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

Aneesh Kumar K.V
2026-01-21T04:51:00Z

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

> On 2026-01-20 2:25 pm, Aneesh Kumar K.V wrote:
>> 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.
>
> See x86's force_dma_unencrypted() implementation - the reason dma-direct
> doesn't need to be so strict for that is because things are the right
> way round that it can always fall back to shared/untrusted DMA as the
> general case, and a device can only access an encrypted page directly if
> it *can* drive the SME bit in the input address to trigger the inline
> encryption engine.
>
> For CCA we have rather the opposite, where dma-direct requires a device
> to be able to address any IPA directly to be sure of working at all, but
> if we do happen to have a stage 1 IOMMU then we could rely on that to
> map smaller IOVAs to the upper IPA space.
>
>> 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?
>
> No, the point of the set_mask check is "is this mask big enough to
> accommodate any *DMA address* we might need to give the device?" - that
> includes offsets, magic bits, magic bits encoded as fake offsets (yes
> you, Raspberry Pi...) and whatever else might comprise an actual DMA
> address as handed off to the hardware. It is *not* directly a "can this
> device DMA to all RAM we know about?" check - that is in the assumption
> that if necessary the SWIOTLB buffer will be reachable at a <=32-bit DMA
> address, and thus we should not *need* to give >32-bit addresses to
> devices. It is only that assumption which fundamentally falls apart for
> CCA (with more than 2GB of IPA space, at least).
>

We need the below change then? commit 91ef26f914171cf753330f13724fd9142b5b1640
discuss some hardware that is broken with the usage of phys_to_dma conversion.

modified kernel/dma/direct.c
@@ -14,6 +14,7 @@
#include <linux/set_memory.h>
#include <linux/slab.h>
#include <linux/pci-p2pdma.h>
+#include <linux/cc_platform.h>
#include "direct.h"

/*
@@ -579,17 +580,23 @@ int dma_direct_mmap(struct device *dev, struct vm_area_struct *vma,

int dma_direct_supported(struct device *dev, u64 mask)
{
- u64 min_mask = (max_pfn - 1) << PAGE_SHIFT;
+ u64 min_mask = DMA_BIT_MASK(32);

/*
- * Because 32-bit DMA masks are so common we expect every architecture
- * to be able to satisfy them - either by not supporting more physical
- * memory, or by providing a ZONE_DMA32. If neither is the case, the
- * architecture needs to use an IOMMU instead of the direct mapping.
+ * Only do the conversion for CC platform, to be compatible
+ * commit 91ef26f914171cf753330f13724fd9142b5b1640
*/
- if (mask >= DMA_BIT_MASK(32))
+ if (cc_platform_has(CC_ATTR_MEM_ENCRYPT))
+ min_mask = phys_to_dma_unencrypted(dev, min_mask);
+
+ /*
+ * if we support ZONE_DMA32 and device mask can cover the DMA32 range,
+ * then we can support direct dma for any max_pfn value using swiotlb.
+ */
+ if (IS_ENABLED(CONFIG_ZONE_DMA32) && mask >= min_mask)
return 1;

+ min_mask = (max_pfn - 1) << PAGE_SHIFT;
/*
* This check needs to be against the actual bit mask value, so use
* __phys_to_dma() here so that the arch specific encryption mask isn't

-aneesh