[PATCH v15 17/41] KVM: VMX: Set host constant supervisor states to VMCS fields

Sean Christopherson
2025-09-12T23:23:55Z

From: Yang Weijiang <weijiang.yang@intel.com>

Save constant values to HOST_{S_CET,SSP,INTR_SSP_TABLE} field explicitly.
Kernel IBT is supported and the setting in MSR_IA32_S_CET is static after
post-boot(The exception is BIOS call case but vCPU thread never across it)
and KVM doesn't need to refresh HOST_S_CET field before every VM-Enter/
VM-Exit sequence.

Host supervisor shadow stack is not enabled now and SSP is not accessible
to kernel mode, thus it's safe to set host IA32_INT_SSP_TAB/SSP VMCS field
to 0s. When shadow stack is enabled for CPL3, SSP is reloaded from PL3_SSP
before it exits to userspace. Check SDM Vol 2A/B Chapter 3/4 for SYSCALL/
SYSRET/SYSENTER SYSEXIT/RDSSP/CALL etc.

Prevent KVM module loading if host supervisor shadow stack SHSTK_EN is set
in MSR_IA32_S_CET as KVM cannot co-exit with it correctly.

Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
[sean: snapshot host S_CET if SHSTK *or* IBT is supported]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/capabilities.h | 4 ++++
arch/x86/kvm/vmx/vmx.c | 15 +++++++++++++++
arch/x86/kvm/x86.c | 12 ++++++++++++
arch/x86/kvm/x86.h | 1 +
4 files changed, 32 insertions(+)

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 5316c27f6099..7d290b2cb0f4 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -103,6 +103,10 @@ static inline bool cpu_has_load_perf_global_ctrl(void)
return vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL;
}

+static inline bool cpu_has_load_cet_ctrl(void)
+{
+ return (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_CET_STATE);
+}
static inline bool cpu_has_vmx_mpx(void)
{
return vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_BNDCFGS;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index adf5af30e537..e8155635cb42 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4320,6 +4320,21 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)

if (cpu_has_load_ia32_efer())
vmcs_write64(HOST_IA32_EFER, kvm_host.efer);
+
+ /*
+ * Supervisor shadow stack is not enabled on host side, i.e.,
+ * host IA32_S_CET.SHSTK_EN bit is guaranteed to 0 now, per SDM
+ * description(RDSSP instruction), SSP is not readable in CPL0,
+ * so resetting the two registers to 0s at VM-Exit does no harm
+ * to kernel execution. When execution flow exits to userspace,
+ * SSP is reloaded from IA32_PL3_SSP. Check SDM Vol.2A/B Chapter
+ * 3 and 4 for details.
+ */
+ if (cpu_has_load_cet_ctrl()) {
+ vmcs_writel(HOST_S_CET, kvm_host.s_cet);
+ vmcs_writel(HOST_SSP, 0);
+ vmcs_writel(HOST_INTR_SSP_TABLE, 0);
+ }
}

void set_cr4_guest_host_mask(struct vcpu_vmx *vmx)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0b67b1b0e361..15f208c44cbd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9982,6 +9982,18 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
return -EIO;
}

+ if (boot_cpu_has(X86_FEATURE_SHSTK) || boot_cpu_has(X86_FEATURE_IBT)) {
+ rdmsrq(MSR_IA32_S_CET, kvm_host.s_cet);
+ /*
+ * Linux doesn't yet support supervisor shadow stacks (SSS), so
+ * KVM doesn't save/restore the associated MSRs, i.e. KVM may
+ * clobber the host values. Yell and refuse to load if SSS is
+ * unexpectedly enabled, e.g. to avoid crashing the host.
+ */
+ if (WARN_ON_ONCE(kvm_host.s_cet & CET_SHSTK_EN))
+ return -EIO;
+ }
+
memset(&kvm_caps, 0, sizeof(kvm_caps));

x86_emulator_cache = kvm_alloc_emulator_cache();
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 076eccba0f7e..65cbd454c4f1 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -50,6 +50,7 @@ struct kvm_host_values {
u64 efer;
u64 xcr0;
u64 xss;
+ u64 s_cet;
u64 arch_capabilities;
};

--
2.51.0.384.g4c02a37b29-goog