Commit 3eda69c9 authored by Mark Rutland's avatar Mark Rutland Committed by Linus Torvalds

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: default avatarMark Rutland <>
Reviewed-by: default avatarAndrew Morton <>
Cc: Ingo Molnar <>
Cc: Mathieu Desnoyers <>
Cc: Michal Hocko <>
Cc: Peter Zijlstra <>
Cc: Rik van Riel <>
Cc: Will Deacon <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 0c7c1bed
...@@ -595,6 +595,8 @@ static void check_mm(struct mm_struct *mm) ...@@ -595,6 +595,8 @@ static void check_mm(struct mm_struct *mm)
void __mmdrop(struct mm_struct *mm) void __mmdrop(struct mm_struct *mm)
{ {
BUG_ON(mm == &init_mm); BUG_ON(mm == &init_mm);
WARN_ON_ONCE(mm == current->mm);
WARN_ON_ONCE(mm == current->active_mm);
mm_free_pgd(mm); mm_free_pgd(mm);
destroy_context(mm); destroy_context(mm);
hmm_mm_destroy(mm); hmm_mm_destroy(mm);
...@@ -5560,6 +5560,7 @@ void idle_task_exit(void) ...@@ -5560,6 +5560,7 @@ void idle_task_exit(void)
if (mm != &init_mm) { if (mm != &init_mm) {
switch_mm(mm, &init_mm, current); switch_mm(mm, &init_mm, current);
current->active_mm = &init_mm;
finish_arch_post_lock_switch(); finish_arch_post_lock_switch();
} }
mmdrop(mm); mmdrop(mm);
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