virt: sev-guest: Choose the VMPCK key based on executing VMPL

Currently, the sev-guest driver uses the vmpck-0 key by default. When an
SVSM is present, the kernel is running at a VMPL other than 0 and the
vmpck-0 key is no longer available. If a specific vmpck key has not be
requested by the user via the vmpck_id module parameter, choose the
vmpck key based on the active VMPL level.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/b88081c5d88263176849df8ea93e90a404619cab.1717600736.git.thomas.lendacky@amd.com

4 files changed, 36 insertions, 6 deletions

diff --git a/Documentation/virt/coco/sev-guest.rst b/Documentation/virt/coco/sev-guest.rst
index e1eaf6a830ce..9d00967a5b2b 100644
--- a/Documentation/virt/coco/sev-guest.rst
+++ b/Documentation/virt/coco/sev-guest.rst
@@ -204,6 +204,17 @@ has taken care to make use of the SEV-SNP CPUID throughout all stages of boot.
 Otherwise, guest owner attestation provides no assurance that the kernel wasn't
 fed incorrect values at some point during boot.
 
+4. SEV Guest Driver Communication Key
+=====================================
+
+Communication between an SEV guest and the SEV firmware in the AMD Secure
+Processor (ASP, aka PSP) is protected by a VM Platform Communication Key
+(VMPCK). By default, the sev-guest driver uses the VMPCK associated with the
+VM Privilege Level (VMPL) at which the guest is running. Should this key be
+wiped by the sev-guest driver (see the driver for reasons why a VMPCK can be
+wiped), a different key can be used by reloading the sev-guest driver and
+specifying the desired key using the vmpck_id module parameter.
+
 
 Reference
 ---------
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 1d0b1b23fafb..9c6f269d4bf5 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -237,6 +237,9 @@ struct svsm_call {
 #define SVSM_CORE_DELETE_VCPU		3
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT
+
+extern u8 snp_vmpl;
+
 extern void __sev_es_ist_enter(struct pt_regs *regs);
 extern void __sev_es_ist_exit(void);
 static __always_inline void sev_es_ist_enter(struct pt_regs *regs)
@@ -319,7 +322,10 @@ u64 snp_get_unsupported_features(u64 status);
 u64 sev_get_status(void);
 void sev_show_status(void);
 void snp_update_svsm_ca(void);
-#else
+
+#else	/* !CONFIG_AMD_MEM_ENCRYPT */
+
+#define snp_vmpl 0
 static inline void sev_es_ist_enter(struct pt_regs *regs) { }
 static inline void sev_es_ist_exit(void) { }
 static inline int sev_es_setup_ap_jump_table(struct real_mode_header *rmh) { return 0; }
@@ -349,7 +355,8 @@ static inline u64 snp_get_unsupported_features(u64 status) { return 0; }
 static inline u64 sev_get_status(void) { return 0; }
 static inline void sev_show_status(void) { }
 static inline void snp_update_svsm_ca(void) { }
-#endif
+
+#endif	/* CONFIG_AMD_MEM_ENCRYPT */
 
 #ifdef CONFIG_KVM_AMD_SEV
 bool snp_probe_rmptable_info(void);
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 2f509109c4f2..71de53194089 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -36,7 +36,8 @@
  *   early boot, both with identity mapped virtual addresses and proper kernel
  *   virtual addresses.
  */
-static u8 snp_vmpl __ro_after_init;
+u8 snp_vmpl __ro_after_init;
+EXPORT_SYMBOL_GPL(snp_vmpl);
 static struct svsm_ca *boot_svsm_caa __ro_after_init;
 static u64 boot_svsm_caa_pa __ro_after_init;
 
diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 654290a8e1ba..4597042f31e4 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -2,7 +2,7 @@
 /*
  * AMD Secure Encrypted Virtualization (SEV) guest driver interface
  *
- * Copyright (C) 2021 Advanced Micro Devices, Inc.
+ * Copyright (C) 2021-2024 Advanced Micro Devices, Inc.
  *
  * Author: Brijesh Singh <brijesh.singh@amd.com>
  */
@@ -70,8 +70,15 @@ struct snp_guest_dev {
 	u8 *vmpck;
 };
 
-static u32 vmpck_id;
-module_param(vmpck_id, uint, 0444);
+/*
+ * The VMPCK ID represents the key used by the SNP guest to communicate with the
+ * SEV firmware in the AMD Secure Processor (ASP, aka PSP). By default, the key
+ * used will be the key associated with the VMPL at which the guest is running.
+ * Should the default key be wiped (see snp_disable_vmpck()), this parameter
+ * allows for using one of the remaining VMPCKs.
+ */
+static int vmpck_id = -1;
+module_param(vmpck_id, int, 0444);
 MODULE_PARM_DESC(vmpck_id, "The VMPCK ID to use when communicating with the PSP.");
 
 /* Mutex to serialize the shared buffer access and command handling. */
@@ -923,6 +930,10 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	if (!snp_dev)
 		goto e_unmap;
 
+	/* Adjust the default VMPCK key based on the executing VMPL level */
+	if (vmpck_id == -1)
+		vmpck_id = snp_vmpl;
+
 	ret = -EINVAL;
 	snp_dev->vmpck = get_vmpck(vmpck_id, secrets, &snp_dev->os_area_msg_seqno);
 	if (!snp_dev->vmpck) {