Re: [PATCH v3 3/3] x86/hyperv: Make encrypted/decrypted changes safe for load_unaligned_zeropad()

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 1/8/2024 11:13 AM, Michael Kelley wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> Sent: Monday, January 8, 2024 10:37 AM
>>
>> On 1/5/2024 10:30 AM, mhkelley58@gmail.com wrote:
>>> From: Michael Kelley <mhklinux@outlook.com>
>>>
>>> In a CoCo VM, when transitioning memory from encrypted to decrypted, or
>>> vice versa, the caller of set_memory_encrypted() or set_memory_decrypted()
>>> is responsible for ensuring the memory isn't in use and isn't referenced
>>> while the transition is in progress.  The transition has multiple steps,
>>> and the memory is in an inconsistent state until all steps are complete.
>>> A reference while the state is inconsistent could result in an exception
>>> that can't be cleanly fixed up.
>>>
>>> However, the kernel load_unaligned_zeropad() mechanism could cause a stray
>>> reference that can't be prevented by the caller of set_memory_encrypted()
>>> or set_memory_decrypted(), so there's specific code to handle this case.
>>> But a CoCo VM running on Hyper-V may be configured to run with a paravisor,
>>> with the #VC or #VE exception routed to the paravisor. There's no
>>> architectural way to forward the exceptions back to the guest kernel, and
>>> in such a case, the load_unaligned_zeropad() specific code doesn't work.
>>>
>>> To avoid this problem, mark pages as "not present" while a transition
>>> is in progress. If load_unaligned_zeropad() causes a stray reference, a
>>> normal page fault is generated instead of #VC or #VE, and the
>>> page-fault-based fixup handlers for load_unaligned_zeropad() resolve the
>>> reference. When the encrypted/decrypted transition is complete, mark the
>>> pages as "present" again.
>>
>> Change looks good to me. But I am wondering why are adding it part of
>> prepare and finish callbacks instead of directly in set_memory_encrypted() function.
>>
> 
> The prepare/finish callbacks are different for TDX, SEV-SNP, and
> Hyper-V CoCo guests running with a paravisor -- so there are three sets
> of callbacks.  As described in the cover letter, I've given up on using this
> scheme for the TDX and SEV-SNP cases, because of the difficulty with
> the SEV-SNP callbacks needing a valid virtual address (whereas TDX and
> Hyper-V paravisor need only a physical address).  So it seems like the
> callbacks specific to the Hyper-V paravisor are the natural place for the
> code.  That leaves the TDX and SEV-SNP code paths unchanged, which
> was my intent.
> 

Got it. Thanks for clarifying it.

> Or maybe I'm not understanding your comment?  If that's the case,
> please elaborate.
> 
> Michael
> 
>> Reviewed-by: Kuppuswamy Sathyanarayanan
>> <sathyanarayanan.kuppuswamy@linux.intel.com>
>>

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer