[PATCH v5 4/6] swiotlb: Fix alignment checks when both allocation and DMA masks are present

Will Deacon
2024-02-28T13:39:47Z

Nicolin reports that swiotlb buffer allocations fail for an NVME device
behind an IOMMU using 64KiB pages. This is because we end up with a
minimum allocation alignment of 64KiB (for the IOMMU to map the buffer
safely) but a minimum DMA alignment mask corresponding to a 4KiB NVME
page (i.e. preserving the 4KiB page offset from the original allocation).
If the original address is not 4KiB-aligned, the allocation will fail
because swiotlb_search_pool_area() erroneously compares these unmasked
bits with the 64KiB-aligned candidate allocation.

Tweak swiotlb_search_pool_area() so that the DMA alignment mask is
reduced based on the required alignment of the allocation.

Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
Reported-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/cover.1707851466.git.nicolinc@nvidia.com
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
kernel/dma/swiotlb.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index c20324fba814..c381a7ed718f 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -981,8 +981,7 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
dma_addr_t tbl_dma_addr =
phys_to_dma_unencrypted(dev, pool->start) & boundary_mask;
unsigned long max_slots = get_max_slots(boundary_mask);
- unsigned int iotlb_align_mask =
- dma_get_min_align_mask(dev) & ~(IO_TLB_SIZE - 1);
+ unsigned int iotlb_align_mask = dma_get_min_align_mask(dev);
unsigned int nslots = nr_slots(alloc_size), stride;
unsigned int offset = swiotlb_align_offset(dev, orig_addr);
unsigned int index, slots_checked, count = 0, i;
@@ -993,6 +992,14 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
BUG_ON(!nslots);
BUG_ON(area_index >= pool->nareas);

+ /*
+ * Ensure that the allocation is at least slot-aligned and update
+ * 'iotlb_align_mask' to ignore bits that will be preserved when
+ * offsetting into the allocation.
+ */
+ alloc_align_mask |= (IO_TLB_SIZE - 1);
+ iotlb_align_mask &= ~alloc_align_mask;
+
/*
* For mappings with an alignment requirement don't bother looping to
* unaligned slots once we found an aligned one.
--
2.44.0.rc1.240.g4c46232300-goog