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

[Chris Wilson]

Quoting Tvrtko Ursulin (2017-07-27 10:05:02)
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> (v2)
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> ---
>  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@xxxxxxxxxxxxxxxxxx>
-Chris