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
