[PATCH v5 14/19] KVM:VMX: Set host constant supervisor states to VMCS fields

From: Yang Weijiang
Date: Thu Aug 03 2023 - 03:38:14 EST


Set constant values to HOST_{S_CET,SSP,INTR_SSP_TABLE} VMCS
fields explicitly. Kernel IBT is supported and the setting in
MSR_IA32_S_CET is static after post-boot(except is BIOS call
case but vCPU thread never across it.), i.e. 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 fields to 0s. When shadow stack is enabled for
CPL3, SSP is reloaded from IA32_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 and 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@xxxxxxxxxx>
Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
arch/x86/kvm/vmx/capabilities.h | 4 ++++
arch/x86/kvm/vmx/vmx.c | 15 +++++++++++++++
arch/x86/kvm/x86.c | 14 ++++++++++++++
arch/x86/kvm/x86.h | 1 +
4 files changed, 34 insertions(+)

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index d0abee35d7ba..b1883f6c08eb 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -106,6 +106,10 @@ 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 6779b8a63789..99bf63b2a779 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4341,6 +4341,21 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)

if (cpu_has_load_ia32_efer())
vmcs_write64(HOST_IA32_EFER, 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, 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 56aa5a3d3913..01b4f10fa8ab 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -113,6 +113,8 @@ static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
#endif

static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
+u64 __read_mostly host_s_cet;
+EXPORT_SYMBOL_GPL(host_s_cet);

#define KVM_EXIT_HYPERCALL_VALID_MASK (1 << KVM_HC_MAP_GPA_RANGE)

@@ -9615,6 +9617,18 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
return -EIO;
}

+ if (boot_cpu_has(X86_FEATURE_SHSTK)) {
+ rdmsrl(MSR_IA32_S_CET, 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(host_s_cet & CET_SHSTK_EN))
+ return -EIO;
+ }
+
x86_emulator_cache = kvm_alloc_emulator_cache();
if (!x86_emulator_cache) {
pr_err("failed to allocate cache for x86 emulator\n");
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 3b79d6db2f83..e42e5263fcf7 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -323,6 +323,7 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);

extern u64 host_xcr0;
extern u64 host_xss;
+extern u64 host_s_cet;

extern struct kvm_caps kvm_caps;

--
2.27.0