linux-kernel - Re: [RFC PATCH RESEND] mm: munlock: Prevent walking off the end of a pagetable in no-pmd configuration

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Message-ID: <5238FF3A.2070500@oracle.com>
Date:	Wed, 18 Sep 2013 09:17:46 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
CC:	Fengguang Wu <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Jörn Engel <joern@...fs.org>,
	Mel Gorman <mgorman@...e.de>,
	Michel Lespinasse <walken@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [RFC PATCH RESEND] mm: munlock: Prevent walking off the end of
 a pagetable in no-pmd configuration

On 09/17/2013 10:22 PM, Vlastimil Babka wrote:
> The function __munlock_pagevec_fill() introduced in commit 7a8010cd3
> ("mm: munlock: manual pte walk in fast path instead of follow_page_mask()")
> uses pmd_addr_end() for restricting its operation within current page table.
> This is insufficient on architectures/configurations where pmd is folded
> and pmd_addr_end() just returns the end of the full range to be walked. In
> this case, it allows pte++ to walk off the end of a page table resulting in
> unpredictable behaviour.
> 
> This patch fixes the function by using pgd_addr_end() and pud_addr_end()
> before pmd_addr_end(), which will yield correct page table boundary on all
> configurations. This is similar to what existing page walkers do when walking
> each level of the page table.
> 
> Additionaly, the patch clarifies a comment for get_locked_pte() call in the
> function.
> 
> Reported-by: Fengguang Wu <fengguang.wu@...el.com>
> Cc: Jörn Engel <joern@...fs.org>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Michel Lespinasse <walken@...gle.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/mlock.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index d638026..758c0fc 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -379,10 +379,14 @@ static unsigned long __munlock_pagevec_fill(struct pagevec *pvec,
>  
>  	/*
>  	 * Initialize pte walk starting at the already pinned page where we
> -	 * are sure that there is a pte.
> +	 * are sure that there is a pte, as it was pinned under the same
> +	 * mmap_sem write op.
>  	 */
>  	pte = get_locked_pte(vma->vm_mm, start,	&ptl);
> -	end = min(end, pmd_addr_end(start, end));
> +	/* Make sure we do not cross the page table boundary */
> +	end = pgd_addr_end(start, end);
> +	end = pud_addr_end(start, end);
> +	end = pmd_addr_end(start, end);
>  

Nitpick, how about unfolding pmd_addr_end(start, end) directly? Like:

--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -376,13 +376,14 @@ static unsigned long __munlock_pagevec_fill(struct
pagevec *pvec,
 {
        pte_t *pte;
        spinlock_t *ptl;
+       unsigned long pmd_end = (start + PMD_SIZE) & PMD_MASK;
+       end = (pmd_end - 1 < end - 1) ? pmd_end : end;

        /*
         * Initialize pte walk starting at the already pinned page where we
         * are sure that there is a pte.
         */
        pte = get_locked_pte(vma->vm_mm, start, &ptl);
-       end = min(end, pmd_addr_end(start, end));

        /* The page next to the pinned page is the first we will try to
get */
        start += PAGE_SIZE;

Anyway,
Reviewed-by: Bob Liu <bob.liu@...cle.com>

-- 
Regards,
-Bob
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