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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Robin Murphy <robin.murphy@arm.com> writes:

> On 2026-01-20 9:59 am, Suzuki K Poulose wrote:
>> 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 ?
>
> Yes, it's much the same as for AMD SEV (albeit the other way round) - 
> the encryption/decryption bit is part of the DMA address because it 
> needs to be driven by the device, so it is crucial that the device's DMA 
> mask is capable of expressing that.
>

Commit c92a54cfa0257e8ffd66b2a17d49e9c0bd4b769f explains these details.

I was wondering whether the DMA-enable operation should live outside the
set_mask operation. Conceptually, set_mask should be derived purely from
max_pfn, while the DMA enablement path could additionally verify whether
the device requires access to an alias address, depending on whether it
is operating in trusted or untrusted mode?

>
> Hence, as I think we've discussed before, for CCA we can't really 
> support 32-bit devices doing DMA to shared pages at all, unless the 
> whole VM is limited to a 31-bit IPA space.
>

-aneesh