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

[Suzuki K Poulose]

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@xxxxxxxxxx>
> > Cc: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Robin Murphy <robin.murphy@xxxxxxx>
> > Cc: Steven Price <steven.price@xxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >   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 */