Re: [PATCH] iommu/dma: Fix calculation overflow in __finalise_sg()

[Nicolin Chen]

On Tue, Jul 02, 2019 at 11:40:02AM +0100, Robin Murphy wrote:
> On reflection, I don't really think that size_t fits here anyway, since
> all the members of the incoming struct scatterlist are unsigned int too.
> Does the patch below work?

Yes.

> ----->8-----
> From: Robin Murphy <robin.murphy@xxxxxxx>
> Subject: [PATCH] iommu/dma: Handle SG length overflow better
> 
> Since scatterlist dimensions are all unsigned ints, in the relatively
> rare cases where a device's max_segment_size is set to UINT_MAX, then
> the "cur_len + s_length <= max_len" check in __finalise_sg() will always
> return true. As a result, the corner case of such a device mapping an
> excessively large scatterlist which is mergeable to or beyond a total
> length of 4GB can lead to overflow and a bogus truncated dma_length in
> the resulting segment.
> 
> As we already assume that any single segment must be no longer than
> max_len to begin with, this can easily be addressed by reshuffling the
> comparison.
> 
> Fixes: 809eac54cdd6 ("iommu/dma: Implement scatterlist segment merging")
> Reported-by: Nicolin Chen <nicoleotsuka@xxxxxxxxx>
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>

Tested-by: Nicolin Chen <nicoleotsuka@xxxxxxxxx>

Thank you!

> ---
>  drivers/iommu/dma-iommu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 129c4badf9ae..8de6cf623362 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -721,7 +721,7 @@ static int __finalise_sg(struct device *dev, struct scatterlist *sg, int nents,
>  		 * - and wouldn't make the resulting output segment too long
>  		 */
>  		if (cur_len && !s_iova_off && (dma_addr & seg_mask) &&
> -		    (cur_len + s_length <= max_len)) {
> +		    (max_len - cur_len >= s_length)) {
>  			/* ...then concatenate it with the previous one */
>  			cur_len += s_length;
>  		} else {