linux-kernel - Re: [PATCH 1/2] dma-direct: Validate DMA mask against canonical DMA addresses

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Message-ID: <2a0b6d1b-875a-4075-8fc9-a8534afc9168@arm.com>
Date: Tue, 20 Jan 2026 09:59:07 +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,
 robin.murphy@....com, jgg@...pe.ca, steven.price@....com,
 Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 1/2] dma-direct: Validate DMA mask against canonical DMA
 addresses

On 20/01/2026 06:42, Aneesh Kumar K.V (Arm) wrote:
> On systems that apply an address encryption tag or mask to DMA addresses,
> DMA mask validation must be performed against the canonical DMA address.
> Using a non-canonical (e.g. encrypted or unencrypted) DMA address
> can incorrectly fail capability checks, since architecture-specific
> encryption bits are not part of the device’s actual DMA addressing
> capability. For example, arm64 adds PROT_NS_SHARED to unencrypted DMA
> addresses.
> 
> Fix this by validating device DMA masks against __phys_to_dma(), ensuring
> that the architecture encryption mask does not influence the check.
> 
> Fixes: b66e2ee7b6c8 ("dma: Introduce generic dma_addr_*crypted helpers")


> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> ---
>   kernel/dma/direct.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 8e04f72baaa3..a5639e9415f5 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -580,12 +580,12 @@ int dma_direct_supported(struct device *dev, u64 mask)
>   
>   	/*
>   	 * This check needs to be against the actual bit mask value, so use
> -	 * phys_to_dma_unencrypted() here so that the SME encryption mask isn't
> +	 * __phys_to_dma() here so that the arch specific encryption mask isn't
>   	 * part of the check.
>   	 */
>   	if (IS_ENABLED(CONFIG_ZONE_DMA))
>   		min_mask = min_t(u64, min_mask, zone_dma_limit);
> -	return mask >= phys_to_dma_unencrypted(dev, min_mask);
> +	return mask >= __phys_to_dma(dev, min_mask);

This is wrong, isn't it ? For e.g., for CCA, even though the "Flag" is
added to the PA, it is really part of the actual "PA" and thus must be
checked against the full PA ?

Suzuki


>   }
>   
>   static const struct bus_dma_region *dma_find_range(struct device *dev,