[PATCH v1 2/2] swiotlb: Fix slot alignment checks

[Petr Tesarik]

From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>

Explicit alignment and page alignment are used only to calculate
the stride, not when checking actual slot physical address.

Originally, only page alignment was implemented, and that worked,
because the whole SWIOTLB is allocated on a page boundary, so
aligning the start index was sufficient to ensure a page-aligned
slot.

When Christoph Hellwig added support for min_align_mask, the index
could be incremented in the search loop, potentially finding an
unaligned slot if minimum device alignment is between IO_TLB_SIZE
and PAGE_SIZE. The bug could go unnoticed, because the slot size
is 2 KiB, and the most common page size is 4 KiB, so there is no
alignment value in between.

IIUC the intention has been to find a slot that conforms to all
alignment constraints: device minimum alignment, an explicit
alignment (given as function parameter) and optionally page
alignment (if allocation size is >= PAGE_SIZE). The most
restrictive mask can be trivially computed with logical AND. The
rest can stay.

Fixes: 1f221a0d0dbf ("swiotlb: respect min_align_mask")
Fixes: e81e99bacc9f ("swiotlb: Support aligned swiotlb buffers")
Signed-off-by: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
---
 kernel/dma/swiotlb.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 3856e2b524b4..5b919ef832b6 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -634,22 +634,26 @@ static int swiotlb_do_find_slots(struct device *dev, int area_index,
 	BUG_ON(!nslots);
 	BUG_ON(area_index >= mem->nareas);
 
+	/*
+	 * For allocations of PAGE_SIZE or larger only look for page aligned
+	 * allocations.
+	 */
+	if (alloc_size >= PAGE_SIZE)
+		iotlb_align_mask &= PAGE_MASK;
+	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.  For allocations of
-	 * PAGE_SIZE or larger only look for page aligned allocations.
+	 * unaligned slots once we found an aligned one.
 	 */
 	stride = (iotlb_align_mask >> IO_TLB_SHIFT) + 1;
-	if (alloc_size >= PAGE_SIZE)
-		stride = max(stride, stride << (PAGE_SHIFT - IO_TLB_SHIFT));
-	stride = max(stride, (alloc_align_mask >> IO_TLB_SHIFT) + 1);
 
 	spin_lock_irqsave(&area->lock, flags);
 	if (unlikely(nslots > mem->area_nslabs - area->used))
 		goto not_found;
 
 	slot_base = area_index * mem->area_nslabs;
-	index = wrap_area_index(mem, ALIGN(area->index, stride));
+	index = area->index;
 
 	for (slots_checked = 0; slots_checked < mem->area_nslabs; ) {
 		slot_index = slot_base + index;
-- 
2.39.2