Re: [PATCH v8 08/26] tcp: authopt: Disable via sysctl by default

From: Eric Dumazet
Date: Tue Sep 06 2022 - 19:12:18 EST

On Mon, Sep 5, 2022 at 12:06 AM Leonard Crestez <cdleonard@xxxxxxxxx> 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@xxxxxxxxx>
> ---
> 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
>