x86/sev: Check for the presence of an SVSM in the SNP secrets page

During early boot phases, check for the presence of an SVSM when running
as an SEV-SNP guest.

An SVSM is present if not running at VMPL0 and the 64-bit value at offset
0x148 into the secrets page is non-zero. If an SVSM is present, save the
SVSM Calling Area address (CAA), located at offset 0x150 into the secrets
page, and set the VMPL level of the guest, which should be non-zero, to
indicate the presence of an SVSM.

  [ bp: Touchups. ]

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/9d3fe161be93d4ea60f43c2a3f2c311fe708b63b.1717600736.git.thomas.lendacky@amd.com

2 files changed, 83 insertions, 0 deletions

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index b4f8fa0f722c..06a5078150b5 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -23,6 +23,21 @@
 #define sev_printk_rtl(fmt, ...)
 #endif
 
+/*
+ * SVSM related information:
+ *   When running under an SVSM, the VMPL that Linux is executing at must be
+ *   non-zero. The VMPL is therefore used to indicate the presence of an SVSM.
+ *
+ *   During boot, the page tables are set up as identity mapped and later
+ *   changed to use kernel virtual addresses. Maintain separate virtual and
+ *   physical addresses for the CAA to allow SVSM functions to be used during
+ *   early boot, both with identity mapped virtual addresses and proper kernel
+ *   virtual addresses.
+ */
+static u8 snp_vmpl __ro_after_init;
+static struct svsm_ca *boot_svsm_caa __ro_after_init;
+static u64 boot_svsm_caa_pa __ro_after_init;
+
 /* I/O parameters for CPUID-related helpers */
 struct cpuid_leaf {
 	u32 fn;
@@ -1269,3 +1284,64 @@ static enum es_result vc_check_opcode_bytes(struct es_em_ctxt *ctxt,
 
 	return ES_UNSUPPORTED;
 }
+
+/*
+ * Maintain the GPA of the SVSM Calling Area (CA) in order to utilize the SVSM
+ * services needed when not running in VMPL0.
+ */
+static void __head svsm_setup_ca(const struct cc_blob_sev_info *cc_info)
+{
+	struct snp_secrets_page *secrets_page;
+	u64 caa;
+
+	BUILD_BUG_ON(sizeof(*secrets_page) != PAGE_SIZE);
+
+	/*
+	 * Check if running at VMPL0.
+	 *
+	 * Use RMPADJUST (see the rmpadjust() function for a description of what
+	 * the instruction does) to update the VMPL1 permissions of a page. If
+	 * the guest is running at VMPL0, this will succeed and implies there is
+	 * no SVSM. If the guest is running at any other VMPL, this will fail.
+	 * Linux SNP guests only ever run at a single VMPL level so permission mask
+	 * changes of a lesser-privileged VMPL are a don't-care.
+	 *
+	 * Use a rip-relative reference to obtain the proper address, since this
+	 * routine is running identity mapped when called, both by the decompressor
+	 * code and the early kernel code.
+	 */
+	if (!rmpadjust((unsigned long)&RIP_REL_REF(boot_ghcb_page), RMP_PG_SIZE_4K, 1))
+		return;
+
+	/*
+	 * Not running at VMPL0, ensure everything has been properly supplied
+	 * for running under an SVSM.
+	 */
+	if (!cc_info || !cc_info->secrets_phys || cc_info->secrets_len != PAGE_SIZE)
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SECRETS_PAGE);
+
+	secrets_page = (struct snp_secrets_page *)cc_info->secrets_phys;
+	if (!secrets_page->svsm_size)
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_NO_SVSM);
+
+	if (!secrets_page->svsm_guest_vmpl)
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SVSM_VMPL0);
+
+	RIP_REL_REF(snp_vmpl) = secrets_page->svsm_guest_vmpl;
+
+	caa = secrets_page->svsm_caa;
+
+	/*
+	 * An open-coded PAGE_ALIGNED() in order to avoid including
+	 * kernel-proper headers into the decompressor.
+	 */
+	if (caa & (PAGE_SIZE - 1))
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_SVSM_CAA);
+
+	/*
+	 * The CA is identity mapped when this routine is called, both by the
+	 * decompressor code and the early kernel code.
+	 */
+	RIP_REL_REF(boot_svsm_caa) = (struct svsm_ca *)caa;
+	RIP_REL_REF(boot_svsm_caa_pa) = caa;
+}
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 3342ed58e168..36a117a38b10 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2109,6 +2109,13 @@ bool __head snp_init(struct boot_params *bp)
 	setup_cpuid_table(cc_info);
 
 	/*
+	 * Record the SVSM Calling Area address (CAA) if the guest is not
+	 * running at VMPL0. The CA will be used to communicate with the
+	 * SVSM to perform the SVSM services.
+	 */
+	svsm_setup_ca(cc_info);
+
+	/*
 	 * The CC blob will be used later to access the secrets page. Cache
 	 * it here like the boot kernel does.
 	 */