Re: [PATCH] mm: fix balloon_page_movable() page->flags check

[Rafael Aquini <aquini@redhat.com>]

On Tue, Nov 27, 2012 at 09:31:10PM -0200, Rafael Aquini wrote:
> This patch fixes the following crash by fixing and enhancing the way 
> page->flags are tested to identify a ballooned page.
> 
> ---8<---
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000194
> IP: [<ffffffff8122b354>] isolate_migratepages_range+0x344/0x7b0
> --->8---
> 
> The NULL pointer deref was taking place because balloon_page_movable()
> page->flags tests were incomplete and we ended up 
> inadvertently poking at private pages.
> 
> Reported-by: Sasha Levin <levinsasha928@gmail.com>
> Signed-off-by: Rafael Aquini <aquini@redhat.com>
> ---

Here it is Andrew, sorry by the lagged reply

Cheers!
--Rafael


>  include/linux/balloon_compaction.h | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 68893bc..634a19b 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -107,6 +107,23 @@ static inline void balloon_mapping_free(struct address_space *balloon_mapping)
>  }
>  
>  /*
> + * __balloon_page_flags - helper to perform balloon @page ->flags tests.
> + *
> + * As balloon pages are got from Buddy, and we do not play with page->flags
> + * at driver level (exception made when we get the page lock for compaction),
> + * therefore we can safely identify a ballooned page by checking if the
> + * NR_PAGEFLAGS rightmost bits from the page->flags are all cleared.
> + * This approach also helps on skipping ballooned pages that are locked for
> + * compaction or release, thus mitigating their racy check at
> + * balloon_page_movable()
> + */
> +#define BALLOON_PAGE_FLAGS_MASK       ((1UL << NR_PAGEFLAGS) - 1)
> +static inline bool __balloon_page_flags(struct page *page)
> +{
> +	return page->flags & BALLOON_PAGE_FLAGS_MASK ? false : true;
> +}
> +
> +/*
>   * __is_movable_balloon_page - helper to perform @page mapping->flags tests
>   */
>  static inline bool __is_movable_balloon_page(struct page *page)
> @@ -135,8 +152,8 @@ static inline bool balloon_page_movable(struct page *page)
>  	 * Before dereferencing and testing mapping->flags, lets make sure
>  	 * this is not a page that uses ->mapping in a different way
>  	 */
> -	if (!PageSlab(page) && !PageSwapCache(page) && !PageAnon(page) &&
> -	    !page_mapped(page))
> +	if (__balloon_page_flags(page) && !page_mapped(page) &&
> +	    page_count(page) == 1)
>  		return __is_movable_balloon_page(page);
>  
>  	return false;
> -- 
> 1.7.11.7
> 

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