Re: [PATCH v6 22/25] KVM: VMX: Set host constant supervisor states to VMCS fields

[Maxim Levitsky]

On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> 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@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 41a4533f9989..ee8938818c8a 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 30373258573d..9ccc2c552f55 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4375,6 +4375,21 @@ 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 c85ee42ab4f1..231d4a7b6f3d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -114,6 +114,8 @@ 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)
>  
> @@ -9618,6 +9620,18 @@ 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;
This is a good idea.

> +	}
> +
>  	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 9a8e3a84eaf4..0d5f673338dd 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -324,6 +324,7 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);
>  extern u64 host_xcr0;
>  extern u64 host_xss;
>  extern u64 host_arch_capabilities;
> +extern u64 host_s_cet;
>  
>  extern struct kvm_caps kvm_caps;
>  


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky