x86/sev: Register SEV-SNP guest request platform device

Version 2 of the GHCB specification provides a Non Automatic Exit (NAE)
event type that can be used by the SEV-SNP guest to communicate with the
PSP without risk from a malicious hypervisor who wishes to read, alter,
drop or replay the messages sent.

SNP_LAUNCH_UPDATE can insert two special pages into the guest’s memory:
the secrets page and the CPUID page. The PSP firmware populates the
contents of the secrets page. The secrets page contains encryption keys
used by the guest to interact with the firmware. Because the secrets
page is encrypted with the guest’s memory encryption key, the hypervisor
cannot read the keys. See SEV-SNP firmware spec for further details on
the secrets page format.

Create a platform device that the SEV-SNP guest driver can bind to get
the platform resources such as encryption key and message id to use to
communicate with the PSP. The SEV-SNP guest driver provides a userspace
interface to get the attestation report, key derivation, extended
attestation report etc.
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-43-brijesh.singh@amd.com

arch/x86/include/asm/sev.h

@@ -95,6 +95,10 @@ struct snp_req_data {
 	unsigned int data_npages;
 };
 
+struct snp_guest_platform_data {
+	u64 secrets_gpa;
+};
+
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 extern struct static_key_false sev_es_enable_key;
 extern void __sev_es_ist_enter(struct pt_regs *regs);

arch/x86/kernel/sev.c

@@ -19,6 +19,9 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/cpumask.h>
+#include <linux/efi.h>
+#include <linux/platform_device.h>
+#include <linux/io.h>
 
 #include <asm/cpu_entry_area.h>
 #include <asm/stacktrace.h>
@@ -2163,3 +2166,56 @@ int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned
 	return ret;
 }
 EXPORT_SYMBOL_GPL(snp_issue_guest_request);
+
+static struct platform_device guest_req_device = {
+	.name		= "snp-guest",
+	.id		= -1,
+};
+
+static u64 get_secrets_page(void)
+{
+	u64 pa_data = boot_params.cc_blob_address;
+	struct cc_blob_sev_info info;
+	void *map;
+
+	/*
+	 * The CC blob contains the address of the secrets page, check if the
+	 * blob is present.
+	 */
+	if (!pa_data)
+		return 0;
+
+	map = early_memremap(pa_data, sizeof(info));
+	memcpy(&info, map, sizeof(info));
+	early_memunmap(map, sizeof(info));
+
+	/* smoke-test the secrets page passed */
+	if (!info.secrets_phys || info.secrets_len != PAGE_SIZE)
+		return 0;
+
+	return info.secrets_phys;
+}
+
+static int __init snp_init_platform_device(void)
+{
+	struct snp_guest_platform_data data;
+	u64 gpa;
+
+	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
+		return -ENODEV;
+
+	gpa = get_secrets_page();
+	if (!gpa)
+		return -ENODEV;
+
+	data.secrets_gpa = gpa;
+	if (platform_device_add_data(&guest_req_device, &data, sizeof(data)))
+		return -ENODEV;
+
+	if (platform_device_register(&guest_req_device))
+		return -ENODEV;
+
+	pr_info("SNP guest platform device initialized.\n");
+	return 0;
+}
+device_initcall(snp_init_platform_device);