[PATCH v4 1/4] lib/scatterlist: Fix mapping iterator when sg->offset is greater than PAGE_SIZE

[Herbert Xu]

On Mon, Jun 17, 2019 at 09:15:02PM +0000, Christophe Leroy wrote:
> All mapping iterator logic is based on the assumption that sg->offset
> is always lower than PAGE_SIZE.
> 
> But there are situations where sg->offset is such that the SG item
> is on the second page. In that case sg_copy_to_buffer() fails
> properly copying the data into the buffer. One of the reason is
> that the data will be outside the kmapped area used to access that
> data.
> 
> This patch fixes the issue by adjusting the mapping iterator
> offset and pgoffset fields such that offset is always lower than
> PAGE_SIZE.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> Fixes: 4225fc8555a9 ("lib/scatterlist: use page iterator in the mapping iterator")
> Cc: stable at vger.kernel.org
> ---
>  lib/scatterlist.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)

Good catch.

> @@ -686,7 +686,12 @@ static bool sg_miter_get_next_page(struct sg_mapping_iter *miter)
>  		sg = miter->piter.sg;
>  		pgoffset = miter->piter.sg_pgoffset;
>  
> -		miter->__offset = pgoffset ? 0 : sg->offset;
> +		offset = pgoffset ? 0 : sg->offset;
> +		while (offset >= PAGE_SIZE) {
> +			miter->piter.sg_pgoffset = ++pgoffset;
> +			offset -= PAGE_SIZE;
> +		}

How about

	miter->piter.sg_pgoffset += offset >> PAGE_SHIFT;
	offset &= PAGE_SIZE - 1;

Thanks,
-- 
Email: Herbert Xu <herbert at gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt