Re: [PATCH 2/2] swiotlb: Enforce page alignment in swiotlb_alloc()

From: Christoph Hellwig <hch@lst.de>
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()
Date: Mon, 29 Jan 2024 07:08:53 +0100	[thread overview]
Message-ID: <20240129060853.GB19258@lst.de> (raw)
In-Reply-To: <20240126151956.10014-3-will@kernel.org>

On Fri, Jan 26, 2024 at 03:19:56PM +0000, Will Deacon wrote:
> 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;
> +	}
>  
>  	return pfn_to_page(PFN_DOWN(tlb_addr));

So PFN_DOWN aligns the address and thus per se converting the unaligned
address isn't a problem.  That being said swiotlb obviously should never
allocate unaligned addresses, but the placement of this check feels
odd to me.  Also because it only catches swiotlb_alloc and not the
map side.

Maybe just throw a WARN_ON_ONCE into slot_addr() ?

>  }
> -- 
> 2.43.0.429.g432eaa2c6b-goog
---end quoted text---