[PATCH 5/6] swiotlb: fix a braino in the alignment check fix

Tim Gardner tim.gardner at canonical.com
Wed Jul 12 18:01:40 UTC 2023

From: Petr Tesarik <petr.tesarik.ext at huawei.com>

BugLink: https://bugs.launchpad.net/bugs/2026736

The alignment mask in swiotlb_do_find_slots() masks off the high
bits which are not relevant for the alignment, so multiple
requirements are combined with a bitwise OR rather than AND.
In plain English, the stricter the alignment, the more bits must
be set in iotlb_align_mask.

Confusion may arise from the fact that the same variable is also
used to mask off the offset within a swiotlb slot, which is
achieved with a bitwise AND.

Fixes: 0eee5ae10256 ("swiotlb: fix slot alignment checks")
Reported-by: Dexuan Cui <decui at microsoft.com>
Link: https://lore.kernel.org/all/CAA42JLa1y9jJ7BgQvXeUYQh-K2mDNHd2BYZ4iZUz33r5zY7oAQ@mail.gmail.com/
Reported-by: Kelsey Steele <kelseysteele at linux.microsoft.com>
Link: https://lore.kernel.org/all/20230405003549.GA21326@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net/
Signed-off-by: Petr Tesarik <petr.tesarik.ext at huawei.com>
Tested-by: Dexuan Cui <decui at microsoft.com>
Signed-off-by: Christoph Hellwig <hch at lst.de>
(cherry picked from commit bbb73a103fbbed6f63cb738d3783261c4241b4b2)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
 kernel/dma/swiotlb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 9f0eacabe8e2..2048709cd733 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -583,7 +583,7 @@ static int swiotlb_do_find_slots(struct device *dev, int area_index,
 		phys_to_dma_unencrypted(dev, mem->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);
+		dma_get_min_align_mask(dev) | alloc_align_mask;
 	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;
@@ -599,8 +599,8 @@ static int swiotlb_do_find_slots(struct device *dev, int area_index,
 	 * allocations.
 	if (alloc_size >= PAGE_SIZE)
-		iotlb_align_mask &= PAGE_MASK;
-	iotlb_align_mask &= alloc_align_mask;
+		iotlb_align_mask |= ~PAGE_MASK;
+	iotlb_align_mask &= ~(IO_TLB_SIZE - 1);
 	 * For mappings with an alignment requirement don't bother looping to

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