Re: [PATCH v2 3/3] arm64: realm: Use aliased addresses for device DMA to shared buffers

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 25/02/2025 13:04, Robin Murphy wrote:
> On 2025-02-19 10:07 pm, Suzuki K Poulose wrote:
>> When a device performs DMA to a shared buffer using physical addresses,
>> (without Stage1 translation), the device must use the "{I}PA address" 
>> with the
>> top bit set in Realm. This is to make sure that a trusted device will 
>> be able
>> to write to shared buffers as well as the protected buffers. Thus, a 
>> Realm must
>> always program the full address including the "protection" bit, like 
>> AMD SME
>> encryption bits.
>>
>> Enable this by providing arm64 specific 
>> dma_{encrypted,decrypted,clear_encryption}
>> helpers for Realms. Please note that the VMM needs to similarly make 
>> sure that
>> the SMMU Stage2 in the Non-secure world is setup accordingly to map 
>> IPA at the
>> unprotected alias.
>>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Robin Murphy <robin.murphy@arm.com>
>> Cc: Steven Price <steven.price@arm.com>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>> Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>>   arch/arm64/include/asm/mem_encrypt.h | 22 ++++++++++++++++++++++
>>   1 file changed, 22 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/mem_encrypt.h b/arch/arm64/ 
>> include/asm/mem_encrypt.h
>> index f8f78f622dd2..aeda3bba255e 100644
>> --- a/arch/arm64/include/asm/mem_encrypt.h
>> +++ b/arch/arm64/include/asm/mem_encrypt.h
>> @@ -21,4 +21,26 @@ static inline bool force_dma_unencrypted(struct 
>> device *dev)
>>       return is_realm_world();
>>   }
>> +static inline dma_addr_t dma_decrypted(dma_addr_t daddr)
>> +{
>> +    if (is_realm_world())
>> +        daddr |= prot_ns_shared;
>> +    return daddr;
>> +}
>> +#define dma_decrypted dma_decrypted
>> +
>> +static inline dma_addr_t dma_encrypted(dma_addr_t daddr)
>> +{
>> +    if (is_realm_world())
>> +        daddr &= prot_ns_shared - 1;
> 
> Nit: is there a reason this isn't the direct inverse of the other 
> operation, i.e. "daddr &= ~prot_ns_shared"? If so, it might be worth 

It could be. The IPA size for the realm is split into half with the
lower half protected/encrypted and anything above that unprotected. 
Technically any addr >= prot_ns_shared is "unencrypted" (even though it
may be invalid, if >= BIT(IPA_Size) - 1), so to cover that, I masked
anything above the MS. But now when I think of it, it is much better to
trigger a Stage2 fault if the address is illegal (i.e., > BIT(IPA_Size) 
- 1) than corrupting some valid memory, by masking the top bits (beyond 
prot_ns_shared).

Cheers
Suzuki




> dropping a comment why we're doing slightly unintuitive arithmetic on a 
> pagetable attribute (and if not then maybe just do the more obvious 
> thing). I doubt anyone's in a rush to support TBI for DMA, and this 
> would be far from the only potential hiccup for that, but still... :)
> 
> Thanks,
> Robin.
> 
>> +    return daddr;
>> +}
>> +#define dma_encrypted dma_encrypted
>> +
>> +static inline dma_addr_t dma_clear_encryption(dma_addr_t daddr)
>> +{
>> +    return dma_encrypted(daddr);
>> +}
>> +#define dma_clear_encryption dma_clear_encryption
>> +
>>   #endif    /* __ASM_MEM_ENCRYPT_H */
>