Re: [PATCH v3 3/8] KVM:CPUID: Add CPUID support for Guest CET

[Yang Weijiang]

On Fri, Mar 01, 2019 at 06:53:23AM -0800, Sean Christopherson wrote:
> On Thu, Feb 28, 2019 at 04:28:32PM +0800, Yang Weijiang wrote:
> > On Thu, Feb 28, 2019 at 07:59:40AM -0800, Sean Christopherson wrote:
> > > On Mon, Feb 25, 2019 at 09:27:11PM +0800, Yang Weijiang wrote:
> > > > Guest CET SHSTK and IBT capability are reported via
> > > > CPUID.(EAX=7, ECX=0):ECX[bit 7] and EDX[bit 20] respectively.
> > > > Guest user mode and supervisor mode xsaves component size
> > > > is reported via CPUID.(EAX=0xD, ECX=1):ECX[bit 11] and ECX[bit 12]
> > > > respectively.
> > > > 
> > > > Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > > > ---
> > > >  arch/x86/kvm/cpuid.c | 60 +++++++++++++++++++++++++++++++++-----------
> > > >  arch/x86/kvm/x86.h   |  4 +++
> > > >  2 files changed, 50 insertions(+), 14 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > > index cb1aece25b17..5e05756cc6db 100644
> > > > --- a/arch/x86/kvm/cpuid.c
> > > > +++ b/arch/x86/kvm/cpuid.c
> > > > @@ -65,6 +65,16 @@ u64 kvm_supported_xcr0(void)
> > > >  	return xcr0;
> > > >  }
> > > >  
> > > > +u64 kvm_supported_xss(void)
> > > > +{
> > > > +	u64 xss;
> > > > +
> > > > +	rdmsrl(MSR_IA32_XSS, xss);
> > > > +	xss &= KVM_SUPPORTED_XSS;
> > > > +	return xss;
> > > > +}
> > > > +EXPORT_SYMBOL(kvm_supported_xss);
> > > > +
> > > >  #define F(x) bit(X86_FEATURE_##x)
> > > >  
> > > >  /* For scattered features from cpufeatures.h; we currently expose none */
> > > > @@ -323,6 +333,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> > > >  				 u32 index, int *nent, int maxnent)
> > > >  {
> > > >  	int r;
> > > > +	u32 eax, ebx, ecx, edx;
> > > >  	unsigned f_nx = is_efer_nx() ? F(NX) : 0;
> > > >  #ifdef CONFIG_X86_64
> > > >  	unsigned f_gbpages = (kvm_x86_ops->get_lpage_level() == PT_PDPE_LEVEL)
> > > > @@ -503,6 +514,20 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> > > >  			 * if the host doesn't support it.
> > > >  			 */
> > > >  			entry->edx |= F(ARCH_CAPABILITIES);
> > > > +
> > > > +			/*
> > > > +			 * Guest OS CET enabling is designed independent to
> > > > +			 * host enabling, it only has dependency on Host HW
> > > > +			 * capability, if it has, report CET support to
> > > > +			 * Guest.
> > > > +			 */
> > > > +			cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
> > > > +			if (ecx & F(SHSTK))
> > > > +				entry->ecx |= F(SHSTK);
> > > > +
> > > > +			if (edx & F(IBT))
> > > > +				entry->edx |= F(IBT);
> > > 
> > > There's no need to manually add these flags.  They will be automatically
> > > kept if supported in hardware because your previous patch, 02/08, added
> > > them to the mask of features that can be exposed to the guest,
> > > i.e. set them in kvm_cpuid_7_0_e{c,d}x_x86_features.
> > > 
> > I shared the same thought as you before, but after I took a closer look at the
> > kernel code, actually, when host CET feature is disabled by user via
> > cmdline options(no_cet_shstk and no_cet_ibt), it'll mask out CET feature bits in
> > boot_cpu_data.x86_capbility[] array, and cpuid_mask() will make the bits
> > in previous definition lost, so these lines actually add them back when
> > host CET is disabled.
> 
> 'entry' is filled by do_cpuid_1_ent(), which does cpuid_count(), same as
> your code, i.e. it's not affected by whether or not the host kernel is
> using each feature.
>
I checked CET kernel patch:
#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
static __init int setup_disable_shstk(char *s) {
	/* require an exact match without trailing characters */
	if (s[0] != '\0')
		return 0;

	if (!boot_cpu_has(X86_FEATURE_SHSTK))
		return 1;

	setup_clear_cpu_cap(X86_FEATURE_SHSTK);
	pr_info("x86: 'no_cet_shstk' specified, disabling Shadow Stack\n");
	return 1;
}
__setup("no_cet_shstk", setup_disable_shstk); #endif

setup_disable_shstk()->setup_clear_cpu_cap(X86_FEATURE_SHSTK)->do_clear_cpu_cap(NULL,
feature)->clear_feature(c, feature)->clear_cpu_cap(&boot_cpu_data, feature);

this path will clear boot_cpu_data.x86_capability[X86_FEATURE_SHSTK] if "no_cet_shstk" is set.
but in cpuid_mask(), it will "AND" the bit with SHSTK bit set 
in kvm_cpuid_7_0_ecx_x86_features, so the bit in ecx is cleared, 
need to add the bit back according to host cpuid_count(). 
the CET kernel patch can be seen in below patch link.

> > please check CET kernel patch here:
> > https://lkml.org/lkml/2018/11/20/204
> > 
> > > > +
> > > >  		} else {
> > > >  			entry->ebx = 0;
> > > >  			entry->ecx = 0;
> > > > @@ -564,14 +589,17 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> > > >  	}
> > > >  	case 0xd: {
> > > >  		int idx, i;
> > > > -		u64 supported = kvm_supported_xcr0();
> > > > +		u64 u_supported = kvm_supported_xcr0();
> > > > +		u64 s_supported = kvm_supported_xss();
> > > > +		u64 supported;
> > > > +		int compacted;
> > > >  
> > > > -		entry->eax &= supported;
> > > > -		entry->ebx = xstate_required_size(supported, false);
> > > > +		entry->eax &= u_supported;
> > > > +		entry->ebx = xstate_required_size(u_supported, false);
> > > >  		entry->ecx = entry->ebx;
> > > > -		entry->edx &= supported >> 32;
> > > > +		entry->edx &= u_supported >> 32;
> > > >  		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > > > -		if (!supported)
> > > > +		if (!u_supported && !s_supported)
> > > >  			break;
> > > >  
> > > >  		for (idx = 1, i = 1; idx < 64; ++idx) {
> > > > @@ -583,19 +611,23 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> > > >  			if (idx == 1) {
> > > >  				entry[i].eax &= kvm_cpuid_D_1_eax_x86_features;
> > > >  				cpuid_mask(&entry[i].eax, CPUID_D_1_EAX);
> > > > -				entry[i].ebx = 0;
> > > > -				if (entry[i].eax & (F(XSAVES)|F(XSAVEC)))
> > > > -					entry[i].ebx =
> > > > -						xstate_required_size(supported,
> > > > -								     true);
> > > > +				supported = u_supported | s_supported;
> > > > +				compacted = entry[i].eax &
> > > > +					(F(XSAVES) | F(XSAVEC));
> > > > +				entry[i].ebx = xstate_required_size(supported,
> > > > +								    compacted);
> > > > +				entry[i].ecx &= s_supported;
> > > > +				entry[i].edx = 0;
> > > >  			} else {
> > > > +				supported = (entry[i].ecx & 1) ? s_supported :
> > > > +								 u_supported;
> > > >  				if (entry[i].eax == 0 || !(supported & mask))
> > > >  					continue;
> > > > -				if (WARN_ON_ONCE(entry[i].ecx & 1))
> > > > -					continue;
> > > > +				entry[i].ecx &= 1;
> > > > +				entry[i].edx = 0;
> > > > +				if (entry[i].ecx)
> > > > +					entry[i].ebx = 0;
> > > >  			}
> > > > -			entry[i].ecx = 0;
> > > > -			entry[i].edx = 0;
> > > >  			entry[i].flags |=
> > > >  			       KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > > >  			++*nent;
> > > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > > > index 224cd0a47568..c61da41c3c5c 100644
> > > > --- a/arch/x86/kvm/x86.h
> > > > +++ b/arch/x86/kvm/x86.h
> > > > @@ -283,6 +283,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, unsigned long cr2,
> > > >  				| XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
> > > >  				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> > > >  				| XFEATURE_MASK_PKRU)
> > > > +
> > > > +#define KVM_SUPPORTED_XSS	(XFEATURE_MASK_SHSTK_USER \
> > > > +				| XFEATURE_MASK_SHSTK_KERNEL)
> > > > +
> > > >  extern u64 host_xcr0;
> > > >  
> > > >  extern u64 kvm_supported_xcr0(void);
> > > > -- 
> > > > 2.17.1
> > > >