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From:   Christoph Hellwig <hch@lst.de>
To:     Lu Baolu <baolu.lu@linux.intel.com>
Cc:     David Woodhouse <dwmw2@infradead.org>,
        Joerg Roedel <joro@8bytes.org>,
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Subject: Re: [PATCH v5 06/10] swiotlb: Zero out bounce buffer for untrusted
 device
Message-ID: <20190725114903.GB31065@lst.de>
References: <20190725031717.32317-1-baolu.lu@linux.intel.com> <20190725031717.32317-7-baolu.lu@linux.intel.com>
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> index 43c88626a1f3..edc84a00b9f9 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -35,6 +35,7 @@
>  #include <linux/scatterlist.h>
>  #include <linux/mem_encrypt.h>
>  #include <linux/set_memory.h>
> +#include <linux/pci.h>
>  #ifdef CONFIG_DEBUG_FS
>  #include <linux/debugfs.h>
>  #endif
> @@ -562,6 +563,11 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>  	 */
>  	for (i = 0; i < nslots; i++)
>  		io_tlb_orig_addr[index+i] = orig_addr + (i << IO_TLB_SHIFT);
> +
> +	/* Zero out the bounce buffer if the consumer is untrusted. */
> +	if (dev_is_untrusted(hwdev))
> +		memset(phys_to_virt(tlb_addr), 0, alloc_size);

Hmm.  Maybe we need to move the untrusted flag to struct device?
Directly poking into the pci_dev from swiotlb is a bit of a layering
violation.

> +
>  	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
>  	    (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL))
>  		swiotlb_bounce(orig_addr, tlb_addr, mapping_size, DMA_TO_DEVICE);

Also for the case where we bounce here we only need to zero the padding
(if there is any), so I think we could optimize this a bit.