Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when changing C-bit

Borislav Petkov
2022-02-10T16:49:06Z

On Wed, Feb 09, 2022 at 12:10:15PM -0600, Brijesh Singh wrote:
> The set_memory_{encrypted,decrypted}() are used for changing the pages
> from decrypted (shared) to encrypted (private) and vice versa.
> When SEV-SNP is active, the page state transition needs to go through
> additional steps done by the guest.
>
> If the page is transitioned from shared to private, then perform the
> following after the encryption attribute is set in the page table:
>
> 1. Issue the page state change VMGEXIT to add the memory region in
> the RMP table.
> 2. Validate the memory region after the RMP entry is added.
>
> To maintain the security guarantees, if the page is transitioned from
> private to shared, then perform the following before encryption attribute
> is removed from the page table:
>
> 1. Invalidate the page.
> 2. Issue the page state change VMGEXIT to remove the page from RMP table.
>
> To change the page state in the RMP table, use the Page State Change
> VMGEXIT defined in the GHCB specification.
>
> The GHCB specification provides the flexibility to use either 4K or 2MB
> page size in during the page state change (PSC) request. For now use the
> 4K page size for all the PSC until RMP page size tracking is supported
> in the kernel.

This commit message sounds familiar because I've read it before - patch
18 - and it looks copied. So I've turned into a simple one which says it
all:

x86/mm: Validate memory when changing the C-bit

Add the needed functionality to change pages state from shared
to private and vice-versa using the Page State Change VMGEXIT as
documented in the GHCB spec.

Thx.

--
Regards/Gruss,
Boris.