From 0c228e833c88e3aa029250f5db77d5968c5ce5b5 Mon Sep 17 00:00:00 2001 From: Calvin Owens Date: Thu, 20 Nov 2014 15:09:53 -0800 Subject: tcp: Restore RFC5961-compliant behavior for SYN packets Commit c3ae62af8e755 ("tcp: should drop incoming frames without ACK flag set") was created to mitigate a security vulnerability in which a local attacker is able to inject data into locally-opened sockets by using TCP protocol statistics in procfs to quickly find the correct sequence number. This broke the RFC5961 requirement to send a challenge ACK in response to spurious RST packets, which was subsequently fixed by commit 7b514a886ba50 ("tcp: accept RST without ACK flag"). Unfortunately, the RFC5961 requirement that spurious SYN packets be handled in a similar manner remains broken. RFC5961 section 4 states that: ... the handling of the SYN in the synchronized state SHOULD be performed as follows: 1) If the SYN bit is set, irrespective of the sequence number, TCP MUST send an ACK (also referred to as challenge ACK) to the remote peer: After sending the acknowledgment, TCP MUST drop the unacceptable segment and stop processing further. By sending an ACK, the remote peer is challenged to confirm the loss of the previous connection and the request to start a new connection. A legitimate peer, after restart, would not have a TCB in the synchronized state. Thus, when the ACK arrives, the peer should send a RST segment back with the sequence number derived from the ACK field that caused the RST. This RST will confirm that the remote peer has indeed closed the previous connection. Upon receipt of a valid RST, the local TCP endpoint MUST terminate its connection. The local TCP endpoint should then rely on SYN retransmission from the remote end to re-establish the connection. This patch lets SYN packets through the discard added in c3ae62af8e755, so that spurious SYN packets are properly dealt with as per the RFC. The challenge ACK is sent unconditionally and is rate-limited, so the original vulnerability is not reintroduced by this patch. Signed-off-by: Calvin Owens Acked-by: Eric Dumazet Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'net') diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 88fa2d160685..d107ee246a1d 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5231,7 +5231,7 @@ slow_path: if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb)) goto csum_error; - if (!th->ack && !th->rst) + if (!th->ack && !th->rst && !th->syn) goto discard; /* @@ -5650,7 +5650,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb, goto discard; } - if (!th->ack && !th->rst) + if (!th->ack && !th->rst && !th->syn) goto discard; if (!tcp_validate_incoming(sk, skb, th, 0)) -- cgit v1.2.1