Re: [PATCH net 2/4] tcp: tcp_fragment() should apply sane memory limits

Jonathan Looney reported that a malicious peer can force a sender
to fragment its retransmit queue into tiny skbs, inflating memory
usage and/or overflow 32bit counters.

TCP allows an application to queue up to sk_sndbuf bytes,
so we need to give some allowance for non malicious splitting
of retransmit queue.

A new SNMP counter is added to monitor how many times TCP
did not allow to split an skb if the allowance was exceeded.

Note that this counter might increase in the case applications
use SO_SNDBUF socket option to lower sk_sndbuf.

CVE-2019-11478 : tcp_fragment, prevent fragmenting a packet when the
        socket is already using more than half the allowed space

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jonathan Looney <redacted>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <redacted>
Reviewed-by: Tyler Hicks <redacted>
Cc: Bruce Curtis <redacted>
Cc: Jonathan Lemon <redacted>
---
 include/uapi/linux/snmp.h | 1 +
 net/ipv4/proc.c           | 1 +
 net/ipv4/tcp_output.c     | 5 +++++
 3 files changed, 7 insertions(+)

diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
index 86dc24a96c90ab047d5173d625450facd6c6dd79..fd42c1316d3d112ecd8a00d2b499d6f6901c5e81 100644
--- a/include/uapi/linux/snmp.h
+++ b/include/uapi/linux/snmp.h

@@ -283,6 +283,7 @@ enum
        LINUX_MIB_TCPACKCOMPRESSED,             /* TCPAckCompressed */
        LINUX_MIB_TCPZEROWINDOWDROP,            /* TCPZeroWindowDrop */
        LINUX_MIB_TCPRCVQDROP,                  /* TCPRcvQDrop */
+       LINUX_MIB_TCPWQUEUETOOBIG,              /* TCPWqueueTooBig */
        __LINUX_MIB_MAX
 };

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 4370f4246e86dfe06a9e07cace848baeaf6cc4da..073273b751f8fcda1c9c79cd1ab566f2939b2517 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c

@@ -287,6 +287,7 @@ static const struct snmp_mib snmp4_net_list[] = {
        SNMP_MIB_ITEM("TCPAckCompressed", LINUX_MIB_TCPACKCOMPRESSED),
        SNMP_MIB_ITEM("TCPZeroWindowDrop", LINUX_MIB_TCPZEROWINDOWDROP),
        SNMP_MIB_ITEM("TCPRcvQDrop", LINUX_MIB_TCPRCVQDROP),
+       SNMP_MIB_ITEM("TCPWqueueTooBig", LINUX_MIB_TCPWQUEUETOOBIG),
        SNMP_MIB_SENTINEL
 };

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b8e3bbb852117459d131fbb41d69ae63bd251a3e..1bb1c46b4abad100622d3f101a0a3ca0a6c8e881 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c

@@ -1296,6 +1296,11 @@ int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
        if (nsize < 0)
                nsize = 0;

+       if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf)) {
+               NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
+               return -ENOMEM;
+       }
+