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authorEric Dumazet <edumazet@google.com>2019-06-06 09:15:31 -0700
committerDavid S. Miller <davem@davemloft.net>2019-06-15 18:47:31 -0700
commit5f3e2bf008c2221478101ee72f5cb4654b9fc363 (patch)
tree7be18a95aa24dc68564ab7d84bb158468efb8a09
parenttcp: tcp_fragment() should apply sane memory limits (diff)
downloadlinux-dev-5f3e2bf008c2221478101ee72f5cb4654b9fc363.tar.xz
linux-dev-5f3e2bf008c2221478101ee72f5cb4654b9fc363.zip
tcp: add tcp_min_snd_mss sysctl
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>
-rw-r--r--Documentation/networking/ip-sysctl.txt8
-rw-r--r--include/net/netns/ipv4.h1
-rw-r--r--net/ipv4/sysctl_net_ipv4.c11
-rw-r--r--net/ipv4/tcp_ipv4.c1
-rw-r--r--net/ipv4/tcp_output.c3
5 files changed, 22 insertions, 2 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 288aa264ac26..22f6b8b1110a 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -255,6 +255,14 @@ 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 7698460a3dd1..623cfbb7b8dc 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -117,6 +117,7 @@ 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 08a428a7b274..9e5257251163 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -39,6 +39,8 @@ 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_privileged_port_min;
static int ip_privileged_port_max = 65535;
static int ip_ttl_min = 1;
@@ -775,6 +777,15 @@ static struct ctl_table ipv4_net_table[] = {
.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,
.maxlen = sizeof(int),
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index bc86f9735f45..cfa81190a1b1 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2628,6 +2628,7 @@ 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 1bb1c46b4aba..00c01a01b547 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1459,8 +1459,7 @@ 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;
}