[tip: x86/urgent] x86/sev: Check SME/SEV support in CPUID first

[tip-bot2 for Pu Wen]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     009767dbf42ac0dbe3cf48c1ee224f6b778aa85a
Gitweb:        https://git.kernel.org/tip/009767dbf42ac0dbe3cf48c1ee224f6b778aa85a
Author:        Pu Wen <puwen@xxxxxxxx>
AuthorDate:    Wed, 02 Jun 2021 15:02:07 +08:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Fri, 04 Jun 2021 18:39:09 +02:00

x86/sev: Check SME/SEV support in CPUID first

The first two bits of the CPUID leaf 0x8000001F EAX indicate whether SEV
or SME is supported, respectively. It's better to check whether SEV or
SME is actually supported before accessing the MSR_AMD64_SEV to check
whether SEV or SME is enabled.

This is both a bare-metal issue and a guest/VM issue. Since the first
generation Hygon Dhyana CPU doesn't support the MSR_AMD64_SEV, reading that
MSR results in a #GP - either directly from hardware in the bare-metal
case or via the hypervisor (because the RDMSR is actually intercepted)
in the guest/VM case, resulting in a failed boot. And since this is very
early in the boot phase, rdmsrl_safe()/native_read_msr_safe() can't be
used.

So check the CPUID bits first, before accessing the MSR.

 [ tlendacky: Expand and improve commit message. ]
 [ bp: Massage commit message. ]

Fixes: eab696d8e8b9 ("x86/sev: Do not require Hypervisor CPUID bit for SEV guests")
Signed-off-by: Pu Wen <puwen@xxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
Link: https://lkml.kernel.org/r/20210602070207.2480-1-puwen@xxxxxxxx
---
 arch/x86/mm/mem_encrypt_identity.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index a9639f6..470b202 100644
--- a/arch/x86/mm/mem_encrypt_identity.c
+++ b/arch/x86/mm/mem_encrypt_identity.c
@@ -504,10 +504,6 @@ void __init sme_enable(struct boot_params *bp)
 #define AMD_SME_BIT	BIT(0)
 #define AMD_SEV_BIT	BIT(1)
 
-	/* Check the SEV MSR whether SEV or SME is enabled */
-	sev_status   = __rdmsr(MSR_AMD64_SEV);
-	feature_mask = (sev_status & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
-
 	/*
 	 * Check for the SME/SEV feature:
 	 *   CPUID Fn8000_001F[EAX]
@@ -519,11 +515,16 @@ void __init sme_enable(struct boot_params *bp)
 	eax = 0x8000001f;
 	ecx = 0;
 	native_cpuid(&eax, &ebx, &ecx, &edx);
-	if (!(eax & feature_mask))
+	/* Check whether SEV or SME is supported */
+	if (!(eax & (AMD_SEV_BIT | AMD_SME_BIT)))
 		return;
 
 	me_mask = 1UL << (ebx & 0x3f);
 
+	/* Check the SEV MSR whether SEV or SME is enabled */
+	sev_status   = __rdmsr(MSR_AMD64_SEV);
+	feature_mask = (sev_status & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
+
 	/* Check if memory encryption is enabled */
 	if (feature_mask == AMD_SME_BIT) {
 		/*