[PATCH Part1 RFC v2 09/20] x86/sev: check SEV-SNP features support

[Brijesh Singh <brijesh.singh@amd.com>]

Version 2 of the GHCB specification added the advertisement of features
that are supported by the hypervisor. If hypervisor supports the SEV-SNP
then it must set the SEV-SNP features bit to indicate that the base
SEV-SNP is supported.

Check the SEV-SNP feature while establishing the GHCB, if failed,
terminate the guest.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 arch/x86/boot/compressed/sev.c | 21 +++++++++++++++++++++
 arch/x86/kernel/sev-shared.c   | 13 ++++++++++++-
 arch/x86/kernel/sev.c          |  4 ++++
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 6d9055427f37..7badbeb6cb95 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -25,6 +25,8 @@
 
 struct ghcb boot_ghcb_page __aligned(PAGE_SIZE);
 struct ghcb *boot_ghcb;
+static u64 sev_status_val;
+static bool sev_status_checked;
 
 /*
  * Copy a version of this function here - insn-eval.c can't be used in
@@ -119,11 +121,30 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 /* Include code for early handlers */
 #include "../../kernel/sev-shared.c"
 
+static inline bool sev_snp_enabled(void)
+{
+	unsigned long low, high;
+
+	if (!sev_status_checked) {
+		asm volatile("rdmsr\n"
+			     : "=a" (low), "=d" (high)
+			     : "c" (MSR_AMD64_SEV));
+		sev_status_val = (high << 32) | low;
+		sev_status_checked = true;
+	}
+
+	return sev_status_val & MSR_AMD64_SEV_SNP_ENABLED ? true : false;
+}
+
 static bool early_setup_sev_es(void)
 {
 	if (!sev_es_negotiate_protocol())
 		sev_es_terminate(0, GHCB_SEV_ES_REASON_PROTOCOL_UNSUPPORTED);
 
+	/* If SEV-SNP is enabled then check if the hypervisor supports the SEV-SNP features. */
+	if (sev_snp_enabled() && !sev_snp_check_hypervisor_features())
+		sev_es_terminate(0, GHCB_SEV_ES_REASON_SNP_UNSUPPORTED);
+
 	if (set_page_decrypted((unsigned long)&boot_ghcb_page))
 		return false;
 
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 3f9b06a04395..085d3d724bc8 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -32,7 +32,18 @@ static bool __init sev_es_check_cpu_features(void)
 	return true;
 }
 
-+static void __noreturn sev_es_terminate(unsigned int set, unsigned int reason)
+static bool __init sev_snp_check_hypervisor_features(void)
+{
+	if (ghcb_version < 2)
+		return false;
+
+	if (!(hv_features & GHCB_HV_FEATURES_SNP))
+		return false;
+
+	return true;
+}
+
+static void __noreturn sev_es_terminate(unsigned int set, unsigned int reason)
 {
 	u64 val = GHCB_MSR_TERM_REQ;
 
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 97be0fe666ab..8c8c939a1754 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -609,6 +609,10 @@ static bool __init sev_es_setup_ghcb(void)
 	if (!sev_es_negotiate_protocol())
 		return false;
 
+	/* If SNP is active, make sure that hypervisor supports the feature. */
+	if (sev_snp_active() && !sev_snp_check_hypervisor_features())
+		sev_es_terminate(0, GHCB_SEV_ES_REASON_SNP_UNSUPPORTED);
+
 	/*
 	 * Clear the boot_ghcb. The first exception comes in before the bss
 	 * section is cleared.
-- 
2.17.1