Re: [PATCH v5 14/16] dma-direct: Allocate memory from restricted DMA pool if available

[Robin Murphy <robin.murphy@xxxxxxx>]

On 2021-04-22 09:15, Claire Chang wrote:
> The restricted DMA pool is preferred if available.
>
> The restricted DMA pools provide a basic level of protection against the
> DMA overwriting buffer contents at unexpected times. However, to protect
> against general data leakage and system memory corruption, the system
> needs to provide a way to lock down the memory access, e.g., MPU.
>
> Signed-off-by: Claire Chang <tientzu@xxxxxxxxxxxx>
> ---
>   kernel/dma/direct.c | 35 ++++++++++++++++++++++++++---------
>   1 file changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 7a27f0510fcc..29523d2a9845 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -78,6 +78,10 @@ static bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
>   static void __dma_direct_free_pages(struct device *dev, struct page *page,
>   				    size_t size)
>   {
> +#ifdef CONFIG_DMA_RESTRICTED_POOL
> +	if (swiotlb_free(dev, page, size))
> +		return;
> +#endif
>   	dma_free_contiguous(dev, page, size);
>   }
>   
> @@ -92,7 +96,17 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
>   
>   	gfp |= dma_direct_optimal_gfp_mask(dev, dev->coherent_dma_mask,
>   					   &phys_limit);
> -	page = dma_alloc_contiguous(dev, size, gfp);
> +
> +#ifdef CONFIG_DMA_RESTRICTED_POOL
> +	page = swiotlb_alloc(dev, size);
> +	if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
> +		__dma_direct_free_pages(dev, page, size);
> +		page = NULL;
> +	}
> +#endif
> +
> +	if (!page)
> +		page = dma_alloc_contiguous(dev, size, gfp);
>   	if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
>   		dma_free_contiguous(dev, page, size);
>   		page = NULL;
> @@ -148,7 +162,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   		gfp |= __GFP_NOWARN;
>   
>   	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> -	    !force_dma_unencrypted(dev)) {
> +	    !force_dma_unencrypted(dev) && !is_dev_swiotlb_force(dev)) {
>   		page = __dma_direct_alloc_pages(dev, size, gfp & ~__GFP_ZERO);
>   		if (!page)
>   			return NULL;
> @@ -161,8 +175,8 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	}
>   
>   	if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
> -	    !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> -	    !dev_is_dma_coherent(dev))
> +	    !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev) &&
> +	    !is_dev_swiotlb_force(dev))
>   		return arch_dma_alloc(dev, size, dma_handle, gfp, attrs);
>   
>   	/*
> @@ -172,7 +186,9 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
>   	    !gfpflags_allow_blocking(gfp) &&
>   	    (force_dma_unencrypted(dev) ||
> -	     (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev))))
> +	     (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> +	      !dev_is_dma_coherent(dev))) &&
> +	    !is_dev_swiotlb_force(dev))
>   		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>   
>   	/* we always manually zero the memory once we are done */
> @@ -253,15 +269,15 @@ void dma_direct_free(struct device *dev, size_t size,
>   	unsigned int page_order = get_order(size);
>   
>   	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> -	    !force_dma_unencrypted(dev)) {
> +	    !force_dma_unencrypted(dev) && !is_dev_swiotlb_force(dev)) {
>   		/* cpu_addr is a struct page cookie, not a kernel address */
>   		dma_free_contiguous(dev, cpu_addr, size);
>   		return;
>   	}
>   
>   	if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
> -	    !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> -	    !dev_is_dma_coherent(dev)) {
> +	    !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && !dev_is_dma_coherent(dev) &&
> +	    !is_dev_swiotlb_force(dev)) {
>   		arch_dma_free(dev, size, cpu_addr, dma_addr, attrs);
>   		return;
>   	}
> @@ -289,7 +305,8 @@ struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
>   	void *ret;
>   
>   	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
> -	    force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp))
> +	    force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp) &&
> +	    !is_dev_swiotlb_force(dev))
>   		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);

Wait, this seems broken for non-coherent devices - in that case we need 
to return a non-cacheable address, but we can't simply fall through into 
the remapping path below in GFP_ATOMIC context. That's why we need the 
atomic pool concept in the first place :/

Unless I've overlooked something, we're still using the regular 
cacheable linear map address of the dma_io_tlb_mem buffer, no?

Robin.

>   
>   	page = __dma_direct_alloc_pages(dev, size, gfp);