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

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Message-ID: <0da8b73c-5bec-44c3-9902-221a11142c34@arm.com>
Date: Tue, 20 Jan 2026 14:39:14 +0000
From: Suzuki K Poulose <suzuki.poulose@....com>
To: "Aneesh Kumar K.V" <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 14:18, Aneesh Kumar K.V wrote:
> 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?

Irrespective of the mode in which the device is operating, the DMA
address must include the fully qualified "{I}PA" address, right ?
i.e., "the Unencrypted" bit is only a software construct and the full
PA must be used, irrespective of the mode of the device.

Suzuki

> 
> -aneesh