Re: [PATCH] iommu/dma: Fix iova map result check bug

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

On 2022-05-07 09:52, yf.wang@mediatek.com wrote:
> From: Yunfei Wang <yf.wang@mediatek.com>
> 
> The data type of the return value of the iommu_map_sg_atomic
> is ssize_t, but the data type of iova size is size_t,
> e.g. one is int while the other is unsigned int.
> 
> When iommu_map_sg_atomic return value is compared with iova size,
> it will force the signed int to be converted to unsigned int, if
> iova map fails and iommu_map_sg_atomic return error code is less
> than 0, then (ret < iova_len) is false, which will to cause not
> do free iova, and the master can still successfully get the iova
> of map fail, which is not expected.
> 
> Therefore, we need to check the return value of iommu_map_sg_atomic
> in two cases according to whether it is less than 0.

Heh, it's always a fun day when I have to go back to the C standard to 
remind myself of the usual arithmetic conversions. But indeed this seems 
correct, and even though the double comparisons look a little 
non-obvious on their own I can't think of an objectively better 
alternative, so:

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Fixes: ad8f36e4b6b1 ("iommu: return full error code from iommu_map_sg[_atomic]()")
> Signed-off-by: Yunfei Wang <yf.wang@mediatek.com>
> Cc: <stable@vger.kernel.org> # 5.15.*
> ---
>   drivers/iommu/dma-iommu.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 09f6e1c0f9c0..2932281e93fc 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -776,6 +776,7 @@ static struct page **__iommu_dma_alloc_noncontiguous(struct device *dev,
>   	unsigned int count, min_size, alloc_sizes = domain->pgsize_bitmap;
>   	struct page **pages;
>   	dma_addr_t iova;
> +	ssize_t ret;
>   
>   	if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
>   	    iommu_deferred_attach(dev, domain))
> @@ -813,8 +814,8 @@ static struct page **__iommu_dma_alloc_noncontiguous(struct device *dev,
>   			arch_dma_prep_coherent(sg_page(sg), sg->length);
>   	}
>   
> -	if (iommu_map_sg_atomic(domain, iova, sgt->sgl, sgt->orig_nents, ioprot)
> -			< size)
> +	ret = iommu_map_sg_atomic(domain, iova, sgt->sgl, sgt->orig_nents, ioprot);
> +	if (ret < 0 || ret < size)
>   		goto out_free_sg;
>   
>   	sgt->sgl->dma_address = iova;
> @@ -1209,7 +1210,7 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
>   	 * implementation - it knows better than we do.
>   	 */
>   	ret = iommu_map_sg_atomic(domain, iova, sg, nents, prot);
> -	if (ret < iova_len)
> +	if (ret < 0 || ret < iova_len)
>   		goto out_free_iova;
>   
>   	return __finalise_sg(dev, sg, nents, iova);
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