Re: [PATCH v8 08/40] x86/sev: Check the vmpl level

Venu Busireddy
2021-12-16T20:24:53Z

On 2021-12-10 09:43:00 -0600, Brijesh Singh wrote:
> Virtual Machine Privilege Level (VMPL) feature in the SEV-SNP architecture
> allows a guest VM to divide its address space into four levels. The level
> can be used to provide the hardware isolated abstraction layers with a VM.
> The VMPL0 is the highest privilege, and VMPL3 is the least privilege.
> Certain operations must be done by the VMPL0 software, such as:
>
> * Validate or invalidate memory range (PVALIDATE instruction)
> * Allocate VMSA page (RMPADJUST instruction when VMSA=1)
>
> The initial SEV-SNP support requires that the guest kernel is running on
> VMPL0. Add a check to make sure that kernel is running at VMPL0 before
> continuing the boot. There is no easy method to query the current VMPL
> level, so use the RMPADJUST instruction to determine whether the guest is
> running at the VMPL0.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/boot/compressed/sev.c | 34 ++++++++++++++++++++++++++++---
> arch/x86/include/asm/sev-common.h | 1 +
> arch/x86/include/asm/sev.h | 16 +++++++++++++++
> 3 files changed, 48 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> index a0708f359a46..9be369f72299 100644
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -212,6 +212,31 @@ static inline u64 rd_sev_status_msr(void)
> return ((high << 32) | low);
> }
>
> +static void enforce_vmpl0(void)
> +{
> + u64 attrs;
> + int err;
> +
> + /*
> + * There is no straightforward way to query the current VMPL level. The
> + * simplest method is to use the RMPADJUST instruction to change a page
> + * permission to a VMPL level-1, and if the guest kernel is launched at
> + * a level <= 1, then RMPADJUST instruction will return an error.

Perhaps a nit. When you say "level <= 1", do you mean a level lower than or
equal to 1 semantically, or numerically?

Venu