From: Thomas Gleixner tglx@linutronix.de
commit af8cbda2cfcaa5515d61ec500498d46e9a8247e2 upstream.
exec() attempts to handle potentially held futexes gracefully by running the futex exit handling code like exit() does.
The current implementation has no protection against concurrent incoming waiters. The reason is that the futex state cannot be set to FUTEX_STATE_DEAD after the cleanup because the task struct is still active and just about to execute the new binary.
While its arguably buggy when a task holds a futex over exec(), for consistency sake the state handling can at least cover the actual futex exit cleanup section. This provides state consistency protection accross the cleanup. As the futex state of the task becomes FUTEX_STATE_OK after the cleanup has been finished, this cannot prevent subsequent attempts to attach to the task in case that the cleanup was not successfull in mopping up all leftovers.
Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Ingo Molnar mingo@kernel.org Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/20191106224556.753355618@linutronix.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/futex.c | 38 ++++++++++++++++++++++++++++++++++---- 1 file changed, 34 insertions(+), 4 deletions(-)
--- a/kernel/futex.c +++ b/kernel/futex.c @@ -3661,7 +3661,7 @@ static void exit_robust_list(struct task } }
-void futex_exec_release(struct task_struct *tsk) +static void futex_cleanup(struct task_struct *tsk) { if (unlikely(tsk->robust_list)) { exit_robust_list(tsk); @@ -3701,7 +3701,7 @@ void futex_exit_recursive(struct task_st tsk->futex_state = FUTEX_STATE_DEAD; }
-void futex_exit_release(struct task_struct *tsk) +static void futex_cleanup_begin(struct task_struct *tsk) { /* * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock. @@ -3717,10 +3717,40 @@ void futex_exit_release(struct task_stru raw_spin_lock_irq(&tsk->pi_lock); tsk->futex_state = FUTEX_STATE_EXITING; raw_spin_unlock_irq(&tsk->pi_lock); +}
- futex_exec_release(tsk); +static void futex_cleanup_end(struct task_struct *tsk, int state) +{ + /* + * Lockless store. The only side effect is that an observer might + * take another loop until it becomes visible. + */ + tsk->futex_state = state; +}
- tsk->futex_state = FUTEX_STATE_DEAD; +void futex_exec_release(struct task_struct *tsk) +{ + /* + * The state handling is done for consistency, but in the case of + * exec() there is no way to prevent futher damage as the PID stays + * the same. But for the unlikely and arguably buggy case that a + * futex is held on exec(), this provides at least as much state + * consistency protection which is possible. + */ + futex_cleanup_begin(tsk); + futex_cleanup(tsk); + /* + * Reset the state to FUTEX_STATE_OK. The task is alive and about + * exec a new binary. + */ + futex_cleanup_end(tsk, FUTEX_STATE_OK); +} + +void futex_exit_release(struct task_struct *tsk) +{ + futex_cleanup_begin(tsk); + futex_cleanup(tsk); + futex_cleanup_end(tsk, FUTEX_STATE_DEAD); }
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,