[PATCH Part1 RFC v3 06/22] x86/sev: check SEV-SNP features support

[Brijesh Singh <brijesh.singh@xxxxxxx>]

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@xxxxxxx>
---
 arch/x86/boot/compressed/sev.c    | 22 ++++++++++++++++++++++
 arch/x86/include/asm/sev-common.h |  3 +++
 arch/x86/kernel/sev-shared.c      | 11 +++++++++++
 arch/x86/kernel/sev.c             |  4 ++++
 4 files changed, 40 insertions(+)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 87621f4e4703..0745ea61d32e 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -25,6 +25,7 @@
 
 struct ghcb boot_ghcb_page __aligned(PAGE_SIZE);
 struct ghcb *boot_ghcb;
+static u64 msr_sev_status;
 
 /*
  * Copy a version of this function here - insn-eval.c can't be used in
@@ -119,11 +120,32 @@ 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 (!msr_sev_status) {
+		asm volatile("rdmsr\n"
+			     : "=a" (low), "=d" (high)
+			     : "c" (MSR_AMD64_SEV));
+		msr_sev_status = (high << 32) | low;
+	}
+
+	return msr_sev_status & MSR_AMD64_SEV_SNP_ENABLED;
+}
+
 static bool early_setup_sev_es(void)
 {
 	if (!sev_es_negotiate_protocol())
 		sev_es_terminate(0, GHCB_SEV_ES_PROT_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_SNP_UNSUPPORTED);
+
 	if (set_page_decrypted((unsigned long)&boot_ghcb_page))
 		return false;
 
diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index 981fff2257b9..3ebf00772f26 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -51,6 +51,8 @@
 #define GHCB_MSR_HV_FT_POS	12
 #define GHCB_MSR_HV_FT_MASK	GENMASK_ULL(51, 0)
 
+#define GHCB_HV_FT_SNP		BIT_ULL(0)
+
 #define GHCB_MSR_HV_FT_RESP_VAL(v)	\
 	(((unsigned long)((v) & GHCB_MSR_HV_FT_MASK) >> GHCB_MSR_HV_FT_POS))
 
@@ -65,6 +67,7 @@
 
 #define GHCB_SEV_ES_GEN_REQ		0
 #define GHCB_SEV_ES_PROT_UNSUPPORTED	1
+#define GHCB_SEV_ES_SNP_UNSUPPORTED	2
 
 #define GHCB_RESP_CODE(v)		((v) & GHCB_MSR_INFO_MASK)
 
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 94957c5bdb51..b8312ad66120 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -32,6 +32,17 @@ static bool __init sev_es_check_cpu_features(void)
 	return true;
 }
 
+static bool __init sev_snp_check_hypervisor_features(void)
+{
+	if (ghcb_version < 2)
+		return false;
+
+	if (!(hv_features & GHCB_HV_FT_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 77a754365ba9..9b70b7332614 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_feature_enabled(SEV_SNP) && !sev_snp_check_hypervisor_features())
+		sev_es_terminate(0, GHCB_SEV_ES_SNP_UNSUPPORTED);
+
 	/*
 	 * Clear the boot_ghcb. The first exception comes in before the bss
 	 * section is cleared.
-- 
2.17.1