kernel/fork.c: detect early free of a live mm

KASAN splats indicate that in some cases we free a live mm, then
continue to access it, with potentially disastrous results.  This is
likely due to a mismatched mmdrop() somewhere in the kernel, but so far
the culprit remains elusive.

Let's have __mmdrop() verify that the mm isn't live for the current
task, similar to the existing check for init_mm.  This way, we can catch
this class of issue earlier, and without requiring KASAN.

Currently, idle_task_exit() leaves active_mm stale after it switches to
init_mm.  This isn't harmful, but will trigger the new assertions, so we
must adjust idle_task_exit() to update active_mm.

Link: http://lkml.kernel.org/r/20180312140103.19235-1-mark.rutland@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -595,6 +595,8 @@ static void check_mm(struct mm_struct *mm)
 void __mmdrop(struct mm_struct *mm)
 {
 	BUG_ON(mm == &init_mm);
+	WARN_ON_ONCE(mm == current->mm);
+	WARN_ON_ONCE(mm == current->active_mm);
 	mm_free_pgd(mm);
 	destroy_context(mm);
 	hmm_mm_destroy(mm);

kernel/sched/core.c

@@ -5560,6 +5560,7 @@ void idle_task_exit(void)
 
 	if (mm != &init_mm) {
 		switch_mm(mm, &init_mm, current);
+		current->active_mm = &init_mm;
 		finish_arch_post_lock_switch();
 	}
 	mmdrop(mm);