RE: [Patch v3 05/14] x86/mm: Handle decryption/re-encryption of bss_decrypted consistently

Michael Kelley (LINUX)
2022-11-22T17:59:22Z

From: Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>
>
> On 11/21/22 6:39 AM, Borislav Petkov wrote:
> > On Fri, Nov 18, 2022 at 02:55:32AM +0000, Michael Kelley (LINUX) wrote:
> >> But I had not thought about TDX. In the TDX case, it appears that
> >> sme_postprocess_startup() will not decrypt the bss_decrypted section.
> >> The corresponding mem_encrypt_free_decrypted_mem() is a no-op unless
> >> CONFIG_AMD_MEM_ENCRYPT is set. But maybe if someone builds a
> >> kernel image that supports both TDX and AMD encryption, it could break
> >
> > sme_me_mask better be 0 on a kernel with both built in and running as a
> > TDX guest.
> >
>
> Yes. It will be 0 in TDX. In sme_enable(), AMD code checks for CPUID
> support before updating the sme_me_mask.
>

Right. But here's my point: With current code and an image built with
CONFIG_AMD_MEM_ENCRYPT=y and running as a TDX guest,
sme_postprocess_startup() will not decrypt the bss_decrypted section.
Then later mem_encrypt_free_decrypted_mem() will run, see that
CC_ATTR_MEM_ENCRYPT is true, and try to re-encrypt the memory.
In other words, a TDX guest would break in the same way as a Hyper-V
vTOM guest would break. This patch fixes the problem for both cases.

The only things I see in the bss_decrypted section are two clock structures
In arch/x86/kernel/kvmclock.c, which aren't needed when Hyper-V is the
hypervisor. But with a TDX guest on KVM, will *not* decrypting the
bss_decrypted section be a problem? I don't know that kvmclock
code or why the two clock structures need to be decrypted for
AMD mem encryption.

Michael