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@gmail.com>
Tested-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index 6441197..4ea9cf0 100644 (file)
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -762,7 +762,7 @@ 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 {