linux-kernel - Re: DMA error when sg->offset value is greater than PAGE

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Date:   Wed, 20 Sep 2017 11:12:08 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Harsh Jain <Harsh@...lsio.com>
Cc:     leedom@...lsio.com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-crypto@...r.kernel.org,
        dwmw2@...radead.org
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU

On 20/09/17 09:01, Herbert Xu wrote:
> Harsh Jain <Harsh@...lsio.com> wrote:
>>
>> While debugging DMA mapping error in chelsio crypto driver we observed that when scatter/gather list received by driver has some entry with page->offset > 4096 (PAGE_SIZE). It starts giving DMA error.  Without IOMMU it works fine.
> 
> This is not a bug.  The network stack can and will feed us such
> SG lists.
> 
>> 2) It cannot be driver's responsibilty to update received sg entries to adjust offset and page 
>> because we are not the only one who directly uses received sg list.
> 
> No the driver must deal with this.  Having said that, if we can
> improve our driver helper interface to make this easier then we
> should do that too.  What we certainly shouldn't do is to take a
> whack-a-mole approach like this patch does.

AFAICS this is entirely on intel-iommu - from a brief look it appears
that all the IOVA calculations would handle the offset correctly, but
then __domain_mapping() blindly uses sg_page() for the physical address,
so if offset is larger than a page it would end up with the DMA mapping
covering the wrong part of the buffer.

Does the diff below help?

Robin.

----->8-----
diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index b3914fce8254..2ed43d928135 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -2253,7 +2253,7 @@ static int __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
 			sg_res = aligned_nrpages(sg->offset, sg->length);
 			sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + sg->offset;
 			sg->dma_length = sg->length;
-			pteval = page_to_phys(sg_page(sg)) | prot;
+			pteval = (sg_phys(sg) & PAGE_MASK) | prot;
 			phys_pfn = pteval >> VTD_PAGE_SHIFT;
 		}