In the Linux kernel, the following vulnerability has been resolved: posix-cpu-timers: Prevent freeing a timer which is queued on the expiry

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:

 1expiry handler              timer_settime()        timer_delete()
 2--------------              ---------------        --------------
 3collect timer onto
 4private firing list
 5firing = true
 6                            observes firing = true
 7                            firing = false
 8                            return TIMER_RETRY
 9                            wait for handler
10                                                   observes firing = false
11                                                   finish deletion
12                                                   unhash and free timer
13resume list traversal
14read freed elist.next
15-> 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.