[PATCH v2 bpf-next 4/5] bpf: tcp: Stop bpf_setsockopt(TCP_CONGESTION) in init ops to recur itself

[Martin KaFai Lau <kafai@fb.com>]

From: Martin KaFai Lau <martin.lau@kernel.org>

When a bad bpf prog '.init' calls
bpf_setsockopt(TCP_CONGESTION, "itself"), it will trigger this loop:

.init => bpf_setsockopt(tcp_cc) => .init => bpf_setsockopt(tcp_cc) ...
... => .init => bpf_setsockopt(tcp_cc).

It was prevented by the prog->active counter before but the prog->active
detection cannot be used in struct_ops as explained in the earlier
patch of the set.

In this patch, the second bpf_setsockopt(tcp_cc) is not allowed
in order to break the loop.  This is done by using a bit of
an existing 1 byte hole in tcp_sock to check if there is
on-going bpf_setsockopt(TCP_CONGESTION) in this tcp_sock.

Note that this essentially limits only the first '.init' can
call bpf_setsockopt(TCP_CONGESTION) to pick a fallback cc (eg. peer
does not support ECN) and the second '.init' cannot fallback to
another cc.  This applies even the second
bpf_setsockopt(TCP_CONGESTION) will not cause a loop.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
 include/linux/tcp.h |  6 ++++++
 net/core/filter.c   | 28 +++++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index a9fbe22732c3..3bdf687e2fb3 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -388,6 +388,12 @@ struct tcp_sock {
 	u8	bpf_sock_ops_cb_flags;  /* Control calling BPF programs
 					 * values defined in uapi/linux/tcp.h
 					 */
+	u8	bpf_chg_cc_inprogress:1; /* In the middle of
+					  * bpf_setsockopt(TCP_CONGESTION),
+					  * it is to avoid the bpf_tcp_cc->init()
+					  * to recur itself by calling
+					  * bpf_setsockopt(TCP_CONGESTION, "itself").
+					  */
 #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) (TP->bpf_sock_ops_cb_flags & ARG)
 #else
 #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) 0
diff --git a/net/core/filter.c b/net/core/filter.c
index 96f2f7a65e65..ac4c45c02da5 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5105,6 +5105,9 @@ static int bpf_sol_tcp_setsockopt(struct sock *sk, int optname,
 static int sol_tcp_sockopt_congestion(struct sock *sk, char *optval,
 				      int *optlen, bool getopt)
 {
+	struct tcp_sock *tp;
+	int ret;
+
 	if (*optlen < 2)
 		return -EINVAL;
 
@@ -5125,8 +5128,31 @@ static int sol_tcp_sockopt_congestion(struct sock *sk, char *optval,
 	if (*optlen >= sizeof("cdg") - 1 && !strncmp("cdg", optval, *optlen))
 		return -ENOTSUPP;
 
-	return do_tcp_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
+	/* It stops this looping
+	 *
+	 * .init => bpf_setsockopt(tcp_cc) => .init =>
+	 * bpf_setsockopt(tcp_cc)" => .init => ....
+	 *
+	 * The second bpf_setsockopt(tcp_cc) is not allowed
+	 * in order to break the loop when both .init
+	 * are the same bpf prog.
+	 *
+	 * This applies even the second bpf_setsockopt(tcp_cc)
+	 * does not cause a loop.  This limits only the first
+	 * '.init' can call bpf_setsockopt(TCP_CONGESTION) to
+	 * pick a fallback cc (eg. peer does not support ECN)
+	 * and the second '.init' cannot fallback to
+	 * another.
+	 */
+	tp = tcp_sk(sk);
+	if (tp->bpf_chg_cc_inprogress)
+		return -EBUSY;
+
+	tp->bpf_chg_cc_inprogress = 1;
+	ret = do_tcp_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
 				KERNEL_SOCKPTR(optval), *optlen);
+	tp->bpf_chg_cc_inprogress = 0;
+	return ret;
 }
 
 static int sol_tcp_sockopt(struct sock *sk, int optname,
-- 
2.30.2