netdev - [PATCH bpf] bpf: Check sk_fullsock() before returning from bpf_sk

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Date:   Fri, 17 May 2019 14:21:17 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Joe Stringer <joe@...valent.com>
Subject: [PATCH bpf] bpf: Check sk_fullsock() before returning from bpf_sk_lookup()

The BPF_FUNC_sk_lookup_xxx helpers return RET_PTR_TO_SOCKET_OR_NULL.
Meaning a fullsock ptr and its fullsock's fields in bpf_sock can be
accessed, e.g. type, protocol, mark and priority.
Some new helper, like bpf_sk_storage_get(), also expects
ARG_PTR_TO_SOCKET is a fullsock.

bpf_sk_lookup() currently calls sk_to_full_sk() before returning.
However, the ptr returned from sk_to_full_sk() is not guaranteed
to be a fullsock.  For example, it cannot get a fullsock if sk
is in TCP_TIME_WAIT.

This patch checks for sk_fullsock() before returning. If it is not
a fullsock, sock_gen_put() is called if needed and then returns NULL.

Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
Cc: Joe Stringer <joe@...valent.com>
Signed-off-by: Martin KaFai Lau <kafai@...com>
---
 net/core/filter.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 55bfc941d17a..85def5a20aaf 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5337,8 +5337,14 @@ __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);
 
-	if (sk)
+	if (sk) {
 		sk = sk_to_full_sk(sk);
+		if (!sk_fullsock(sk)) {
+			if (!sock_flag(sk, SOCK_RCU_FREE))
+				sock_gen_put(sk);
+			return NULL;
+		}
+	}
 
 	return sk;
 }
@@ -5369,8 +5375,14 @@ 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);
 
-	if (sk)
+	if (sk) {
 		sk = sk_to_full_sk(sk);
+		if (!sk_fullsock(sk)) {
+			if (!sock_flag(sk, SOCK_RCU_FREE))
+				sock_gen_put(sk);
+			return NULL;
+		}
+	}
 
 	return sk;
 }
-- 
2.17.1