virt/coco/sev-guest: Double-buffer messages

virt/coco/sev-guest: Double-buffer messages

[ Upstream commit 965006103a14703cc42043bbf9b5e0cdf7a468ad ]

The encryption algorithms read and write directly to shared unencrypted
memory, which may leak information as well as permit the host to tamper
with the message integrity. Instead, copy whole messages in or out as
needed before doing any computation on them.

Fixes: d5af44dde546 ("x86/sev: Provide support for SNP guest request NAEs")
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230214164638.1189804-3-dionnaglaze@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>