diff mbox series

[v10,21/27] KVM: x86: Save and reload SSP to/from SMRAM

Message ID 20240219074733.122080-22-weijiang.yang@intel.com (mailing list archive)
State New, archived
Headers show
Series Enable CET Virtualization | expand

Commit Message

Yang, Weijiang Feb. 19, 2024, 7:47 a.m. UTC
Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
one of such registers on 64-bit Arch, and add the support for SSP. Note,
on 32-bit Arch, SSP is not defined in SMRAM, so fail 32-bit CET guest
launch.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/cpuid.c | 11 +++++++++++
 arch/x86/kvm/smm.c   |  8 ++++++++
 arch/x86/kvm/smm.h   |  2 +-
 3 files changed, 20 insertions(+), 1 deletion(-)

Comments

Sean Christopherson May 1, 2024, 10:50 p.m. UTC | #1
On Sun, Feb 18, 2024, Yang Weijiang wrote:
> Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
> behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
> at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
> one of such registers on 64-bit Arch, and add the support for SSP. Note,
> on 32-bit Arch, SSP is not defined in SMRAM, so fail 32-bit CET guest
> launch.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/cpuid.c | 11 +++++++++++
>  arch/x86/kvm/smm.c   |  8 ++++++++
>  arch/x86/kvm/smm.h   |  2 +-
>  3 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2bb1931103ad..c0e13040e35b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -149,6 +149,17 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
>  		if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
>  			return -EINVAL;
>  	}
> +	/*
> +	 * Prevent 32-bit guest launch if shadow stack is exposed as SSP
> +	 * state is not defined for 32-bit SMRAM.

Why?  Lack of save/restore for SSP on 32-bit guests is a gap in Intel's
architecture, I don't see why KVM should diverge from hardware.  I.e. just do
nothing for SSP on SMI/RSM, because that's exactly what the architecture says
will happen.

> +	 */
> +	best = cpuid_entry2_find(entries, nent, 0x80000001,
> +				 KVM_CPUID_INDEX_NOT_SIGNIFICANT);
> +	if (best && !(best->edx & F(LM))) {
> +		best = cpuid_entry2_find(entries, nent, 0x7, 0);
> +		if (best && (best->ecx & F(SHSTK)))
> +			return -EINVAL;
> +	}
>  
>  	/*
>  	 * Exposing dynamic xfeatures to the guest requires additional
> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
> index 45c855389ea7..7aac9c54c353 100644
> --- a/arch/x86/kvm/smm.c
> +++ b/arch/x86/kvm/smm.c
> @@ -275,6 +275,10 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
>  	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
>  
>  	smram->int_shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
> +
> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
> +		KVM_BUG_ON(kvm_msr_read(vcpu, MSR_KVM_SSP, &smram->ssp),
> +			   vcpu->kvm);
>  }
>  #endif
>  
> @@ -564,6 +568,10 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
>  	static_call(kvm_x86_set_interrupt_shadow)(vcpu, 0);
>  	ctxt->interruptibility = (u8)smstate->int_shadow;
>  
> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
> +		KVM_BUG_ON(kvm_msr_write(vcpu, MSR_KVM_SSP, smstate->ssp),
> +			   vcpu->kvm);


This should synthesize triple-fault, not WARN and kill the VM, as the value to
be restored is guest controlled (the guest can scribble SMRAM from within the
SMI handler).

At that point, I would just synthesize triple-fault for the read path too.
Yang, Weijiang May 6, 2024, 8:41 a.m. UTC | #2
On 5/2/2024 6:50 AM, Sean Christopherson wrote:
> On Sun, Feb 18, 2024, Yang Weijiang wrote:
>> Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
>> behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
>> at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
>> one of such registers on 64-bit Arch, and add the support for SSP. Note,
>> on 32-bit Arch, SSP is not defined in SMRAM, so fail 32-bit CET guest
>> launch.
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>> ---
>>   arch/x86/kvm/cpuid.c | 11 +++++++++++
>>   arch/x86/kvm/smm.c   |  8 ++++++++
>>   arch/x86/kvm/smm.h   |  2 +-
>>   3 files changed, 20 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 2bb1931103ad..c0e13040e35b 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -149,6 +149,17 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
>>   		if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
>>   			return -EINVAL;
>>   	}
>> +	/*
>> +	 * Prevent 32-bit guest launch if shadow stack is exposed as SSP
>> +	 * state is not defined for 32-bit SMRAM.
> Why?  Lack of save/restore for SSP on 32-bit guests is a gap in Intel's
> architecture, I don't see why KVM should diverge from hardware.  I.e. just do
> nothing for SSP on SMI/RSM, because that's exactly what the architecture says
> will happen.

OK, will remove the check. I just wanted to avoid any undocumented hole if SHSTK is
exposed in CPUID.

>
>> +	 */
>> +	best = cpuid_entry2_find(entries, nent, 0x80000001,
>> +				 KVM_CPUID_INDEX_NOT_SIGNIFICANT);
>> +	if (best && !(best->edx & F(LM))) {
>> +		best = cpuid_entry2_find(entries, nent, 0x7, 0);
>> +		if (best && (best->ecx & F(SHSTK)))
>> +			return -EINVAL;
>> +	}
>>   
>>   	/*
>>   	 * Exposing dynamic xfeatures to the guest requires additional
>> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
>> index 45c855389ea7..7aac9c54c353 100644
>> --- a/arch/x86/kvm/smm.c
>> +++ b/arch/x86/kvm/smm.c
>> @@ -275,6 +275,10 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
>>   	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
>>   
>>   	smram->int_shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
>> +
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>> +		KVM_BUG_ON(kvm_msr_read(vcpu, MSR_KVM_SSP, &smram->ssp),
>> +			   vcpu->kvm);
>>   }
>>   #endif
>>   
>> @@ -564,6 +568,10 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
>>   	static_call(kvm_x86_set_interrupt_shadow)(vcpu, 0);
>>   	ctxt->interruptibility = (u8)smstate->int_shadow;
>>   
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>> +		KVM_BUG_ON(kvm_msr_write(vcpu, MSR_KVM_SSP, smstate->ssp),
>> +			   vcpu->kvm);
>
> This should synthesize triple-fault, not WARN and kill the VM, as the value to
> be restored is guest controlled (the guest can scribble SMRAM from within the
> SMI handler).
>
> At that point, I would just synthesize triple-fault for the read path too.

Ah, yes, will fail with triple-fault in next version, thanks!

>
diff mbox series

Patch

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2bb1931103ad..c0e13040e35b 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -149,6 +149,17 @@  static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
 		if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
 			return -EINVAL;
 	}
+	/*
+	 * Prevent 32-bit guest launch if shadow stack is exposed as SSP
+	 * state is not defined for 32-bit SMRAM.
+	 */
+	best = cpuid_entry2_find(entries, nent, 0x80000001,
+				 KVM_CPUID_INDEX_NOT_SIGNIFICANT);
+	if (best && !(best->edx & F(LM))) {
+		best = cpuid_entry2_find(entries, nent, 0x7, 0);
+		if (best && (best->ecx & F(SHSTK)))
+			return -EINVAL;
+	}
 
 	/*
 	 * Exposing dynamic xfeatures to the guest requires additional
diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
index 45c855389ea7..7aac9c54c353 100644
--- a/arch/x86/kvm/smm.c
+++ b/arch/x86/kvm/smm.c
@@ -275,6 +275,10 @@  static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
 	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
 
 	smram->int_shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
+
+	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
+		KVM_BUG_ON(kvm_msr_read(vcpu, MSR_KVM_SSP, &smram->ssp),
+			   vcpu->kvm);
 }
 #endif
 
@@ -564,6 +568,10 @@  static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
 	static_call(kvm_x86_set_interrupt_shadow)(vcpu, 0);
 	ctxt->interruptibility = (u8)smstate->int_shadow;
 
+	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
+		KVM_BUG_ON(kvm_msr_write(vcpu, MSR_KVM_SSP, smstate->ssp),
+			   vcpu->kvm);
+
 	return X86EMUL_CONTINUE;
 }
 #endif
diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
index a1cf2ac5bd78..1e2a3e18207f 100644
--- a/arch/x86/kvm/smm.h
+++ b/arch/x86/kvm/smm.h
@@ -116,8 +116,8 @@  struct kvm_smram_state_64 {
 	u32 smbase;
 	u32 reserved4[5];
 
-	/* ssp and svm_* fields below are not implemented by KVM */
 	u64 ssp;
+	/* svm_* fields below are not implemented by KVM */
 	u64 svm_guest_pat;
 	u64 svm_host_efer;
 	u64 svm_host_cr4;