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AJvYcCW5p7CBymMSgLMbjWc+4XhJwFbz8N/+Zzuvu/c/55zzYbNZvoCJrFtsvdXIHyr9Ill43H1PgdZDl+pDHrc=@vger.kernel.org X-Gm-Message-State: AOJu0YzhRObST0xer+JBN/HvcrXqn1T2ZGrYfIB4V2kxrghIG1HV0UdJ tW0F3jBHXP0kbb54JRpqg5M51DZI8Xc0umMscP/2Jduimhqthl2qf05DqQrMTpOgn8OIdhMlwSq ircRUpQ== X-Google-Smtp-Source: AGHT+IFE/67yba6hPA+b8vLirH7hId58WfEp5916YRvDTSOyjQ2NfnsTDXh1javTa5MMXSzGv8biAwA4laI= X-Received: from pjbiq12.prod.google.com ([2002:a17:90a:fb4c:b0:34e:9b4f:a5a6]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:5287:b0:343:7714:4caa with SMTP id 98e67ed59e1d1-34f68b4c72dmr7671978a91.3.1767932130761; Thu, 08 Jan 2026 20:15:30 -0800 (PST) Reply-To: Sean Christopherson Date: Thu, 8 Jan 2026 20:15:22 -0800 In-Reply-To: <20260109041523.1027323-1-seanjc@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260109041523.1027323-1-seanjc@google.com> X-Mailer: git-send-email 2.52.0.457.g6b5491de43-goog Message-ID: <20260109041523.1027323-4-seanjc@google.com> Subject: [PATCH v3 3/4] KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware" From: Sean Christopherson To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Xiaoyao Li , Chao Gao , Xin Li , Yosry Ahmed Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Disallow access (VMREAD/VMWRITE), both emulated and via a shadow VMCS, to VMCS fields that the loaded incarnation of KVM doesn't support, e.g. due to lack of hardware support, as a middle ground between allowing access to any vmcs12 field defined by KVM (current behavior) and gating access based on the userspace-defined vCPU model (the most functionally correct, but very costly, implementation). Disallowing access to unsupported fields helps a tiny bit in terms of closing the virtualization hole (see below), but the main motivation is to avoid having to weed out unsupported fields when synchronizing between vmcs12 and a shadow VMCS. Because shadow VMCS accesses are done via VMREAD and VMWRITE, KVM _must_ filter out unsupported fields (or eat VMREAD/VMWRITE failures), and filtering out just shadow VMCS fields is about the same amount of effort, and arguably much more confusing. As a bonus, this also fixes a KVM-Unit-Test failure bug when running on _hardware_ without support for TSC Scaling, which fails with the same signature as the bug fixed by commit ba1f82456ba8 ("KVM: nVMX: Dynamically compute max VMCS index for vmcs12"): FAIL: VMX_VMCS_ENUM.MAX_INDEX expected: 19, actual: 17 Dynamically computing the max VMCS index only resolved the issue where KVM was hardcoding max index, but for CPUs with TSC Scaling, that was "good enough". Reviewed-by: Chao Gao Reviewed-by: Xin Li Cc: Yosry Ahmed Link: https://lore.kernel.org/all/20251026201911.505204-22-xin@zytor.com Link: https://lore.kernel.org/all/YR2Tf9WPNEzrE7Xg@google.com Signed-off-by: Sean Christopherson --- arch/x86/kvm/vmx/nested.c | 15 +++++---- arch/x86/kvm/vmx/vmcs.h | 8 +++++ arch/x86/kvm/vmx/vmcs12.c | 70 +++++++++++++++++++++++++++++++++++++-- arch/x86/kvm/vmx/vmcs12.h | 6 ++-- 4 files changed, 89 insertions(+), 10 deletions(-) diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index 61113ead3d7b..ac7a17560c8f 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -111,6 +111,9 @@ static void init_vmcs_shadow_fields(void) field <=3D GUEST_TR_AR_BYTES, "Update vmcs12_write_any() to drop reserved bits from AR_BYTES"); =20 + if (get_vmcs12_field_offset(field) < 0) + continue; + /* * PML and the preemption timer can be emulated, but the * processor cannot vmwrite to fields that don't exist @@ -7074,12 +7077,6 @@ void nested_vmx_set_vmcs_shadowing_bitmap(void) } } =20 -/* - * Indexing into the vmcs12 uses the VMCS encoding rotated left by 6. Undo - * that madness to get the encoding for comparison. - */ -#define VMCS12_IDX_TO_ENC(idx) ((u16)(((u16)(idx) >> 6) | ((u16)(idx) << 1= 0))) - static u64 nested_vmx_calc_vmcs_enum_msr(void) { /* @@ -7407,6 +7404,12 @@ __init int nested_vmx_hardware_setup(int (*exit_hand= lers[])(struct kvm_vcpu *)) { int i; =20 + /* + * Note! The set of supported vmcs12 fields is consumed by both VMX + * MSR and shadow VMCS setup. + */ + nested_vmx_setup_vmcs12_fields(); + nested_vmx_setup_ctls_msrs(&vmcs_config, vmx_capability.ept); =20 if (!cpu_has_vmx_shadow_vmcs()) diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h index 9aa204c87661..66d747e265b1 100644 --- a/arch/x86/kvm/vmx/vmcs.h +++ b/arch/x86/kvm/vmx/vmcs.h @@ -11,7 +11,15 @@ =20 #include "capabilities.h" =20 +/* + * Indexing into the vmcs12 uses the VMCS encoding rotated left by 6 as a = very + * rudimentary compression of the range of indices. The compression ratio= is + * good enough to allow KVM to use a (very sparsely populated) array witho= ut + * wasting too much memory, while the "algorithm" is fast enough to be use= d to + * lookup vmcs12 fields on-demand, e.g. for emulation. + */ #define ROL16(val, n) ((u16)(((u16)(val) << (n)) | ((u16)(val) >> (16 - (n= ))))) +#define VMCS12_IDX_TO_ENC(idx) ROL16(idx, 10) #define ENC_TO_VMCS12_IDX(enc) ROL16(enc, 6) =20 struct vmcs_hdr { diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c index c2ac9e1a50b3..1ebe67c384ad 100644 --- a/arch/x86/kvm/vmx/vmcs12.c +++ b/arch/x86/kvm/vmx/vmcs12.c @@ -9,7 +9,7 @@ FIELD(number, name), \ [ENC_TO_VMCS12_IDX(number##_HIGH)] =3D VMCS12_OFFSET(name) + sizeof(u32) =20 -const unsigned short vmcs12_field_offsets[] =3D { +static const u16 kvm_supported_vmcs12_field_offsets[] __initconst =3D { FIELD(VIRTUAL_PROCESSOR_ID, virtual_processor_id), FIELD(POSTED_INTR_NV, posted_intr_nv), FIELD(GUEST_ES_SELECTOR, guest_es_selector), @@ -158,4 +158,70 @@ const unsigned short vmcs12_field_offsets[] =3D { FIELD(HOST_SSP, host_ssp), FIELD(HOST_INTR_SSP_TABLE, host_ssp_tbl), }; -const unsigned int nr_vmcs12_fields =3D ARRAY_SIZE(vmcs12_field_offsets); + +u16 vmcs12_field_offsets[ARRAY_SIZE(kvm_supported_vmcs12_field_offsets)] _= _ro_after_init; +unsigned int nr_vmcs12_fields __ro_after_init; + +#define VMCS12_CASE64(enc) case enc##_HIGH: case enc + +static __init bool cpu_has_vmcs12_field(unsigned int idx) +{ + switch (VMCS12_IDX_TO_ENC(idx)) { + case VIRTUAL_PROCESSOR_ID: + return cpu_has_vmx_vpid(); + case POSTED_INTR_NV: + return cpu_has_vmx_posted_intr(); + VMCS12_CASE64(TSC_MULTIPLIER): + return cpu_has_vmx_tsc_scaling(); + case TPR_THRESHOLD: + VMCS12_CASE64(VIRTUAL_APIC_PAGE_ADDR): + return cpu_has_vmx_tpr_shadow(); + VMCS12_CASE64(APIC_ACCESS_ADDR): + return cpu_has_vmx_virtualize_apic_accesses(); + VMCS12_CASE64(POSTED_INTR_DESC_ADDR): + return cpu_has_vmx_posted_intr(); + case GUEST_INTR_STATUS: + return cpu_has_vmx_virtual_intr_delivery(); + VMCS12_CASE64(VM_FUNCTION_CONTROL): + VMCS12_CASE64(EPTP_LIST_ADDRESS): + return cpu_has_vmx_vmfunc(); + VMCS12_CASE64(EPT_POINTER): + return cpu_has_vmx_ept(); + VMCS12_CASE64(XSS_EXIT_BITMAP): + return cpu_has_vmx_xsaves(); + VMCS12_CASE64(ENCLS_EXITING_BITMAP): + return cpu_has_vmx_encls_vmexit(); + VMCS12_CASE64(GUEST_IA32_PERF_GLOBAL_CTRL): + VMCS12_CASE64(HOST_IA32_PERF_GLOBAL_CTRL): + return cpu_has_load_perf_global_ctrl(); + case SECONDARY_VM_EXEC_CONTROL: + return cpu_has_secondary_exec_ctrls(); + case GUEST_S_CET: + case GUEST_SSP: + case GUEST_INTR_SSP_TABLE: + case HOST_S_CET: + case HOST_SSP: + case HOST_INTR_SSP_TABLE: + return cpu_has_load_cet_ctrl(); + + /* KVM always emulates PML and the VMX preemption timer in software. */ + case GUEST_PML_INDEX: + case VMX_PREEMPTION_TIMER_VALUE: + default: + return true; + } +} + +void __init nested_vmx_setup_vmcs12_fields(void) +{ + unsigned int i; + + for (i =3D 0; i < ARRAY_SIZE(kvm_supported_vmcs12_field_offsets); i++) { + if (!kvm_supported_vmcs12_field_offsets[i] || + !cpu_has_vmcs12_field(i)) + continue; + + vmcs12_field_offsets[i] =3D kvm_supported_vmcs12_field_offsets[i]; + nr_vmcs12_fields =3D i + 1; + } +} diff --git a/arch/x86/kvm/vmx/vmcs12.h b/arch/x86/kvm/vmx/vmcs12.h index 7a5fdd9b27ba..21cd1b75e4fd 100644 --- a/arch/x86/kvm/vmx/vmcs12.h +++ b/arch/x86/kvm/vmx/vmcs12.h @@ -374,8 +374,10 @@ static inline void vmx_check_vmcs12_offsets(void) CHECK_OFFSET(guest_pml_index, 996); } =20 -extern const unsigned short vmcs12_field_offsets[]; -extern const unsigned int nr_vmcs12_fields; +extern u16 vmcs12_field_offsets[] __ro_after_init; +extern unsigned int nr_vmcs12_fields __ro_after_init; + +void __init nested_vmx_setup_vmcs12_fields(void); =20 static inline short get_vmcs12_field_offset(unsigned long field) { --=20 2.52.0.457.g6b5491de43-goog