[RFC PATCH 05/12] x86/sev: Use GHCB protocol version 2 if supported

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

Check whether the hypervisor supports GHCB version 2 and use it if
available.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/boot/compressed/sev.c | 10 ++++++++--
 arch/x86/include/asm/sev.h     |  4 ++--
 arch/x86/kernel/sev-shared.c   | 17 ++++++++++++++---
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 101e08c67296..7f8416f76be7 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -119,16 +119,22 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 /* Include code for early handlers */
 #include "../../kernel/sev-shared.c"
 
+static unsigned int ghcb_protocol;
+
 static u64 sev_get_ghcb_proto_ver(void)
 {
-	return GHCB_PROTOCOL_MAX;
+	return ghcb_protocol;
 }
 
 static bool early_setup_sev_es(void)
 {
-	if (!sev_es_negotiate_protocol(NULL))
+	struct sev_ghcb_protocol_info info;
+
+	if (!sev_es_negotiate_protocol(&info))
 		sev_es_terminate(GHCB_SEV_ES_REASON_PROTOCOL_UNSUPPORTED);
 
+	ghcb_protocol = info.vm_proto;
+
 	if (set_page_decrypted((unsigned long)&boot_ghcb_page))
 		return false;
 
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index fa5cd05d3b5b..134a7c9d91b6 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -12,8 +12,8 @@
 #include <asm/insn.h>
 #include <asm/sev-common.h>
 
-#define GHCB_PROTO_OUR		0x0001UL
-#define GHCB_PROTOCOL_MAX	1ULL
+#define GHCB_PROTOCOL_MIN	1ULL
+#define GHCB_PROTOCOL_MAX	2ULL
 #define GHCB_DEFAULT_USAGE	0ULL
 
 #define	VMGEXIT()			{ asm volatile("rep; vmmcall\n\r"); }
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 36eaac2773ed..40a1ca81bdb8 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -61,6 +61,7 @@ static void __noreturn sev_es_terminate(unsigned int reason)
 
 static bool sev_es_negotiate_protocol(struct sev_ghcb_protocol_info *info)
 {
+	unsigned int protocol;
 	u64 val;
 
 	/* Do the GHCB protocol version negotiation */
@@ -71,14 +72,24 @@ static bool sev_es_negotiate_protocol(struct sev_ghcb_protocol_info *info)
 	if (GHCB_MSR_INFO(val) != GHCB_MSR_SEV_INFO_RESP)
 		return false;
 
-	if (GHCB_MSR_PROTO_MAX(val) < GHCB_PROTO_OUR ||
-	    GHCB_MSR_PROTO_MIN(val) > GHCB_PROTO_OUR)
+	/* Sanity check untrusted input */
+	if (GHCB_MSR_PROTO_MIN(val) > GHCB_MSR_PROTO_MAX(val))
+		return false;
+
+	/* Use maximum supported protocol version */
+	protocol = min_t(unsigned int, GHCB_MSR_PROTO_MAX(val), GHCB_PROTOCOL_MAX);
+
+	/*
+	 * Hypervisor does not support any protocol version required for this
+	 * kernel.
+	 */
+	if (protocol < GHCB_MSR_PROTO_MIN(val))
 		return false;
 
 	if (info) {
 		info->hv_proto_min = GHCB_MSR_PROTO_MIN(val);
 		info->hv_proto_max = GHCB_MSR_PROTO_MAX(val);
-		info->vm_proto	   = GHCB_PROTO_OUR;
+		info->vm_proto	   = protocol;
 	}
 
 	return true;
-- 
2.31.1