a bug detected in dropbear v071

[Thomas De Schampheleire]

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