linux-kernel - Re: [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still

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Date:   Sun, 30 Aug 2020 23:06:47 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Jann Horn <jannh@...gle.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still_valid()
 hack

On Thu, 27 Aug 2020, Jann Horn wrote:

> The preceding patches have ensured that core dumping properly takes the
> mmap_lock. Thanks to that, we can now remove mmget_still_valid() and all
> its users.

Hi Jann, while the only tears to be shed over losing mmget_still_valid()
will be tears of joy, I think you need to explain why you believe it's
safe to remove the instance in mm/khugepaged.c: which you'll have found
I moved just recently, to cover an extra case (sorry for not Cc'ing you).

> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
>  
>  static inline int khugepaged_test_exit(struct mm_struct *mm)
>  {
> -	return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm);
> +	return atomic_read(&mm->mm_users) == 0;
>  }
>  
>  static bool hugepage_vma_check(struct vm_area_struct *vma,

The movement (which you have correctly followed) was in
bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
but the "pmd .. physical page 0" issue is explained better in its parent
18e77600f7a1 ("khugepaged: retract_page_tables() remember to test exit")

I think your core dumping is still reading the page tables without
holding mmap_lock, so still vulnerable to that extra issue.  It won't
be as satisfying as removing all traces of mmget_still_valid(), but
right now I think you should add an mm->core_state check there instead.

(I do have a better solution in use, but it's a much larger patch, that
will take a lot more effort to get in: checks in pte_offset_map_lock(),
perhaps returning NULL when pmd is transitioning, requiring retry.)

Or maybe it's me who has missed what you're doing instead.

Hugh