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

[Nicolin Chen]

Sorry to ping this but it's been a while.

Robin, did you get a chance to resend your version?

Thanks
Nicolin

On Tue, Jul 02, 2019 at 02:04:01PM -0700, Nicolin Chen wrote:
> 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 {