In the Linux kernel, the following vulnerability has been resolved: posix-cpu-timers: Prevent...

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.