x86/sev: Do not require Hypervisor CPUID bit for SEV guests

A malicious hypervisor could disable the CPUID intercept for an SEV or
SEV-ES guest and trick it into the no-SEV boot path, where it could
potentially reveal secrets. This is not an issue for SEV-SNP guests,
as the CPUID intercept can't be disabled for those.

Remove the Hypervisor CPUID bit check from the SEV detection code to
protect against this kind of attack and add a Hypervisor bit equals zero
check to the SME detection path to prevent non-encrypted guests from
trying to enable SME.

This handles the following cases:

	1) SEV(-ES) guest where CPUID intercept is disabled. The guest
	   will still see leaf 0x8000001f and the SEV bit. It can
	   retrieve the C-bit and boot normally.

	2) Non-encrypted guests with intercepted CPUID will check
	   the SEV_STATUS MSR and find it 0 and will try to enable SME.
	   This will fail when the guest finds MSR_K8_SYSCFG to be zero,
	   as it is emulated by KVM. But we can't rely on that, as there
	   might be other hypervisors which return this MSR with bit
	   23 set. The Hypervisor bit check will prevent that the guest
	   tries to enable SME in this case.

	3) Non-encrypted guests on SEV capable hosts with CPUID intercept
	   disabled (by a malicious hypervisor) will try to boot into
	   the SME path. This will fail, but it is also not considered
	   a problem because non-encrypted guests have no protection
	   against the hypervisor anyway.

 [ bp: s/non-SEV/non-encrypted/g ]

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lkml.kernel.org/r/20210312123824.306-3-joro@8bytes.org

1 files changed, 1 insertions, 5 deletions

diff --git a/arch/x86/kernel/sev-es-shared.c b/arch/x86/kernel/sev-es-shared.c
index cdc04d091242..387b71669818 100644
--- a/arch/x86/kernel/sev-es-shared.c
+++ b/arch/x86/kernel/sev-es-shared.c
@@ -186,7 +186,6 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 	 * make it accessible to the hypervisor.
 	 *
 	 * In particular, check for:
-	 *	- Hypervisor CPUID bit
 	 *	- Availability of CPUID leaf 0x8000001f
 	 *	- SEV CPUID bit.
 	 *
@@ -194,10 +193,7 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 	 * can't be checked here.
 	 */
 
-	if ((fn == 1 && !(regs->cx & BIT(31))))
-		/* Hypervisor bit */
-		goto fail;
-	else if (fn == 0x80000000 && (regs->ax < 0x8000001f))
+	if (fn == 0x80000000 && (regs->ax < 0x8000001f))
 		/* SEV leaf check */
 		goto fail;
 	else if ((fn == 0x8000001f && !(regs->ax & BIT(1))))