On 10/28/20 11:46 AM, Joerg Roedel wrote:
> From: Joerg Roedel
>
> The early #VC handler which doesn't have a GHCB can only handle CPUID
> exit codes. It is needed by the early boot code to handle #VC
> exceptions raised in verify_cpu() and to get the position of the C
> bit.
>
> But the CPUID information comes from the hypervisor, which is untrusted
> and might return results which trick the guest into the no-SEV boot path
> with no C bit set in the page-tables. All data written to memory would
> then be unencrypted and could leak sensitive data to the hypervisor.
>
> Add sanity checks to the early #VC handlers to make sure the hypervisor
> can not pretend that SEV is disabled.
>
> Signed-off-by: Joerg Roedel
Reviewed-by: Tom Lendacky
> ---
> arch/x86/kernel/sev-es-shared.c | 26 ++
> 1 file changed, 26 insertions(+)
>
> diff --git a/arch/x86/kernel/sev-es-shared.c b/arch/x86/kernel/sev-es-shared.c
> index 5f83ccaab877..56d16c405b03 100644
> --- a/arch/x86/kernel/sev-es-shared.c
> +++ b/arch/x86/kernel/sev-es-shared.c
> @@ -178,6 +178,32 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned
> long exit_code)
> goto fail;
> regs->dx = val >> 32;
>
> + /*
> + * This is a VC handler and the #VC is only raised when SEV-ES is
> + * active, which means SEV must be active too. Do sanity checks on the
> + * CPUID results to make sure the hypervisor does not trick the kernel
> + * into the no-sev path. This could map sensitive data unencrypted and
> + * make it accessible to the hypervisor.
> + *
> + * In particular, check for:
> + * - Hypervisor CPUID bit
> + * - Availability of CPUID leaf 0x801f
> + * - SEV CPUID bit.
> + *
> + * The hypervisor might still report the wrong C-bit position, but this
> + * can't be checked here.
> + */
> +
> + if ((fn == 1 && !(regs->cx & BIT(31
> + /* Hypervisor bit */
> + goto fail;
> + else if (fn == 0x8000 && (regs->ax < 0x801f))
> + /* SEV Leaf check */
> + goto fail;
> + else if ((fn == 0x801f && !(regs->ax & BIT(1
> + /* SEV Bit */
> + goto fail;
> +
> /* Skip over the CPUID two-byte opcode */
> regs->ip += 2;
>
>
