x86/sev: Handle failures from snp_init()

Address the ignored failures from snp_init() in sme_enable(). Add error
handling for scenarios where snp_init() fails to retrieve the SEV-SNP CC
blob or encounters issues while parsing the CC blob. Ensure that SNP guests
will error out early, preventing delayed error reporting or undefined
behavior.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20241009092850.197575-3-nikunj@amd.com

Author: nikunjad

arch/x86/mm/mem_encrypt_identity.c

@@ -495,10 +495,10 @@ void __head sme_enable(struct boot_params *bp)
 	unsigned int eax, ebx, ecx, edx;
 	unsigned long feature_mask;
 	unsigned long me_mask;
-	bool snp;
+	bool snp_en;
 	u64 msr;
 
-	snp = snp_init(bp);
+	snp_en = snp_init(bp);
 
 	/* Check for the SME/SEV support leaf */
 	eax = 0x80000000;
@@ -531,8 +531,11 @@ void __head sme_enable(struct boot_params *bp)
 	RIP_REL_REF(sev_status) = msr = __rdmsr(MSR_AMD64_SEV);
 	feature_mask = (msr & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
 
-	/* The SEV-SNP CC blob should never be present unless SEV-SNP is enabled. */
-	if (snp && !(msr & MSR_AMD64_SEV_SNP_ENABLED))
+	/*
+	 * Any discrepancies between the presence of a CC blob and SNP
+	 * enablement abort the guest.
+	 */
+	if (snp_en ^ !!(msr & MSR_AMD64_SEV_SNP_ENABLED))
 		snp_abort();
 
 	/* Check if memory encryption is enabled */