Limit the kex deferred packet queue length - #454
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During a key exchange, non-transport sent packets are enqueued since they are not allowed to be sent. The queue is theoretically unbounded, which could cause unbounded memory use. In normal operation not many packets would be expected from a peer before a reply KEXINIT, but a coincidental flood of channel open requests from a client at the same instant as a rekey begins could cause numerous replies. Perhaps that could happen with hectic SOCKS forwarding, so the limit is set to MAX_CHANNELS. Only reply packets would be enqueue, all of those are relatively small. The rekey KEX can't be directly triggered by a malicious peer, it has to wait for a rekey KEXINIT to be sent by Dropbear (client or server), after 1GB of data or 8 hours. It can only occur post-auth, none of the allowed pre-auth packets can trigger an enqueued reply. Memory is used by the session dropbear process, so the OOM killer would usually sort it out.
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During a key exchange, non-transport sent packets are enqueued since they are not allowed to be sent. The queue is theoretically unbounded, which could cause unbounded memory use.
In normal operation not many packets would be expected from a peer before a reply KEXINIT, but a coincidental flood of channel open requests from a client at the same instant as a rekey begins could cause numerous replies. Perhaps that could happen with hectic SOCKS forwarding, so the limit is set to MAX_CHANNELS.
Only reply packets would be enqueue, all of those are relatively small.
The rekey KEX can't be directly triggered by a malicious peer, it has to wait for a rekey KEXINIT to be sent by Dropbear (client or server), after 1GB of data or 8 hours. It can only occur post-auth, none of the allowed pre-auth packets can trigger an enqueued reply. Memory is used by the session dropbear process, so the OOM killer would usually sort it out.