Commit a26552af authored by Hannes Frederic Sowa's avatar Hannes Frederic Sowa Committed by David S. Miller

tcp: don't allow syn packets without timestamps to pass tcp_tw_recycle logic

tcp_tw_recycle heavily relies on tcp timestamps to build a per-host
ordering of incoming connections and teardowns without the need to
hold state on a specific quadruple for TCP_TIMEWAIT_LEN, but only for
the last measured RTO. To do so, we keep the last seen timestamp in a
per-host indexed data structure and verify if the incoming timestamp
in a connection request is strictly greater than the saved one during
last connection teardown. Thus we can verify later on that no old data
packets will be accepted by the new connection.

During moving a socket to time-wait state we already verify if timestamps
where seen on a connection. Only if that was the case we let the
time-wait socket expire after the RTO, otherwise normal TCP_TIMEWAIT_LEN
will be used. But we don't verify this on incoming SYN packets. If a
connection teardown was less than TCP_PAWS_MSL seconds in the past we
cannot guarantee to not accept data packets from an old connection if
no timestamps are present. We should drop this SYN packet. This patch
closes this loophole.

Please note, this patch does not make tcp_tw_recycle in any way more
usable but only adds another safety check:
Sporadic drops of SYN packets because of reordering in the network or
in the socket backlog queues can happen. Users behing NAT trying to
connect to a tcp_tw_recycle enabled server can get caught in blackholes
and their connection requests may regullary get dropped because hosts
behind an address translator don't have synchronized tcp timestamp clocks.
tcp_tw_recycle cannot work if peers don't have tcp timestamps enabled.

In general, use of tcp_tw_recycle is disadvised.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: default avatarHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 4fab9071
...@@ -417,7 +417,7 @@ void tcp_update_metrics(struct sock *sk); ...@@ -417,7 +417,7 @@ void tcp_update_metrics(struct sock *sk);
void tcp_init_metrics(struct sock *sk); void tcp_init_metrics(struct sock *sk);
void tcp_metrics_init(void); void tcp_metrics_init(void);
bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst, bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst,
bool paws_check); bool paws_check, bool timestamps);
bool tcp_remember_stamp(struct sock *sk); bool tcp_remember_stamp(struct sock *sk);
bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw); bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw);
void tcp_fetch_timewait_stamp(struct sock *sk, struct dst_entry *dst); void tcp_fetch_timewait_stamp(struct sock *sk, struct dst_entry *dst);
......
...@@ -5979,12 +5979,14 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, ...@@ -5979,12 +5979,14 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
* timewait bucket, so that all the necessary checks * timewait bucket, so that all the necessary checks
* are made in the function processing timewait state. * are made in the function processing timewait state.
*/ */
if (tmp_opt.saw_tstamp && tcp_death_row.sysctl_tw_recycle) { if (tcp_death_row.sysctl_tw_recycle) {
bool strict; bool strict;
dst = af_ops->route_req(sk, &fl, req, &strict); dst = af_ops->route_req(sk, &fl, req, &strict);
if (dst && strict && if (dst && strict &&
!tcp_peer_is_proven(req, dst, true)) { !tcp_peer_is_proven(req, dst, true,
tmp_opt.saw_tstamp)) {
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSPASSIVEREJECTED); NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSPASSIVEREJECTED);
goto drop_and_release; goto drop_and_release;
} }
...@@ -5993,7 +5995,8 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops, ...@@ -5993,7 +5995,8 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
else if (!sysctl_tcp_syncookies && else if (!sysctl_tcp_syncookies &&
(sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) < (sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
(sysctl_max_syn_backlog >> 2)) && (sysctl_max_syn_backlog >> 2)) &&
!tcp_peer_is_proven(req, dst, false)) { !tcp_peer_is_proven(req, dst, false,
tmp_opt.saw_tstamp)) {
/* Without syncookies last quarter of /* Without syncookies last quarter of
* backlog is filled with destinations, * backlog is filled with destinations,
* proven to be alive. * proven to be alive.
......
...@@ -576,7 +576,8 @@ void tcp_init_metrics(struct sock *sk) ...@@ -576,7 +576,8 @@ void tcp_init_metrics(struct sock *sk)
tp->snd_cwnd_stamp = tcp_time_stamp; tp->snd_cwnd_stamp = tcp_time_stamp;
} }
bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst, bool paws_check) bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst,
bool paws_check, bool timestamps)
{ {
struct tcp_metrics_block *tm; struct tcp_metrics_block *tm;
bool ret; bool ret;
...@@ -589,7 +590,8 @@ bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst, bool pa ...@@ -589,7 +590,8 @@ bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst, bool pa
if (paws_check) { if (paws_check) {
if (tm && if (tm &&
(u32)get_seconds() - tm->tcpm_ts_stamp < TCP_PAWS_MSL && (u32)get_seconds() - tm->tcpm_ts_stamp < TCP_PAWS_MSL &&
(s32)(tm->tcpm_ts - req->ts_recent) > TCP_PAWS_WINDOW) ((s32)(tm->tcpm_ts - req->ts_recent) > TCP_PAWS_WINDOW ||
!timestamps))
ret = false; ret = false;
else else
ret = true; ret = true;
......
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