linux-kernel - Re: [PATCH 06/10] dma: Use free_decrypted

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Message-ID: <b4dc423b-a658-449f-9c6d-1502685a2f1b@arm.com>
Date:   Wed, 18 Oct 2023 18:42:12 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, luto@...nel.org,
        peterz@...radead.org, kirill.shutemov@...ux.intel.com,
        elena.reshetova@...el.com, isaku.yamahata@...el.com,
        seanjc@...gle.com, Michael Kelley <mikelley@...rosoft.com>,
        thomas.lendacky@....com, decui@...rosoft.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux.dev
Subject: Re: [PATCH 06/10] dma: Use free_decrypted_pages()

On 2023-10-17 21:25, Rick Edgecombe wrote:
> On TDX it is possible for the untrusted host to cause
> set_memory_encrypted() or set_memory_decrypted() to fail such that an
> error is returned and the resulting memory is shared. Callers need to take
> care to handle these errors to avoid returning decrypted (shared) memory to
> the page allocator, which could lead to functional or security issues.
> 
> DMA could free decrypted/shared pages if set_memory_decrypted() fails.
> Use the recently added free_decrypted_pages() to avoid this.
> 
> Several paths also result in proper encrypted pages being freed through
> the same freeing function. Rely on free_decrypted_pages() to not leak the
> memory in these cases.

If something's needed in the fallback path here, what about the 
cma_release() paths?

Thanks,
Robin.

> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: iommu@...ts.linux.dev
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
>   include/linux/dma-map-ops.h | 3 ++-
>   kernel/dma/contiguous.c     | 2 +-
>   2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index f2fc203fb8a1..b0800cbbc357 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -9,6 +9,7 @@
>   #include <linux/dma-mapping.h>
>   #include <linux/pgtable.h>
>   #include <linux/slab.h>
> +#include <linux/set_memory.h>
>   
>   struct cma;
>   
> @@ -165,7 +166,7 @@ static inline struct page *dma_alloc_contiguous(struct device *dev, size_t size,
>   static inline void dma_free_contiguous(struct device *dev, struct page *page,
>   		size_t size)
>   {
> -	__free_pages(page, get_order(size));
> +	free_decrypted_pages((unsigned long)page_address(page), get_order(size));
>   }
>   #endif /* CONFIG_DMA_CMA*/
>   
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index f005c66f378c..e962f1f6434e 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -429,7 +429,7 @@ void dma_free_contiguous(struct device *dev, struct page *page, size_t size)
>   	}
>   
>   	/* not in any cma, free from buddy */
> -	__free_pages(page, get_order(size));
> +	free_decrypted_pages((unsigned long)page_address(page), get_order(size));
>   }
>   
>   /*