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 > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
