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 <redacted>
---
 Documentation/networking/ip-sysctl.rst |  6 ++++
 include/net/tcp_authopt.h              |  1 +
 net/ipv4/sysctl_net_ipv4.c             | 39 ++++++++++++++++++++++++++
 net/ipv4/tcp_authopt.c                 | 25 +++++++++++++++++
 4 files changed, 71 insertions(+)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index a759872a2883..41be0e69d767 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst

@@ -1038,10 +1038,16 @@ tcp_challenge_ack_limit - INTEGER
        Note that this per netns rate limit can allow some side channel
        attacks and probably should not be enabled.
        TCP stack implements per TCP socket limits anyway.
        Default: INT_MAX (unlimited)

+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 7ad34a6987ec..1f5020b790dd 100644
--- a/include/net/tcp_authopt.h
+++ b/include/net/tcp_authopt.h

@@ -80,10 +80,11 @@ struct tcphdr_authopt {
 };

 #ifdef CONFIG_TCP_AUTHOPT
 DECLARE_STATIC_KEY_FALSE(tcp_authopt_needed_key);
 #define tcp_authopt_needed (static_branch_unlikely(&tcp_authopt_needed_key))
+extern int sysctl_tcp_authopt;
 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);
 int tcp_get_authopt_val(struct sock *sk, struct tcp_authopt *key);
 int tcp_set_authopt_key(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 5490c285668b..908a3ef15b47 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 tcp_retr1_max = 255;
 static int ip_local_port_range_min[] = { 1, 1 };
 static int ip_local_port_range_max[] = { 65535, 65535 };
 static int tcp_adv_win_scale_min = -31;

@@ -413,10 +414,37 @@ static int proc_fib_multipath_hash_fields(struct ctl_table *table, int write,

        return ret;
 }
 #endif

+#ifdef CONFIG_TCP_AUTHOPT
+static int proc_tcp_authopt(struct ctl_table *ctl,
+                           int write, void *buffer, size_t *lenp,
+                           loff_t *ppos)
+{
+       int val = sysctl_tcp_authopt;

+       return 0;
+}
+#endif
+
 static struct ctl_table ipv4_table[] = {
        {
                .procname       = "tcp_max_orphans",
                .data           = &sysctl_tcp_max_orphans,
                .maxlen         = sizeof(int),

@@ -524,10 +552,21 @@ 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_tcp_authopt,
+               .extra1         = SYSCTL_ZERO,
+               .extra2         = SYSCTL_ONE,
+       },
+#endif
        { }
 };

 static struct ctl_table ipv4_net_table[] = {
        /* tcp_max_tw_buckets must be first in this table. */

diff --git a/net/ipv4/tcp_authopt.c b/net/ipv4/tcp_authopt.c
index 4f7cbe1e17f3..9d02da8d6964 100644
--- a/net/ipv4/tcp_authopt.c
+++ b/net/ipv4/tcp_authopt.c

@@ -4,10 +4,15 @@
 #include <net/ipv6.h>
 #include <net/tcp.h>
 #include <linux/kref.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_key);
 EXPORT_SYMBOL(tcp_authopt_needed_key);

 /* All current algorithms have a mac length of 12 but crypto API digestsize can be larger */

@@ -437,17 +442,30 @@ static int _copy_from_sockptr_tolerant(u8 *dst,
                memset(dst + srclen, 0, dstlen - srclen);

        return err;
 }

+static int check_sysctl_tcp_authopt(void)
+{
+       if (!sysctl_tcp_authopt) {

+               net_warn_ratelimited("TCP Authentication Option disabled by sysctl.\n");
+               return -EPERM;
+       }
+
+       return 0;
+}
+
 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);
+       err = check_sysctl_tcp_authopt();
+       if (err)
+               return err;

        err = _copy_from_sockptr_tolerant((u8 *)&opt, sizeof(opt), optval, optlen);
        if (err)
                return err;

@@ -465,13 +483,17 @@ 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;
+       int err;

        memset(opt, 0, sizeof(*opt));
        sock_owned_by_me(sk);
+       err = check_sysctl_tcp_authopt();
+       if (err)
+               return err;

        info = rcu_dereference_check(tp->authopt_info, lockdep_sock_is_held(sk));
        if (!info)
                return -ENOENT;

@@ -493,10 +515,13 @@ int tcp_set_authopt_key(struct sock *sk, sockptr_t optval, unsigned int optlen)
        struct netns_tcp_authopt *net = sock_net_tcp_authopt(sk);
        struct tcp_authopt_alg_imp *alg;
        int err;

        sock_owned_by_me(sk);
+       err = check_sysctl_tcp_authopt();
+       if (err)
+               return err;
        if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))
                return -EPERM;

        err = _copy_from_sockptr_tolerant((u8 *)&opt, sizeof(opt), optval, optlen);
        if (err)
--

2.25.1