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

Summary

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.

Timeline

1. Oct 6, 2026, 8:45 AM UTCCVE published
2. Oct 19, 2026, 9:16 AM UTCRecord last updated

Affected versions

• Linux Linux

  From bf635681c906ad056d1fda325de8d1c12c9f8201 · fixed in 9971dac1a77845ff467146915fcbb9170f6a8209; From bf635681c906ad056d1fda325de8d1c12c9f8201 · fixed in 4336e3f47d9d516441066e9a65eaf076790d8d45; From bf635681c906ad056d1fda325de8d1c12c9f8201 · fixed in c21eaa72f02fc6e85621cbe09d303d8fb8bd39cd

• Linux Linux

  6.13

• KernelLinux

  From 6.13.0 · fixed in 6.18.54; From 6.19.0 · fixed in 7.2.8

References 3