linux-kernel - Re: [PATCH 2/2] hugepages: Fix use after free bug in "quota" handling

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Date:	Fri, 9 Mar 2012 01:19:09 +1100
From:	David Gibson <dwg@....ibm.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	akpm@...ux-foundation.org, hughd@...gle.com, paulus@...ba.org,
	linux-kernel@...r.kernel.org, Andrew Barry <abarry@...y.com>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 2/2] hugepages: Fix use after free bug in "quota" handling

On Thu, Mar 08, 2012 at 07:59:27PM +0800, Hillf Danton wrote:
> On Thu, Mar 8, 2012 at 12:17 PM, Aneesh Kumar K.V
> <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> > On Wed, 7 Mar 2012 20:28:39 +0800, Hillf Danton <dhillf@...il.com> wrote:
> >> On Wed, Mar 7, 2012 at 12:48 PM, David Gibson
> >> > @@ -533,9 +611,9 @@ static void free_huge_page(struct page *page)
> >> >         */
> >> >        struct hstate *h = page_hstate(page);
> >> >        int nid = page_to_nid(page);
> >> > -       struct address_space *mapping;
> >> > +       struct hugepage_subpool *spool =
> >> > +               (struct hugepage_subpool *)page_private(page);
> >> >
> >> > -       mapping = (struct address_space *) page_private(page);
> >> >        set_page_private(page, 0);
> >> >        page->mapping = NULL;
> >> >        BUG_ON(page_count(page));
> >> > @@ -551,8 +629,7 @@ static void free_huge_page(struct page *page)
> >> >                enqueue_huge_page(h, page);
> >> >        }
> >> >        spin_unlock(&hugetlb_lock);
> >> > -       if (mapping)
> >> > -               hugetlb_put_quota(mapping, 1);
> >> > +       hugepage_subpool_put_pages(spool, 1);
> >>
> >> Like current code, quota is handed back *unconditionally*, but ...
> >
> >
> > We will end up doing get_quota for every allocated page. get_quota
> > happens either during mmap() if MAP_NORESERVE is not specified.
> > or during alloc_huge_page if we haven't done a quota reservation during
> > mmap for that range. Are you finding any part of code where we miss that ?
> >
> >
> Thank you, Aneesh, I work it out along the direction of your question.
> 
> Apart from David's approach, quota is no longer reclaimed when pages are freed,
> but when they are truncated, and we end up with smaller .text size, and with
> the use-after-free bug fixed as well.

Sorry, I don't follow what you're saying here.

> --- a/mm/hugetlb.c	Mon Mar  5 20:20:34 2012
> +++ b/mm/hugetlb.c	Thu Mar  8 19:47:26 2012
> @@ -533,9 +533,7 @@ static void free_huge_page(struct page *
>  	 */
>  	struct hstate *h = page_hstate(page);
>  	int nid = page_to_nid(page);
> -	struct address_space *mapping;
> 
> -	mapping = (struct address_space *) page_private(page);
>  	set_page_private(page, 0);
>  	page->mapping = NULL;
>  	BUG_ON(page_count(page));
> @@ -551,8 +549,6 @@ static void free_huge_page(struct page *
>  		enqueue_huge_page(h, page);
>  	}
>  	spin_unlock(&hugetlb_lock);
> -	if (mapping)
> -		hugetlb_put_quota(mapping, 1);
>  }
> 
>  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> @@ -2944,8 +2940,8 @@ void hugetlb_unreserve_pages(struct inod
>  	inode->i_blocks -= (blocks_per_huge_page(h) * freed);
>  	spin_unlock(&inode->i_lock);
> 
> -	hugetlb_put_quota(inode->i_mapping, (chg - freed));
> -	hugetlb_acct_memory(h, -(chg - freed));
> +	hugetlb_put_quota(inode->i_mapping, chg);
> +	hugetlb_acct_memory(h, -chg);
>  }
> 
>  #ifdef CONFIG_MEMORY_FAILURE

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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