Re: [PATCH v4 2/5] x86/boot/compressed/64: Add CPUID sanity check to early #VC handler

[Tom Lendacky <thomas.lendacky@amd.com>]

On 10/28/20 11:46 AM, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> 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 <jroedel@suse.de>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>  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 0x8000001f
> +	 *	- 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 == 0x80000000 && (regs->ax < 0x8000001f))
> +		/* SEV Leaf check */
> +		goto fail;
> +	else if ((fn == 0x8000001f && !(regs->ax & BIT(1))))
> +		/* SEV Bit */
> +		goto fail;
> +
>  	/* Skip over the CPUID two-byte opcode */
>  	regs->ip += 2;
>  
>