Re: [PATCH v1 3/6] KVM: VMX: Add new ops in kvm_x86_ops for LASS violation check

[Zhi Wang]

On Mon, 5 Jun 2023 11:31:48 +0800
Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> wrote:

> 
> 
> On 6/1/2023 10:23 PM, Zeng Guang wrote:
> > Intel introduces LASS (Linear Address Separation) feature providing
>                        ^
>   missing "Space" here
> > an independent mechanism to achieve the mode-based protection.
> >
> > LASS partitions 64-bit linear address space into two halves, user-mode
> > address (LA[bit 63]=0) and supervisor-mode address (LA[bit 63]=1). It
> > stops any code execution or conditional data access[1]
> >      1. from user mode to supervisor-mode address space
> >      2. from supervisor mode to user-mode address space
> > and generates LASS violation fault accordingly.
> >
> > [1]A supervisor mode data access causes a LASS violation only if supervisor
> > mode access protection is enabled (CR4.SMAP = 1) and either RFLAGS.AC = 0
> > or the access implicitly accesses a system data structure.
> >
> > Following are the rules of LASS violation check on the linear address(LA).
> > User access to supervisor-mode address space:
> >      LA[bit 63] && (CPL == 3)
> > Supervisor access to user-mode address space:
> >      Instruction fetch: !LA[bit 63] && (CPL < 3)
> >      Data access: !LA[bit 63] && (CR4.SMAP==1) && ((RFLAGS.AC == 0 &&
> >                   CPL < 3) || Implicit supervisor access)
> >
> > Add new ops in kvm_x86_ops to do LASS violation check.
> >
> > Signed-off-by: Zeng Guang <guang.zeng@xxxxxxxxx>
> > Tested-by: Xuelian Guo <xuelian.guo@xxxxxxxxx>
> > ---
> >   arch/x86/include/asm/kvm-x86-ops.h |  3 +-
> >   arch/x86/include/asm/kvm_host.h    |  2 ++
> >   arch/x86/kvm/kvm_emulate.h         |  1 +
> >   arch/x86/kvm/vmx/vmx.c             | 47 ++++++++++++++++++++++++++++++
> >   arch/x86/kvm/vmx/vmx.h             |  2 ++
> >   5 files changed, 54 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index 13bc212cd4bc..8980a3bfa687 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -132,7 +132,8 @@ KVM_X86_OP_OPTIONAL(migrate_timers)
> >   KVM_X86_OP(msr_filter_changed)
> >   KVM_X86_OP(complete_emulated_msr)
> >   KVM_X86_OP(vcpu_deliver_sipi_vector)
> > -KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
> > +KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons)
> > +KVM_X86_OP_OPTIONAL_RET0(check_lass)
> >   
> >   #undef KVM_X86_OP
> >   #undef KVM_X86_OP_OPTIONAL
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 92d8e65fe88c..98666d1e7727 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1731,6 +1731,8 @@ struct kvm_x86_ops {
> >   	 * Returns vCPU specific APICv inhibit reasons
> >   	 */
> >   	unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
> > +
> > +	bool (*check_lass)(struct kvm_vcpu *vcpu, u64 access, u64 la, u32 flags);
> >   };
> >   
> >   struct kvm_x86_nested_ops {
> > diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
> > index 5b9ec610b2cb..f1439ab7c14b 100644
> > --- a/arch/x86/kvm/kvm_emulate.h
> > +++ b/arch/x86/kvm/kvm_emulate.h
> > @@ -91,6 +91,7 @@ struct x86_instruction_info {
> >   /* x86-specific emulation flags */
> >   #define X86EMUL_F_FETCH			BIT(0)
> >   #define X86EMUL_F_WRITE			BIT(1)
> > +#define X86EMUL_F_SKIPLASS		BIT(2)
> >   
> >   struct x86_emulate_ops {
> >   	void (*vm_bugged)(struct x86_emulate_ctxt *ctxt);
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index a33205ded85c..876997e8448e 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -8130,6 +8130,51 @@ static void vmx_vm_destroy(struct kvm *kvm)
> >   	free_pages((unsigned long)kvm_vmx->pid_table, vmx_get_pid_table_order(kvm));
> >   }
> >   
> > +/*
> > + * Determine whether an access to the linear address causes a LASS violation.
> > + * LASS protection is only effective in long mode. As a prerequisite, caller
> > + * should make sure vCPU running in long mode and invoke this api to do LASS
> > + * violation check.
> > + */
> > +bool vmx_check_lass(struct kvm_vcpu *vcpu, u64 access, u64 la, u32 flags)
> > +{
> > +	bool user_mode, user_as, rflags_ac;
> > +
> > +	if (!!(flags & X86EMUL_F_SKIPLASS) ||
> > +	    !kvm_is_cr4_bit_set(vcpu, X86_CR4_LASS))
> > +		return false;
> > +
> > +	WARN_ON_ONCE(!is_long_mode(vcpu));
> IMHO, it's better to skip the following checks and return false if it is 
> out of long mode.
>
The check of long mode is in the caller implemented in in the next patch. :)

+	if (!is_long_mode(vcpu))
+		return false;

> > +
> > +	user_as = !(la >> 63);
> It's better to describe how LASS treat linear address in compatibility 
> mode in changelog or/and in comment,
> i.e. for a linear address with only 32 bits (or 16 bits), the processor 
> treats bit 63 as if it were 0.
>
> 
> > +
> > +	/*
> > +	 * An access is a supervisor-mode access if CPL < 3 or if it implicitly
> > +	 * accesses a system data structure. For implicit accesses to system
> > +	 * data structure, the processor acts as if RFLAGS.AC is clear.
> > +	 */
> > +	if (access & PFERR_IMPLICIT_ACCESS) {
> > +		user_mode = false;
> > +		rflags_ac = false;
> > +	} else {
> > +		user_mode = vmx_get_cpl(vcpu) == 3;
> > +		if (!user_mode)
> > +			rflags_ac = !!(kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
> > +	}
> > +
> > +	if (user_mode == user_as)
> > +		return false;
> > +
> > +	/*
> > +	 * Supervisor-mode _data_ accesses to user address space
> > +	 * cause LASS violations only if SMAP is enabled.
> > +	 */
> > +	if (!user_mode && !(access & PFERR_FETCH_MASK))
> > +		return kvm_is_cr4_bit_set(vcpu, X86_CR4_SMAP) && !rflags_ac;
> > +
> > +	return true;
> > +}
> > +
> >   static struct kvm_x86_ops vmx_x86_ops __initdata = {
> >   	.name = KBUILD_MODNAME,
> >   
> > @@ -8269,6 +8314,8 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
> >   	.complete_emulated_msr = kvm_complete_insn_gp,
> >   
> >   	.vcpu_deliver_sipi_vector = kvm_vcpu_deliver_sipi_vector,
> > +
> > +	.check_lass = vmx_check_lass,
> >   };
> >   
> >   static unsigned int vmx_handle_intel_pt_intr(void)
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > index 9e66531861cf..f2e775b9849b 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -433,6 +433,8 @@ void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type);
> >   u64 vmx_get_l2_tsc_offset(struct kvm_vcpu *vcpu);
> >   u64 vmx_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu);
> >   
> > +bool vmx_check_lass(struct kvm_vcpu *vcpu, u64 access, u64 la, u32 flags);
> > +
> >   static inline void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr,
> >   					     int type, bool value)
> >   {
>