a bug detected in dropbear v071

[ZHANG Hui P]

Hi ,
         I have verified this commit, it works well. When can we got a formal  release includes this commit?
thanks.

From: Matt Johnston [mailto:matt at ucc.asn.au]
Sent: 2016年4月29日 23:18
To: ZHANG Hui P
Cc: dropbear at ucc.asn.au
Subject: Re: a bug detected in dropbear v071

Hi,

I think this problem should be solved by the commit
https://secure.ucc.asn.au/hg/dropbear/rev/432b0a030fd6

Thank you for the detailed report.

Cheers,
Matt


On Wed 20/4/2016, at 2:44 pm, ZHANG Hui P <Hui.P.Zhang at alcatel-sbell.com.cn<mailto:Hui.P.Zhang at alcatel-sbell.com.cn>> wrote:

Hi:
         I am a software engineer of Alcatel-Lucent. In our product we use dropbear v071 under the OS: Linux version 3.4.24. At most time it works perfectly, but recently we got a problem: sometimes a child-process of dropbear occupied nearly 100% CPU (we use ARM1176, single-core). After I investigated it ,I found it is cause by a misuse of KEX_REKEY_TIMEOUT.
KEX_REKEY_TIMEOUT is defined as 8hours. that means when a session lasts more than 8 hours, the server and client will re-exchange their KEY for security reason. The timestamp of last-time KEY-EXCHANGED is recorded in variable "ses.kexstate.lastkextime".
 The child dropbear process decides the "timeout" parameter of "select" function by calling "select_timeout". we can see it checks the timeout-events like KEX_REKEY_TIMEOUT, AUTH_TIMEOUT, keepalive_secs. If there is a timeout occurs, the "update_timeout" function returns a negative value, then "select_timeout" modifies it to ZREO by this:
/* clamp negative timeouts to zero - event has already triggered */
         return MAX(timeout, 0);
   if "select_timeout" returns ZERO, the next "select" call (in "session_loop") will return immediately. Then it will check timeout events by this:
/* check for auth timeout, rekeying required etc */
                   checktimeouts();
   in the function " checktimeouts ", when it find the timeout is reached or to many data has been sent, it will send a SSH_MSG_KEXINIT message to peer. Normally this message will trigger a new KEY-EXCHANGE. However, when there is a network problem that the peer can't receive the message , this bug occurs: the timestamp ses.kexstate.lastkextime is only updated by calling  "switch_keys"-->" kexinitialise ", unfortunately this calling sequence is driven by ssh-messages, either SSH_MSG_KEXDH_INIT or SSH_MSG_NEWKEYS. When there is no ssh-message received , the child dropbear process enters dead-loop "select" with ZERO-timeout parameter caused by KEX_REKEY_TIMEOUT.
>      So there is a very simple way to reproduce this bug: first define the KEX_REKEY_TIMEOUT as small as possible( I set it to 8 seconds), then start a ssh-session , the child dropbear process is forked. then plug out the network wire, after 8 seconds the child dropbear thread will occupy 100% CPU. Could you kindly check it? thanks.

Best regards

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