x86/sev: Check SEV-SNP features support

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

Check that feature bit while establishing the GHCB; if failed, terminate
the guest.

Version 2 of the GHCB specification adds several new Non-Automatic Exits
(NAEs), most of them are optional except the hypervisor feature. Now
that the hypervisor feature NAE is implemented, bump the GHCB maximum
supported protocol version.

While at it, move the GHCB protocol negotiation check from the #VC
exception handler to sev_enable() so that all feature detection happens
before the first #VC exception.

While at it, document why the GHCB page cannot be setup from
load_stage2_idt().

  [ 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/20220307213356.2797205-13-brijesh.singh@amd.com

1 files changed, 15 insertions, 5 deletions

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 56e941d5e092..5b389310be87 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -116,11 +116,8 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 /* Include code for early handlers */
 #include "../../kernel/sev-shared.c"
 
-static bool early_setup_sev_es(void)
+static bool early_setup_ghcb(void)
 {
-	if (!sev_es_negotiate_protocol())
-		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_PROT_UNSUPPORTED);
-
 	if (set_page_decrypted((unsigned long)&boot_ghcb_page))
 		return false;
 
@@ -171,7 +168,7 @@ void do_boot_stage2_vc(struct pt_regs *regs, unsigned long exit_code)
 	struct es_em_ctxt ctxt;
 	enum es_result result;
 
-	if (!boot_ghcb && !early_setup_sev_es())
+	if (!boot_ghcb && !early_setup_ghcb())
 		sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
 
 	vc_ghcb_invalidate(boot_ghcb);
@@ -235,5 +232,18 @@ void sev_enable(struct boot_params *bp)
 	if (!(sev_status & MSR_AMD64_SEV_ENABLED))
 		return;
 
+	/* Negotiate the GHCB protocol version. */
+	if (sev_status & MSR_AMD64_SEV_ES_ENABLED) {
+		if (!sev_es_negotiate_protocol())
+			sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_PROT_UNSUPPORTED);
+	}
+
+	/*
+	 * 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);
+
 	sme_me_mask = BIT_ULL(ebx & 0x3f);
 }