Re: [PATCH v7 03/13] sev: Remove false abstraction of flash encryption

Cornelia Huck
2021-01-15T12:56:28Z

On Thu, 14 Jan 2021 10:58:01 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> When AMD's SEV memory encryption is in use, flash memory banks (which are
> initialed by pc_system_flash_map()) need to be encrypted with the guest's
> key, so that the guest can read them.
>
> That's abstracted via the kvm_memcrypt_encrypt_data() callback in the KVM
> state.. except, that it doesn't really abstract much at all.
>
> For starters, the only called is in code specific to the 'pc' family of

s/called/call site/

> machine types, so it's obviously specific to those and to x86 to begin
> with. But it makes a bunch of further assumptions that need not be true
> about an arbitrary confidential guest system based on memory encryption,
> let alone one based on other mechanisms:
>
> * it assumes that the flash memory is defined to be encrypted with the
> guest key, rather than being shared with hypervisor
> * it assumes that that hypervisor has some mechanism to encrypt data into
> the guest, even though it can't decrypt it out, since that's the whole
> point
> * the interface assumes that this encrypt can be done in place, which
> implies that the hypervisor can write into a confidential guests's
> memory, even if what it writes isn't meaningful
>
> So really, this "abstraction" is actually pretty specific to the way SEV
> works. So, this patch removes it and instead has the PC flash
> initialization code call into a SEV specific callback.
>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
> accel/kvm/kvm-all.c | 31 ++-----------------------------
> accel/kvm/sev-stub.c | 9 ++-------
> accel/stubs/kvm-stub.c | 10 ----------
> hw/i386/pc_sysfw.c | 17 ++++++-----------
> include/sysemu/kvm.h | 16 ----------------
> include/sysemu/sev.h | 4 ++--
> target/i386/sev-stub.c | 5 +++++
> target/i386/sev.c | 24 ++++++++++++++----------
> 8 files changed, 31 insertions(+), 85 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>