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

Chris Wilson
2017-07-28T10:54:16Z

Quoting Tvrtko Ursulin (2017-07-27 10:05:02)
> 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>
> ---
> 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 205aefb4ed93..6dd2ddbc6230 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 dee0c5004e2f..7b2e74da2c44 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;
> + }
> + }

Ok. Took a moment to realise that it works correctly for a chunk on last
page.

> 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;
> + }

Ok.

>
> 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;

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris