Re: [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still_valid() hack

From: Hugh Dickins <hughd@google.com>
To: Jann Horn <jannh@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still_valid() hack
Date: Sun, 30 Aug 2020 23:06:47 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.2008302225510.1934@eggly.anvils> (raw)
In-Reply-To: <20200827114932.3572699-8-jannh@google.com>

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