linux-kernel - Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL

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Date:   Tue, 2 Nov 2021 07:41:55 +0100
From:   Christoph Hellwig <hch@....de>
To:     Walter Wu <walter-zh.wu@...iatek.com>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        wsd_upstream <wsd_upstream@...iatek.com>,
        linux-mediatek@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING

As others pointed out, DMA_ATTR_NO_KERNEL_MAPPING just means the
caller can't rely on a kernel mapping.  So the "fix" here is
wrong.  That being said for cases where we can easily remove a page
from the kernel mapping it would be nice to do to:

 a) improve security
 b) as a debug check to see that no one actually tries to access it

> +		/* remove kernel mapping for pages */
> +		set_memory_valid((unsigned long)phys_to_virt(dma_to_phys(dev, *dma_handle)),

Please avoid overly long lines.  Also this function only exists for arm64
also and others pointed out won't work for all cases.