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tcp: tighten up checks for received SYN

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Any malicous segment could contain a SYN up to now (no check).
A SYN in the wrong segment could break OOSEQ queueing.
Fix this by allowing SYN only in states where it is required.

See bug #56397: Assert "tcp_receive: ooseq tcplen > rcv_wnd"

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>
This commit is contained in:
Simon Goldschmidt 2020-03-05 21:20:35 +01:00
parent a02f34a193
commit adbc5b5f71
1 changed files with 13 additions and 4 deletions
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17
src/core/tcp_in.c
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@ -852,6 +852,13 @@ tcp_process(struct tcp_pcb *pcb)
tcp_parseopt(pcb);
if (flags & TCP_SYN) {
/* accept SYN only in 2 states: */
if ((pcb->state != SYN_SENT) && (pcb->state != SYN_RCVD)) {
return ERR_OK;
}
}
/* Do different things depending on the TCP state. */
switch (pcb->state) {
case SYN_SENT:
@ -924,7 +931,12 @@ tcp_process(struct tcp_pcb *pcb)
}
break;
case SYN_RCVD:
if (flags & TCP_ACK) {
if (flags & TCP_SYN) {
if (seqno == pcb->rcv_nxt - 1) {
/* Looks like another copy of the SYN - retransmit our SYN-ACK */
tcp_rexmit(pcb);
}
} else if (flags & TCP_ACK) {
/* expected ACK number? */
if (TCP_SEQ_BETWEEN(ackno, pcb->lastack + 1, pcb->snd_nxt)) {
pcb->state = ESTABLISHED;
@ -975,9 +987,6 @@ tcp_process(struct tcp_pcb *pcb)
tcp_rst(pcb, ackno, seqno + tcplen, ip_current_dest_addr(),
ip_current_src_addr(), tcphdr->dest, tcphdr->src);
}
} else if ((flags & TCP_SYN) && (seqno == pcb->rcv_nxt - 1)) {
/* Looks like another copy of the SYN - retransmit our SYN-ACK */
tcp_rexmit(pcb);
}
break;
case CLOSE_WAIT: