x86/sev: Check the VMPL level

The 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 hardware isolated abstraction
layers within a VM. VMPL0 is the highest privilege level, and VMPL3 is
the least privilege level. 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 SNP support requires that the guest kernel is running at
VMPL0. Add such a check to verify the guest is running at level 0 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.

  [ bp: Massage commit message. ]

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-15-brijesh.singh@amd.com

1 files changed, 26 insertions, 2 deletions

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 5b389310be87..eb421787d50a 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -199,6 +199,26 @@ finish:
 		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
 }
 
+static void enforce_vmpl0(void)
+{
+	u64 attrs;
+	int err;
+
+	/*
+	 * RMPADJUST modifies RMP permissions of a lesser-privileged (numerically
+	 * higher) privilege level. Here, clear the VMPL1 permission mask of the
+	 * GHCB page. If the guest is not running at VMPL0, this will fail.
+	 *
+	 * If the guest is running at VMPL0, it will succeed. Even if that operation
+	 * modifies permission bits, it is still ok to do so currently because Linux
+	 * SNP guests are supported only on VMPL0 so VMPL1 or higher permission masks
+	 * changing is a don't-care.
+	 */
+	attrs = 1;
+	if (rmpadjust((unsigned long)&boot_ghcb_page, RMP_PG_SIZE_4K, attrs))
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NOT_VMPL0);
+}
+
 void sev_enable(struct boot_params *bp)
 {
 	unsigned int eax, ebx, ecx, edx;
@@ -242,8 +262,12 @@ void sev_enable(struct boot_params *bp)
 	 * SNP is supported in v2 of the GHCB spec which mandates support for HV
 	 * features.
 	 */
-	if (sev_status & MSR_AMD64_SEV_SNP_ENABLED && !(get_hv_features() & GHCB_HV_FT_SNP))
-		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SNP_UNSUPPORTED);
+	if (sev_status & MSR_AMD64_SEV_SNP_ENABLED) {
+		if (!(get_hv_features() & GHCB_HV_FT_SNP))
+			sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SNP_UNSUPPORTED);
+
+		enforce_vmpl0();
+	}
 
 	sme_me_mask = BIT_ULL(ebx & 0x3f);
 }