Linux: posix-cpu-timers: Prevent freeing a timer which is queued on the expiry list

In the Linux kernel, the following vulnerability has been resolved:

posix-cpu-timers: Prevent freeing a timer which is queued on the expiry list

Kijo analyzed another race in the POSIX CPU timer code:

Commit bf635681c906 converted cpu_timer::firing from a tristate value to a boolean. This lost the distinction between "not owned by the firing list" and "still owned, but delivery was canceled". The resulting race is:

expiry handler timer_settime() timer_delete() -------------- --------------- -------------- collect timer onto private firing list firing = true observes firing = true firing = false return TIMER_RETRY wait for handler observes firing = false finish deletion unhash and free timer resume list traversal read freed elist.next -> UAF

The firing bit is clearly the wrong indicator since that commit.

Check whether the timer is queued on the expiry list or not instead. If it is queued clear the firing bit to prevent signal delivery as before and return TIMER_RETRY so the caller unlocks the timer which allows the expiry code to make progress and remove it from the list.