Re: [PATCH v2 2/3] swiotlb: Enforce page alignment in swiotlb_alloc()

[Robin Murphy <robin.murphy@arm.com>]

On 31/01/2024 12:25 pm, Will Deacon 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 56cc08b1fbd6..4485f216e620 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1642,6 +1642,12 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
>   		return NULL;
>   
>   	tlb_addr = slot_addr(pool->start, index);
> +	if (unlikely(!PAGE_ALIGNED(tlb_addr))) {
> +		dev_WARN_ONCE(dev, 1, "Cannot allocate pages from non page-aligned swiotlb addr 0x%pa.\n",
> +			      &tlb_addr);

Nit: if there's cause for another respin, I'd be inclined to use a 
straightforward "if (WARN_ON(...))" here - this condition should 
represent SWIOTLB itself going badly wrong, which isn't really 
attributable to whatever device happened to be involved in the call.

Cheers,
Robin.

> +		swiotlb_release_slots(dev, tlb_addr);
> +		return NULL;
> +	}
>   
>   	return pfn_to_page(PFN_DOWN(tlb_addr));
>   }