linux-kernel - [PATCH v2] KVM: SVM: Inhibit AVIC on SNP-enabled system without HvInUseWrAllowed feature

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Message-ID: <20241018085037.14131-1-suravee.suthikulpanit@amd.com>
Date: Fri, 18 Oct 2024 08:50:37 +0000
From: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To: <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
CC: <pbonzini@...hat.com>, <seanjc@...gle.com>, <joao.m.martins@...cle.com>,
	<david.kaplan@....com>, <jon.grimm@....com>, <santosh.shukla@....com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Subject: [PATCH v2] KVM: SVM: Inhibit AVIC on SNP-enabled system without HvInUseWrAllowed feature

On SNP-enabled system, VMRUN marks AVIC Backing Page as in-use while
the guest is running for both secure and non-secure guest. Any hypervisor
write to the in-use vCPU's AVIC backing page (e.g. to inject an interrupt)
will generate unexpected #PF in the host.

Currently, attempt to run AVIC guest would result in the following error:

    BUG: unable to handle page fault for address: ff3a442e549cc270
    #PF: supervisor write access in kernel mode
    #PF: error_code(0x80000003) - RMP violation
    PGD b6ee01067 P4D b6ee02067 PUD 10096d063 PMD 11c540063 PTE 80000001149cc163
    SEV-SNP: PFN 0x1149cc unassigned, dumping non-zero entries in 2M PFN region: [0x114800 - 0x114a00]
    ...

Newer AMD system is enhanced to allow hypervisor to modify the backing page
for non-secure guest on SNP-enabled system. This enhancement is available
when the CPUID Fn8000_001F_EAX bit 30 is set (HvInUseWrAllowed).

This table describes AVIC support matrix w.r.t. SNP enablement:

               | Non-SNP system |     SNP system
-----------------------------------------------------
 Non-SNP guest |  AVIC Activate | AVIC Activate iff
               |                | HvInuseWrAllowed=1
-----------------------------------------------------
     SNP guest |      N/A       |    Secure AVIC
               |                |    x2APIC only

Introduce APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED to deactivate AVIC
when the feature is not available on SNP-enabled system.

See the AMD64 Architecture Programmer’s Manual (APM) Volume 2 for detail.
(https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/
programmer-references/40332.pdf)

Fixes: 216d106c7ff7 ("x86/sev: Add SEV-SNP host initialization support")
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
---

Change log v2:
 * Use APICv inhibit bit instead of disabling AVIC in driver.

 arch/x86/include/asm/cpufeatures.h | 1 +
 arch/x86/include/asm/kvm_host.h    | 9 ++++++++-
 arch/x86/kvm/svm/avic.c            | 6 ++++++
 arch/x86/kvm/svm/svm.h             | 3 ++-
 4 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index dd4682857c12..921b6de80e24 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -448,6 +448,7 @@
 #define X86_FEATURE_SME_COHERENT	(19*32+10) /* AMD hardware-enforced cache coherency */
 #define X86_FEATURE_DEBUG_SWAP		(19*32+14) /* "debug_swap" AMD SEV-ES full debug state swap support */
 #define X86_FEATURE_SVSM		(19*32+28) /* "svsm" SVSM present */
+#define X86_FEATURE_HV_INUSE_WR_ALLOWED	(19*32+30) /* Write to in-use hypervisor-owned pages allowed */
 
 /* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
 #define X86_FEATURE_NO_NESTED_DATA_BP	(20*32+ 0) /* No Nested Data Breakpoints */
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4a68cb3eba78..1fef50025512 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1276,6 +1276,12 @@ enum kvm_apicv_inhibit {
 	 */
 	APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED,
 
+	/*
+	 * Non-SNP guest cannot activate AVIC on SNP-enabled system w/o
+	 * CPUID HvInUseWrAllowed feature.
+	 */
+	APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED,
+
 	NR_APICV_INHIBIT_REASONS,
 };
 
@@ -1294,7 +1300,8 @@ enum kvm_apicv_inhibit {
 	__APICV_INHIBIT_REASON(IRQWIN),			\
 	__APICV_INHIBIT_REASON(PIT_REINJ),		\
 	__APICV_INHIBIT_REASON(SEV),			\
-	__APICV_INHIBIT_REASON(LOGICAL_ID_ALIASED)
+	__APICV_INHIBIT_REASON(LOGICAL_ID_ALIASED),	\
+	__APICV_INHIBIT_REASON(HVINUSEWR_NOT_ALLOWED)
 
 struct kvm_arch {
 	unsigned long n_used_mmu_pages;
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 4b74ea91f4e6..cc4f0c00334a 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -202,6 +202,12 @@ int avic_vm_init(struct kvm *kvm)
 	if (!enable_apicv)
 		return 0;
 
+	if (cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
+	    !boot_cpu_has(X86_FEATURE_HV_INUSE_WR_ALLOWED)) {
+		pr_debug("APICv Inhibit due to Missing HvInUseWrAllowed.\n");
+		kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED);
+	}
+
 	/* Allocating physical APIC ID table (4KB) */
 	p_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
 	if (!p_page)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 76107c7d0595..13046bad2d6e 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -682,7 +682,8 @@ extern struct kvm_x86_nested_ops svm_nested_ops;
 	BIT(APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED) |	\
 	BIT(APICV_INHIBIT_REASON_APIC_ID_MODIFIED) |	\
 	BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED) |	\
-	BIT(APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED)	\
+	BIT(APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED) |	\
+	BIT(APICV_INHIBIT_REASON_HVINUSEWR_NOT_ALLOWED)	\
 )
 
 bool avic_hardware_setup(void);
-- 
2.34.1