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

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Message-ID: <yq5atswgjrkp.fsf@kernel.org>
Date: Tue, 20 Jan 2026 19:48:14 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Suzuki K Poulose <suzuki.poulose@....com>, 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

Suzuki K Poulose <suzuki.poulose@....com> writes:

> 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 ?
>

That is true only when the device is operating in untrusted mode?. For a
trusted device that mask is valid mask right?

-aneesh