[PATCH 2/2] swiotlb: Enforce page alignment in swiotlb_alloc()

[Will Deacon]

When allocating pages from a restricted DMA pool in swiotlb_alloc(),
the buffer address is blindly converted to a 'struct page *' that is
returned to the caller. In the unlikely event of an allocation bug,
page-unaligned addresses are not detected and slots can silently be
double-allocated.

Add a simple check of the buffer alignment in swiotlb_alloc() to make
debugging a little easier if something has gone wonky.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 kernel/dma/swiotlb.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 25febb9e670c..92433ea9f2d2 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1647,6 +1647,12 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
 		return NULL;
 
 	tlb_addr = slot_addr(pool->start, index);
+	if (!PAGE_ALIGNED(tlb_addr)) {
+		dev_WARN_ONCE(dev, 1, "Cannot return 'struct page *' for non page-aligned swiotlb addr 0x%pa.\n",
+			      &tlb_addr);
+		swiotlb_release_slots(dev, tlb_addr);
+		return NULL;
+	}
 
 	return pfn_to_page(PFN_DOWN(tlb_addr));
 }
-- 
2.43.0.429.g432eaa2c6b-goog