linux-kernel - Re: [PATCH v2 4/4] dma: direct: set decrypted flag for remapped dma allocations

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Message-ID: <5820e8f3-9cf8-423f-89df-0df7ca78a84b@arm.com>
Date: Mon, 22 Dec 2025 15:05:50 +0000
From: Suzuki K Poulose <suzuki.poulose@....com>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
 linux-coco@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
 maz@...nel.org, tglx@...utronix.de, robin.murphy@....com,
 akpm@...ux-foundation.org, jgg@...pe.ca, steven.price@....com
Subject: Re: [PATCH v2 4/4] dma: direct: set decrypted flag for remapped dma
 allocations

On 21/12/2025 16:09, Aneesh Kumar K.V (Arm) wrote:
> Devices that are DMA non-coherent and need a remap were skipping
> dma_set_decrypted(), leaving buffers encrypted even when the device
> requires unencrypted access. Move the call after the remap
> branch so both paths mark the allocation decrypted (or fail cleanly)
> before use.
> 
> Fixes: f3c962226dbe ("dma-direct: clean up the remapping checks in dma_direct_alloc")
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> ---
>   kernel/dma/direct.c | 8 +++-----
>   1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 3448d877c7c6..a62dc25524cc 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -271,9 +271,6 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	if (remap) {
>   		pgprot_t prot = dma_pgprot(dev, PAGE_KERNEL, attrs);
>   
> -		if (force_dma_unencrypted(dev))
> -			prot = pgprot_decrypted(prot);

This would be problematic, isn't it ? We don't support decrypted on a
vmap area for arm64. If we move this down, we might actually use the
vmapped area. Not sure if other archs are fine with "decrypting" a
"vmap" address.

If we map the "vmap" address with pgprot_decrypted, we could go ahead
and further map the linear map (i.e., page_address(page)) decrypted
and get everything working.

Suzuki


> -
>   		/* remove any dirty cache lines on the kernel alias */
>   		arch_dma_prep_coherent(page, size);
>   
> @@ -284,10 +281,11 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   			goto out_free_pages;
>   	} else {
>   		ret = page_address(page);
> -		if (dma_set_decrypted(dev, ret, size))
> -			goto out_leak_pages;
>   	}
>   
> +	if (dma_set_decrypted(dev, ret, size))
> +		goto out_leak_pages;
> +
>   	memset(ret, 0, size);
>   
>   	if (set_uncached) {