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tcp: implement RFC 5961 4.2

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Implement the RFC 5691 mitigation against Blind
Reset attack using SYN bit.

Section 4.2 of RFC 5961 advises to send a Challenge ACK and drop
incoming packet, instead of resetting the session.

Add a new SNMP counter to count number of challenge acks sent
in response to SYN packets.
(netstat -s | grep TCPSYNChallenge)

Remove obsolete TCPAbortOnSyn, since we no longer abort a TCP session
because of a SYN flag.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kiran Kumar Kella <kkiran@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2012-07-17 01:41:30 +00:00 • committed by David S. Miller
commit 0c24604b68
3 changed files with 17 additions and 19 deletions
Download patch file Download diff file Expand all files Collapse all files

2
include/linux/snmp.h
View file

@ -208,7 +208,6 @@ enum
LINUX_MIB_TCPDSACKOFOSENT, /* TCPDSACKOfoSent */
LINUX_MIB_TCPDSACKRECV, /* TCPDSACKRecv */
LINUX_MIB_TCPDSACKOFORECV, /* TCPDSACKOfoRecv */
LINUX_MIB_TCPABORTONSYN, /* TCPAbortOnSyn */
LINUX_MIB_TCPABORTONDATA, /* TCPAbortOnData */
LINUX_MIB_TCPABORTONCLOSE, /* TCPAbortOnClose */
LINUX_MIB_TCPABORTONMEMORY, /* TCPAbortOnMemory */
@ -238,6 +237,7 @@ enum
LINUX_MIB_TCPOFODROP, /* TCPOFODrop */
LINUX_MIB_TCPOFOMERGE, /* TCPOFOMerge */
LINUX_MIB_TCPCHALLENGEACK, /* TCPChallengeACK */
LINUX_MIB_TCPSYNCHALLENGE, /* TCPSYNChallenge */
__LINUX_MIB_MAX
};

2
net/ipv4/proc.c
View file

@ -232,7 +232,6 @@ static const struct snmp_mib snmp4_net_list[] = {
SNMP_MIB_ITEM("TCPDSACKOfoSent", LINUX_MIB_TCPDSACKOFOSENT),
SNMP_MIB_ITEM("TCPDSACKRecv", LINUX_MIB_TCPDSACKRECV),
SNMP_MIB_ITEM("TCPDSACKOfoRecv", LINUX_MIB_TCPDSACKOFORECV),
SNMP_MIB_ITEM("TCPAbortOnSyn", LINUX_MIB_TCPABORTONSYN),
SNMP_MIB_ITEM("TCPAbortOnData", LINUX_MIB_TCPABORTONDATA),
SNMP_MIB_ITEM("TCPAbortOnClose", LINUX_MIB_TCPABORTONCLOSE),
SNMP_MIB_ITEM("TCPAbortOnMemory", LINUX_MIB_TCPABORTONMEMORY),
@ -262,6 +261,7 @@ static const struct snmp_mib snmp4_net_list[] = {
SNMP_MIB_ITEM("TCPOFODrop", LINUX_MIB_TCPOFODROP),
SNMP_MIB_ITEM("TCPOFOMerge", LINUX_MIB_TCPOFOMERGE),
SNMP_MIB_ITEM("TCPChallengeACK", LINUX_MIB_TCPCHALLENGEACK),
SNMP_MIB_ITEM("TCPSYNChallenge", LINUX_MIB_TCPSYNCHALLENGE),
SNMP_MIB_SENTINEL
};

30
net/ipv4/tcp_input.c
View file

@ -5270,7 +5270,7 @@ static void tcp_send_challenge_ack(struct sock *sk)
/* Does PAWS and seqno based validation of an incoming segment, flags will
* play significant role here.
*/
static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
const struct tcphdr *th, int syn_inerr)
{
const u8 *hash_location;
@ -5323,20 +5323,22 @@ static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
/* step 3: check security and precedence [ignored] */
/* step 4: Check for a SYN in window. */
if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
/* step 4: Check for a SYN
* RFC 5691 4.2 : Send a challenge ack
*/
if (th->syn) {
if (syn_inerr)
TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN);
tcp_reset(sk);
return -1;
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
tcp_send_challenge_ack(sk);
goto discard;
}
return 1;
return true;
discard:
__kfree_skb(skb);
return 0;
return false;
}
/*
@ -5366,7 +5368,6 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
const struct tcphdr *th, unsigned int len)
{
struct tcp_sock *tp = tcp_sk(sk);
int res;
if (sk->sk_rx_dst) {
struct dst_entry *dst = sk->sk_rx_dst;
@ -5555,9 +5556,8 @@ slow_path:
* Standard slow path.
*/
res = tcp_validate_incoming(sk, skb, th, 1);
if (res <= 0)
return -res;
if (!tcp_validate_incoming(sk, skb, th, 1))
return 0;
step5:
if (th->ack && tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
@ -5877,7 +5877,6 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
struct tcp_sock *tp = tcp_sk(sk);
struct inet_connection_sock *icsk = inet_csk(sk);
int queued = 0;
int res;
tp->rx_opt.saw_tstamp = 0;
@ -5932,9 +5931,8 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
return 0;
}
res = tcp_validate_incoming(sk, skb, th, 0);
if (res <= 0)
return -res;
if (!tcp_validate_incoming(sk, skb, th, 0))
return 0;
/* step 5: check the ACK field */
if (th->ack) {