[PATCH] iommu/vt-d: Fix scatterlist offset handling

[Robin Murphy]

The intel-iommu DMA ops fail to correctly handle scatterlists where
sg->offset is greater than PAGE_SIZE - the IOVA allocation is computed
appropriately based on the page-aligned portion of the offset, but the
mapping is set up relative to sg->page, which means it fails to actually
cover the whole buffer (and in the worst case doesn't cover it at all):

    (sg->dma_address + sg->dma_len) ----+
    sg->dma_address ---------+          |
    iov_pfn------+           |          |
                 |           |          |
                 v           v          v
iova:   a        b        c        d        e        f
        |--------|--------|--------|--------|--------|
                          <...calculated....>
                 [_____mapped______]
pfn:    0        1        2        3        4        5
        |--------|--------|--------|--------|--------|
                 ^           ^          ^
                 |           |          |
    sg->page ----+           |          |
    sg->offset --------------+          |
    (sg->offset + sg->length) ----------+

As a result, the caller ends up overrunning the mapping into whatever
lies beyond, which usually goes badly:

[  429.645492] DMAR: DRHD: handling fault status reg 2
[  429.650847] DMAR: [DMA Write] Request device [02:00.4] fault addr f2682000 ...

Whilst this is a fairly rare occurrence, it can happen from the result
of intermediate scatterlist processing such as scatterwalk_ffwd() in the
crypto layer. Whilst that particular site could be fixed up, it still
seems worthwhile to bring intel-iommu in line with other DMA API
implementations in handling this robustly.

To that end, fix the intel_map_sg() path to line up the mapping
correctly (in units of MM pages rather than VT-d pages to match the
aligned_nrpages() calculation) regardless of the offset, and use
sg_phys() consistently for clarity.

Reported-by: Harsh Jain <Harsh@xxxxxxxxxxx>
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
 drivers/iommu/intel-iommu.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index 6784a05dd6b2..83f3d4831f94 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -2254,10 +2254,12 @@ static int __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
 		uint64_t tmp;
 
 		if (!sg_res) {
+			unsigned int pgoff = sg->offset & ~PAGE_MASK;
+
 			sg_res = aligned_nrpages(sg->offset, sg->length);
-			sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
+			sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + pgoff;
 			sg->dma_length = sg->length;
-			pteval = page_to_phys(sg_page(sg)) | prot;
+			pteval = (sg_phys(sg) - pgoff) | prot;
 			phys_pfn = pteval >> VTD_PAGE_SHIFT;
 		}
 
@@ -3790,7 +3792,7 @@ static int intel_nontranslate_map_sg(struct device *hddev,
 
 	for_each_sg(sglist, sg, nelems, i) {
 		BUG_ON(!sg_page(sg));
-		sg->dma_address = page_to_phys(sg_page(sg)) + sg->offset;
+		sg->dma_address = sg_phys(sg);
 		sg->dma_length = sg->length;
 	}
 	return nelems;
-- 
2.13.4.dirty