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Date: Fri, 26 Jan 2024 17:23:55 +0100
From: Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org, kernel-team@android.com,
 iommu@lists.linux.dev, Christoph Hellwig <hch@lst.de>, Marek Szyprowski
 <m.szyprowski@samsung.com>, Robin Murphy <robin.murphy@arm.com>, Petr
 Tesarik <petr.tesarik1@huawei-partners.com>, Dexuan Cui
 <decui@microsoft.com>
Subject: Re: [PATCH 2/2] swiotlb: Enforce page alignment in swiotlb_alloc()
Message-ID: <20240126172355.22a03d13@meshulam.tesarici.cz>
In-Reply-To: <20240126151956.10014-3-will@kernel.org>
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	<20240126151956.10014-3-will@kernel.org>
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On Fri, 26 Jan 2024 15:19:56 +0000
Will Deacon <will@kernel.org> wrote:

> When allocating pages from a restricted DMA pool in swiotlb_alloc(),
> the buffer address is blindly converted to a 'struct page *' that is
> returned to the caller. In the unlikely event of an allocation bug,
> page-unaligned addresses are not detected and slots can silently be
> double-allocated.
> 
> Add a simple check of the buffer alignment in swiotlb_alloc() to make
> debugging a little easier if something has gone wonky.
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Petr Tesarik <petr.tesarik1@huawei-partners.com>
> Cc: Dexuan Cui <decui@microsoft.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  kernel/dma/swiotlb.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 25febb9e670c..92433ea9f2d2 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1647,6 +1647,12 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
>  		return NULL;
>  
>  	tlb_addr = slot_addr(pool->start, index);
> +	if (!PAGE_ALIGNED(tlb_addr)) {
> +		dev_WARN_ONCE(dev, 1, "Cannot return 'struct page *' for non page-aligned swiotlb addr 0x%pa.\n",
> +			      &tlb_addr);
> +		swiotlb_release_slots(dev, tlb_addr);
> +		return NULL;
> +	}

Is there a reason not to use BUG_ON()? If yes, I would at least go for:

+	if (unlikely(!PAGE_ALIGNED(tlb_addr))) {

Other than that, yes, such cheap sanity checking looks like a good idea.

Petr T