk->sk_flags. */
+	return !sk_fullsock(sk) || !sock_flag(sk, SOCK_RCU_FREE);
+}
+
 /**
  * skb_steal_sock
  * @skb to steal the socket from
@@ -2504,6 +2519,8 @@ skb_steal_sock(struct sk_buff *skb, bool *refcounted)
 		struct sock *sk = skb->sk;
 
 		*refcounted = true;
+		if (skb_sk_is_prefetched(skb))
+			*refcounted = sk_is_refcounted(sk);
 		skb->destructor = NULL;
 		skb->sk = NULL;
 		return sk;
@@ -2512,14 +2529,6 @@ skb_steal_sock(struct sk_buff *skb, bool *refcounted)
 	return NULL;
 }
 
-/* This helper checks if a socket is a full socket,
- * ie _not_ a timewait or request socket.
- */
-static inline bool sk_fullsock(const struct sock *sk)
-{
-	return (1 << sk->sk_state) & ~(TCPF_TIME_WAIT | TCPF_NEW_SYN_RECV);
-}
-
 /* Checks if this SKB belongs to an HW offloaded socket
  * and whether any SW fallbacks are required based on dev.
  * Check decrypted mark in case skb_orphan() cleared socket.
diff --git a/net/core/filter.c b/net/core/filter.c
index 4033799dafd5..baaeab75327c 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5468,8 +5468,7 @@ static const struct bpf_func_proto bpf_sk_lookup_udp_proto = {
 
 BPF_CALL_1(bpf_sk_release, struct sock *, sk)
 {
-	/* Only full sockets have sk->sk_flags. */
-	if (!sk_fullsock(sk) || !sock_flag(sk, SOCK_RCU_FREE))
+	if (sk_is_refcounted(sk))
 		sock_gen_put(sk);
 	return 0;
 }
@@ -5995,7 +5994,8 @@ BPF_CALL_3(bpf_sk_assign, struct sk_buff *, skb, struct sock *, sk, u64, flags)
 		return -ENETUNREACH;
 	if (unlikely(sk->sk_reuseport))
 		return -ESOCKTNOSUPPORT;
-	if (unlikely(!refcount_inc_not_zero(&sk->sk_refcnt)))
+	if (sk_is_refcounted(sk) &&
+	    unlikely(!refcount_inc_not_zero(&sk->sk_refcnt)))
 		return -ENOENT;
 
 	skb_orphan(skb);
diff --git a/net/core/sock.c b/net/core/sock.c
index 036ba43d044a..bcb7affcc1e0 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2071,7 +2071,8 @@ EXPORT_SYMBOL(sock_efree);
 #ifdef CONFIG_INET
 void sock_pfree(struct sk_buff *skb)
 {
-	sock_gen_put(skb->sk);
+	if (sk_is_refcounted(skb->sk))
+		sock_gen_put(skb->sk);
 }
 EXPORT_SYMBOL(sock_pfree);
 #endif /* CONFIG_INET */
-- 
2.17.1