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>
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@ -595,6 +595,8 @@ static void check_mm(struct mm_struct *mm)
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void __mmdrop(struct mm_struct *mm)
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void __mmdrop(struct mm_struct *mm)
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{
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{
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BUG_ON(mm == &init_mm);
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BUG_ON(mm == &init_mm);
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WARN_ON_ONCE(mm == current->mm);
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WARN_ON_ONCE(mm == current->active_mm);
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mm_free_pgd(mm);
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mm_free_pgd(mm);
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destroy_context(mm);
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destroy_context(mm);
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hmm_mm_destroy(mm);
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hmm_mm_destroy(mm);
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@ -5560,6 +5560,7 @@ void idle_task_exit(void)
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if (mm != &init_mm) {
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if (mm != &init_mm) {
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switch_mm(mm, &init_mm, current);
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switch_mm(mm, &init_mm, current);
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current->active_mm = &init_mm;
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finish_arch_post_lock_switch();
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finish_arch_post_lock_switch();
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}
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}
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mmdrop(mm);
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mmdrop(mm);
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