Re: [Intel-gfx] [PATCH 2/4] lib/scatterlist: Avoid potential scatterlist entry overflow

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

Hi,

On 16/01/2017 14:12, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>
> Since the scatterlist length field is an unsigned int, make
> sure that sg_alloc_table_from_pages does not overflow it while
> coallescing pages to a single entry.
>
> v2: Drop reference to future use. Use UINT_MAX.
> v3: max_segment must be page aligned.
> v4: Do not rely on compiler to optimise out the rounddown.
>     (Joonas Lahtinen)
> v5: Simplified loops and use post-increments rather than
>     pre-increments. Use PAGE_MASK and fix comment typo.
>     (Andy Shevchenko)
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> Cc: linux-kernel@vger.kernel.org
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Andy Shevchenko <andy.shevchenko@gmail.com>

Anyone in the mood for reviewing from here to the end of the series?

Regards,

Tvrtko

> ---
>  include/linux/scatterlist.h |  6 ++++++
>  lib/scatterlist.c           | 31 ++++++++++++++++++++-----------
>  2 files changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index c981bee1a3ae..4768eeeb7054 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -21,6 +21,12 @@ struct scatterlist {
>  };
>
>  /*
> + * Since the above length field is an unsigned int, below we define the maximum
> + * length in bytes that can be stored in one scatterlist entry.
> + */
> +#define SCATTERLIST_MAX_SEGMENT (UINT_MAX & PAGE_MASK)
> +
> +/*
>   * These macros should be used after a dma_map_sg call has been done
>   * to get bus addresses of each of the SG entries and their lengths.
>   * You should only work with the number of sg entries dma_map_sg
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index e05e7fc98892..65f375645df5 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -394,17 +394,22 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
>  	unsigned int offset, unsigned long size,
>  	gfp_t gfp_mask)
>  {
> -	unsigned int chunks;
> -	unsigned int i;
> -	unsigned int cur_page;
> +	const unsigned int max_segment = SCATTERLIST_MAX_SEGMENT;
> +	unsigned int chunks, cur_page, seg_len, i;
>  	int ret;
>  	struct scatterlist *s;
>
>  	/* compute number of contiguous chunks */
>  	chunks = 1;
> -	for (i = 1; i < n_pages; ++i)
> -		if (page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1)
> -			++chunks;
> +	seg_len = 0;
> +	for (i = 1; i < n_pages; i++) {
> +		seg_len += PAGE_SIZE;
> +		if (seg_len >= max_segment ||
> +		    page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1) {
> +			chunks++;
> +			seg_len = 0;
> +		}
> +	}
>
>  	ret = sg_alloc_table(sgt, chunks, gfp_mask);
>  	if (unlikely(ret))
> @@ -413,17 +418,21 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
>  	/* merging chunks and putting them into the scatterlist */
>  	cur_page = 0;
>  	for_each_sg(sgt->sgl, s, sgt->orig_nents, i) {
> -		unsigned long chunk_size;
> -		unsigned int j;
> +		unsigned int j, chunk_size;
>
>  		/* look for the end of the current chunk */
> -		for (j = cur_page + 1; j < n_pages; ++j)
> -			if (page_to_pfn(pages[j]) !=
> +		seg_len = 0;
> +		for (j = cur_page + 1; j < n_pages; j++) {
> +			seg_len += PAGE_SIZE;
> +			if (seg_len >= max_segment ||
> +			    page_to_pfn(pages[j]) !=
>  			    page_to_pfn(pages[j - 1]) + 1)
>  				break;
> +		}
>
>  		chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
> -		sg_set_page(s, pages[cur_page], min(size, chunk_size), offset);
> +		sg_set_page(s, pages[cur_page],
> +			    min_t(unsigned long, size, chunk_size), offset);
>  		size -= chunk_size;
>  		offset = 0;
>  		cur_page = j;
>