tcp: add tcp_min_snd_mss sysctl

commit 5f3e2bf008c2221478101ee72f5cb4654b9fc363 upstream.

Some TCP peers announce a very small MSS option in their SYN and/or
SYN/ACK messages.

This forces the stack to send packets with a very high network/cpu
overhead.

Linux has enforced a minimal value of 48. Since this value includes
the size of TCP options, and that the options can consume up to 40
bytes, this means that each segment can include only 8 bytes of payload.

In some cases, it can be useful to increase the minimal value
to a saner value.

We still let the default to 48 (TCP_MIN_SND_MSS), for compatibility
reasons.

Note that TCP_MAXSEG socket option enforces a minimal value
of (TCP_MIN_MSS). David Miller increased this minimal value
in commit c39508d6f118 ("tcp: Make TCP_MAXSEG minimum more correct.")
from 64 to 88.

We might in the future merge TCP_MIN_SND_MSS and TCP_MIN_MSS.

CVE-2019-11479 -- tcp mss hardcoded to 48

Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Jonathan Looney <jtl@netflix.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Tyler Hicks <tyhicks@canonical.com>
Cc: Bruce Curtis <brucec@netflix.com>
Cc: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 0335285f3918991e7a8181ab296a44b95c77a596..49935d5bb5c6bd481892aa44db336d45f6d47be3 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -230,6 +230,14 @@ tcp_base_mss - INTEGER
        Path MTU discovery (MTU probing).  If MTU probing is enabled,
        this is the initial MSS used by the connection.
 
+tcp_min_snd_mss - INTEGER
+       TCP SYN and SYNACK messages usually advertise an ADVMSS option,
+       as described in RFC 1122 and RFC 6691.
+       If this ADVMSS option is smaller than tcp_min_snd_mss,
+       it is silently capped to tcp_min_snd_mss.
+
+       Default : 48 (at least 8 bytes of payload per segment)
+
 tcp_congestion_control - STRING
        Set the congestion control algorithm to be used for new
        connections. The algorithm "reno" is always available, but

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 7adf4386ac8fa644cf41198ef028c369dcbe02e0..bf619a67ec0317e6a68c2d19a2f6caa889493cf8 100644 (file)
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -94,6 +94,7 @@ struct netns_ipv4 {
 #endif
        int sysctl_tcp_mtu_probing;
        int sysctl_tcp_base_mss;
+       int sysctl_tcp_min_snd_mss;
        int sysctl_tcp_probe_threshold;
        u32 sysctl_tcp_probe_interval;
 

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 85713adf2770af292cb6733285a3e11bf250b82f..e202babb14d63ce6f3e85bc8fc065a32756fbe05 100644 (file)
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -35,6 +35,8 @@ 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;
 static int tcp_adv_win_scale_max = 31;
+static int tcp_min_snd_mss_min = TCP_MIN_SND_MSS;
+static int tcp_min_snd_mss_max = 65535;
 static int ip_ttl_min = 1;
 static int ip_ttl_max = 255;
 static int tcp_syn_retries_min = 1;
@@ -838,6 +840,15 @@ static struct ctl_table ipv4_net_table[] = {
                .mode           = 0644,
                .proc_handler   = proc_dointvec,
        },
+       {
+               .procname       = "tcp_min_snd_mss",
+               .data           = &init_net.ipv4.sysctl_tcp_min_snd_mss,
+               .maxlen         = sizeof(int),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = &tcp_min_snd_mss_min,
+               .extra2         = &tcp_min_snd_mss_max,
+       },
        {
                .procname       = "tcp_probe_threshold",
                .data           = &init_net.ipv4.sysctl_tcp_probe_threshold,

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 82c1064ff4aa28e3085c23dde2e8a177ae2e2b67..848f2c1da8a5f4a38c22e1f46649df89d7117957 100644 (file)
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2456,6 +2456,7 @@ static int __net_init tcp_sk_init(struct net *net)
        net->ipv4.sysctl_tcp_ecn_fallback = 1;
 
        net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS;
+       net->ipv4.sysctl_tcp_min_snd_mss = TCP_MIN_SND_MSS;
        net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD;
        net->ipv4.sysctl_tcp_probe_interval = TCP_PROBE_INTERVAL;
 

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 123b2d8fde464b14e95d7e89af7b06696243ad15..d8c6b833f0ce61d36db02b98a896271812567fba 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1360,8 +1360,7 @@ static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
        mss_now -= icsk->icsk_ext_hdr_len;
 
        /* Then reserve room for full set of TCP options and 8 bytes of data */
-       if (mss_now < TCP_MIN_SND_MSS)
-               mss_now = TCP_MIN_SND_MSS;
+       mss_now = max(mss_now, sock_net(sk)->ipv4.sysctl_tcp_min_snd_mss);
        return mss_now;
 }