On Mon, Sep 5, 2022 at 12:06 AM Leonard Crestez cdleonard@gmail.com wrote:

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@gmail.com

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;
  

val = READ_ONCE(sysctl_tcp_authopt);

•   struct ctl_table tmp = {
  

•           .data = &val,
  

•           .mode = ctl->mode,
  

•           .maxlen = sizeof(val),
  

•           .extra1 = SYSCTL_ZERO,
  

•           .extra2 = SYSCTL_ONE,
  

•   };
  

•   int err;
  

•   err = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
  

•   if (err)
  

•           return err;
  

•   if (sysctl_tcp_authopt && !val) {
  

READ_ONCE(sysctl_tcp_authopt)

Note that this test would still be racy, because another cpu might change sysctl_tcp_authopt right after the read.

•           net_warn_ratelimited("Enabling TCP Authentication Option is permanent\n");
  

•           return -EINVAL;
  

•   }
  

•   sysctl_tcp_authopt = val;
  

WRITE_ONCE(sysctl_tcp_authopt, val), or even better:

if (val) cmpxchg(&sysctl_tcp_authopt, 0, val);

•   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) {
  

READ_ONCE(...)

•           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