[PATCH v2 09/25] tcp: authopt: Disable via sysctl by default

[Leonard Crestez]

This is mainly intended to protect against local privilege escalations
through a rarely used feature so it is deliberately not namespaced.

Enforcement is only at the setsockopt level, this should be enough to
ensure that the tcp_authopt_needed static key never turns on.

No effort is made to handle disabling when the feature is already in
use.

Signed-off-by: Leonard Crestez <cdleonard@xxxxxxxxx>
---
 Documentation/networking/ip-sysctl.rst |  6 ++++++
 include/net/tcp_authopt.h              |  1 +
 net/ipv4/sysctl_net_ipv4.c             | 10 ++++++++++
 net/ipv4/tcp_authopt.c                 | 13 ++++++++++++-
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 16b8bf72feaf..3f00681f73d7 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -987,10 +987,16 @@ tcp_limit_output_bytes - INTEGER
 tcp_challenge_ack_limit - INTEGER
 	Limits number of Challenge ACK sent per second, as recommended
 	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
 	Default: 1000
 
+tcp_authopt - BOOLEAN
+	Enable the TCP Authentication Option (RFC5925), a replacement for TCP
+	MD5 Signatures (RFC2835).
+
+	Default: 0
+
 UDP variables
 =============
 
 udp_l3mdev_accept - BOOLEAN
 	Enabling this option allows a "global" bound socket to work
diff --git a/include/net/tcp_authopt.h b/include/net/tcp_authopt.h
index 8bb76128ed11..a505db1dd67b 100644
--- a/include/net/tcp_authopt.h
+++ b/include/net/tcp_authopt.h
@@ -65,10 +65,11 @@ struct tcp_authopt_info {
 	/** @dst_isn: Remote Initial Sequence Number */
 	u32 dst_isn;
 };
 
 #ifdef CONFIG_TCP_AUTHOPT
+extern int sysctl_tcp_authopt;
 DECLARE_STATIC_KEY_FALSE(tcp_authopt_needed);
 
 void tcp_authopt_free(struct sock *sk, struct tcp_authopt_info *info);
 void tcp_authopt_clear(struct sock *sk);
 int tcp_set_authopt(struct sock *sk, sockptr_t optval, unsigned int optlen);
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 97eb54774924..cc34de6e4817 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -17,10 +17,11 @@
 #include <net/udp.h>
 #include <net/cipso_ipv4.h>
 #include <net/ping.h>
 #include <net/protocol.h>
 #include <net/netevent.h>
+#include <net/tcp_authopt.h>
 
 static int two = 2;
 static int three __maybe_unused = 3;
 static int four = 4;
 static int thousand = 1000;
@@ -583,10 +584,19 @@ static struct ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_douintvec_minmax,
 		.extra1		= &sysctl_fib_sync_mem_min,
 		.extra2		= &sysctl_fib_sync_mem_max,
 	},
+#ifdef CONFIG_TCP_AUTHOPT
+	{
+		.procname	= "tcp_authopt",
+		.data		= &sysctl_tcp_authopt,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
+#endif
 	{ }
 };
 
 static struct ctl_table ipv4_net_table[] = {
 	{
diff --git a/net/ipv4/tcp_authopt.c b/net/ipv4/tcp_authopt.c
index 5e80e5e5e36e..7c49dcce7d24 100644
--- a/net/ipv4/tcp_authopt.c
+++ b/net/ipv4/tcp_authopt.c
@@ -3,10 +3,15 @@
 #include <linux/kernel.h>
 #include <net/tcp.h>
 #include <net/tcp_authopt.h>
 #include <crypto/hash.h>
 
+/* This is mainly intended to protect against local privilege escalations through
+ * a rarely used feature so it is deliberately not namespaced.
+ */
+int sysctl_tcp_authopt;
+
 /* This is enabled when first struct tcp_authopt_info is allocated and never released */
 DEFINE_STATIC_KEY_FALSE(tcp_authopt_needed);
 EXPORT_SYMBOL(tcp_authopt_needed);
 
 /* All current algorithms have a mac length of 12 but crypto API digestsize can be larger */
@@ -360,10 +365,12 @@ int tcp_set_authopt(struct sock *sk, sockptr_t optval, unsigned int optlen)
 	struct tcp_authopt opt;
 	struct tcp_authopt_info *info;
 	int err;
 
 	sock_owned_by_me(sk);
+	if (!sysctl_tcp_authopt)
+		return -EPERM;
 
 	err = _copy_from_sockptr_tolerant((u8 *)&opt, sizeof(opt), optval, optlen);
 	if (err)
 		return err;
 
@@ -382,13 +389,15 @@ int tcp_set_authopt(struct sock *sk, sockptr_t optval, unsigned int optlen)
 int tcp_get_authopt_val(struct sock *sk, struct tcp_authopt *opt)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct tcp_authopt_info *info;
 
+	memset(opt, 0, sizeof(*opt));
 	sock_owned_by_me(sk);
+	if (!sysctl_tcp_authopt)
+		return -EPERM;
 
-	memset(opt, 0, sizeof(*opt));
 	info = rcu_dereference_check(tp->authopt_info, lockdep_sock_is_held(sk));
 	if (!info)
 		return -ENOENT;
 
 	opt->flags = info->flags & TCP_AUTHOPT_KNOWN_FLAGS;
@@ -451,10 +460,12 @@ int tcp_set_authopt_key(struct sock *sk, sockptr_t optval, unsigned int optlen)
 	struct tcp_authopt_key_info *key_info, *old_key_info;
 	struct tcp_authopt_alg_imp *alg;
 	int err;
 
 	sock_owned_by_me(sk);
+	if (!sysctl_tcp_authopt)
+		return -EPERM;
 
 	err = _copy_from_sockptr_tolerant((u8 *)&opt, sizeof(opt), optval, optlen);
 	if (err)
 		return err;
 
-- 
2.25.1