It is unexpected and not allowed to call TTY buffer helpers like ttyinsertflipstring concurrently. This may lead to crashes when ECHOing is enabled and concurrect writers call ptywrite in the meantime. In that case the two writers: the ECHOing from a workqueue and ptywrite from the process race and can overflow the corresponding TTY buffer.

An unprivileged local user could use this flaw to crash the system or, potentially, escalate their privileges on the system.

References: http://seclists.org/oss-sec/2014/q2/243