Re: [RFC PATCH v1 00/18] x86: Secure Memory Encryption (AMD)

[Tom Lendacky]

On 04/27/2016 09:39 AM, Andy Lutomirski wrote:
> On Tue, Apr 26, 2016 at 3:55 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>> This RFC patch series provides support for AMD's new Secure Memory
>> Encryption (SME) feature.
>>
>> SME can be used to mark individual pages of memory as encrypted through the
>> page tables. A page of memory that is marked encrypted will be automatically
>> decrypted when read from DRAM and will be automatically encrypted when
>> written to DRAM. Details on SME can found in the links below.
> 
> Having read through the docs briefly, some questions:
> 
> 1. How does the crypto work?  Is it straight AES-ECB?  Is it a
> tweakable mode?  If so, what does into the tweak?  For example, if I
> swap the ciphertext of two pages, does the plaintext of the pages get
> swapped?  If not, why not?

The AES crypto uses a tweak such that two identical plaintexts at
different locations will have different ciphertext. So swapping the
ciphertext of two pages will not result in the plaintext being swapped.

> 
> 2. In SEV mode, how does the hypervisor relocate a physical backing
> page?  Does it simple move it and update the 2nd-level page tables?
> If so, is the result of decryption guaranteed to be garbage if it
> relocates a page and re-inserts it at the wrong guest physical
> address?

For SEV mode, relocating a physical backing page takes extra steps.
There are APIs that are used to have the AMD Secure Processor create a
transportable encrypted page that can then be moved to a new location
in memory. After moving it to the new location the APIs are used to
haves the AMD Secure Processor re-encrypt the page for use with the
guests SEV key. Based on #1 above, just moving a page without invoking
the necessary APIs will result in the decryption returning garbage.

> 
> 3. In SEV mode, does anything prevent the hypervisor from resuming a
> guest with the wrong ASID, or is this all relying on the resulting
> corruption of the guest code and data to cause a crash?

There is nothing that prevents resuming a guest with the wrong ASID.
This relies on the resulting corruption of the guest code/data to
cause a crash.

> 
> 4. As I understand it, the caches are all unencrypted, and they're
> tagged with the physical address, *including* the SME bit (bit 47).
> In SEV mode, are they also tagged with the ASID?  I.e. if I have a
> page in cache for ASID 1 and I try to read it with ASID 2, will I get
> a fresh copy decrypted with ASID 2's key?  If so, will the old ASID 1
> copy be evicted, or will it stay in cache and be non-coherent?

In SEV mode, the caches are tagged with the ASID. So if you try to read
a cached page with a different ASID, it would result in a cache miss
for that ASID and will instead fetch from memory and decrypt using
the that ASID's key.

Thanks,
Tom

> 
> --Andy
>