a bug detected in dropbear v071

[Matt Johnston]

Hi Thomas,

Hui's analysis look right, I'll try and test it myself later this week. (Sorry, replied privately).

Cheers,
Matt

On 25 April 2016 11:15:58 pm AWST, Thomas De Schampheleire <patrickdepinguin at gmail.com> wrote:
>ZHANG Hui P <Hui.P.Zhang <at> alcatel-sbell.com.cn> writes:
>
>> 
>> 
>> 
>> 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.
>>  
>
>Any feedback regarding this reported issue?
>
>Thanks,
>Thomas