Don't unset validkey when receiving SPTPS handshakes over ANS_KEY.

This fixes a hairy race condition that was introduced in
1e89a63f1638e43dee79afbb18d5f733b27d830b, which changed
the underlying transport of handshake packets from REQ_KEY to ANS_KEY.
Unfortunately, what I missed in that commit is, on the receiving side,
there is a slight difference between req_key_h() and ans_key_h():
indeed, the latter resets validkey to false.

The reason why this is not a problem during typical operation is
because the normal SPTPS key regeneration procedure looks like this:

    KEX ->
    <- KEX
    SIG ->
    <- SIG

All these messages are sent over ANS_KEY, therefore the receiving side
will unset validkey. However, that's typically not a problem in practice
because upon reception of the last message (SIG), SPTPS will call
sptps_receive_record(), which will set validkey to true again, and
everything works out fine in the end.

However, that was the *typical* scenario. Now let's assume that the
SPTPS channel is in active use at the same time key regeneration
happens. Specifically, let's assume a normal VPN data packet sneaks in
during the key regeneration procedure:

    KEX ->
    <- KEX
    <- (SPTPS packet, over TCP or UDP)
    <- KEX (wtf?)
    SIG -> (refused with Invalid packet seqno: XXX != 0)

At this point, both nodes are extremely confused and the SPTPS channel
becomes unusable with various errors being thrown on both sides. The
channel will stay down until automatic SPTPS channel restart kicks in
after 10 seconds.

(Note: the above is just an example - the race can occur on either side
whenever a packet is sent during the period of time between KEX and SIG
messages are received by the node sending the packet.)

I've seen this race occur in the wild - it is very likely to occur if
key regeneration occurs on a heavily loaded channel. It can be
reproduced fairly easily by setting KeyExpire to a short value (a few
seconds) and then running something like ping -f foobar -i 0.01.

The reason why this occurs is because tinc's TX code path triggers the
following:

 - send_packet()
 - try_tx()
 - try_tx_sptps()
 - validkey is false because we just received an ANS_KEY message
 - waitingforkey is false because it's not used for key regeneration
 - send_req_key()
 - SPTPS channel restart (sptps_stop(), sptps_start()).

Obviously, it all goes downhill from there and the two nodes get very
confused quickly (for example the seqno gets reset, hence the error
messages).

This commit fixes the issue by keeping validkey set when SPTPS data is
received over ANS_KEY messages.

diff --git a/src/protocol_key.c b/src/protocol_key.c
index b409a35..dd54704 100644 (file)
--- a/src/protocol_key.c
+++ b/src/protocol_key.c
@@ -418,7 +418,7 @@ bool ans_key_h(connection_t *c, const char *request) {
        cipher_close(from->outcipher);
        digest_close(from->outdigest);
 #endif
-       from->status.validkey = false;
+       if (!from->status.sptps) from->status.validkey = false;
 
        if(compression < 0 || compression > 11) {
                logger(DEBUG_ALWAYS, LOG_ERR, "Node %s (%s) uses bogus compression level!", from->name, from->hostname);