[PATCH v6 39/42] x86/sev: Register SNP guest request platform device

[Brijesh Singh]

Version 2 of GHCB specification provides Non Automatic Exit (NAE) that can
be used by the 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 populate 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 SNP FW ABI spec for further details about the secrets page.

Create a platform device that the 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 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@xxxxxxx>
---
 arch/x86/include/asm/sev.h |  4 +++
 arch/x86/kernel/sev.c      | 61 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 4ea8e2f73d37..2a9e6ea11242 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -88,6 +88,10 @@ 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);
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c29a78f868ed..01505ac9c7b2 100644
--- a/arch/x86/kernel/sev.c
+++ b/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>
@@ -34,6 +37,7 @@
 #include <asm/cpu.h>
 #include <asm/apic.h>
 #include <asm/cpuid.h>
+#include <asm/setup.h>
 
 #define DR7_RESET_VALUE        0x400
 
@@ -2171,3 +2175,60 @@ 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 init_snp_platform_device(void)
+{
+	struct snp_guest_platform_data data;
+	u64 gpa;
+
+	if (!cc_platform_has(CC_ATTR_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)))
+		goto e_fail;
+
+	if (platform_device_register(&guest_req_device))
+		goto e_fail;
+
+	pr_info("SNP guest platform device initialized.\n");
+	return 0;
+
+e_fail:
+	pr_err("Failed to initialize SNP guest device\n");
+	return -ENODEV;
+}
+device_initcall(init_snp_platform_device);
-- 
2.25.1