linux-kernel - Re: [PATCH v10 21/27] KVM: x86: Save and reload SSP to/from SMRAM

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Date: Mon, 6 May 2024 16:41:04 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <pbonzini@...hat.com>, <dave.hansen@...el.com>, <x86@...nel.org>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peterz@...radead.org>, <chao.gao@...el.com>, <rick.p.edgecombe@...el.com>,
	<mlevitsk@...hat.com>, <john.allen@....com>
Subject: Re: [PATCH v10 21/27] KVM: x86: Save and reload SSP to/from SMRAM

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@...gle.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.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!

>