Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Petr Tesarik <petrtesarik@huaweicloud.com>
To:     Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Petr Tesarik <petr.tesarik.ext@huawei.com>,
        iommu@lists.linux.dev (open list:DMA MAPPING HELPERS),
        linux-kernel@vger.kernel.org (open list)
Cc:     Roberto Sassu <roberto.sassu@huawei.com>, petr@tesarici.cz
Subject: [PATCH v1] swiotlb: fix a braino in the alignment check fix
Date:   Thu,  6 Apr 2023 16:35:39 +0200
Message-Id: <20230406143539.25-1-petrtesarik@huaweicloud.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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

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@microsoft.com>
Link: https://lore.kernel.org/all/CAA42JLa1y9jJ7BgQvXeUYQh-K2mDNHd2BYZ4iZUz33r5zY7oAQ@mail.gmail.com/
Reported-by: Kelsey Steele <kelseysteele@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@huawei.com>
Tested-by: Dexuan Cui <decui@microsoft.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 5b919ef832b6..dac42a2ad588 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -623,7 +623,7 @@ 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;
@@ -639,8 +639,8 @@ 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
-- 
2.25.1