In the Linux kernel, the following vulnerability has been resolved: futex/pi: Plug private futex...

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

futex/pi: Plug private futex exec() race

The check for private futexes whether the waiter's mm, which is stored in the futex_key and copied into the pi_state, is the same as the owner's mm is not sufficient for exec(). exec() has a gap where the mm check fails to give the correct answer:

exec() ... exec_release_mm() futex_exec_release() tsk::futex::exit_state = EXITING; cleanup_robust_list();

1.  tsk::futex::exit_state = OK;
   

   ... old_mm = tsk::mm;
2. tsk::mm = ->mm;

Between #1 and #2 the check for the mm is wrong as that mm is about to be swapped out and eventually freed.

Plug this gap by:

1. Setting tsk::futex::exit_state to FUTEX_STATE_DEAD in futex_exec_release()

2. Setting tsk::futex::exit_state to FUTEX_STATE_OK after the mm has been switched.

From a futex point of view the task is dead after it finished the robust list cleanup up to the point where it sets the state to OK again.