futex: Handle user space corruption gracefully

If the owner of a PI futex dies we fix up the pi_state and set
pi_state->owner to NULL. When a malicious or just sloppy programmed
user space application sets the futex value to 0 e.g. by calling
pthread_mutex_init(), then the futex can be acquired again. A new
waiter manages to enqueue itself on the pi_state w/o damage, but on
unlock the kernel dereferences pi_state->owner and oopses.

Prevent this by checking pi_state->owner in the unlock path. If
pi_state->owner is not current we know that user space manipulated the
futex value. Ignore the mess and return -EINVAL.

This catches the above case and also the case where a task hijacks the
futex by setting the tid value and then tries to unlock it.

Reported-by: Jermome Marchand <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Darren Hart <[e-mail]>
Acked-by: Peter Zijlstra <[e-mail]>
Cc: <[e-mail]>

kernel/futex.c

···
 	if (!pi_state)
 		return -EINVAL;
 
+	/*
+	 * If current does not own the pi_state then the futex is
+	 * inconsistent and user space fiddled with the futex value.
+	 */
+	if (pi_state->owner != current)
+		return -EINVAL;
+
 	raw_spin_lock(&pi_state->pi_mutex.wait_lock);
 	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);