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

Suzuki K Poulose
2026-01-20T14:40:28Z

On 20/01/2026 14:18, Aneesh Kumar K.V wrote:
> Suzuki K Poulose <suzuki.poulose@arm.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@kernel.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