Re: [PATCH 2/6] KVM: VMX: Add new ops in kvm_x86_ops for LASS violation check

[Binbin Wu <binbin.wu@linux.intel.com>]

On 4/20/2023 9:37 PM, Zeng Guang wrote:
> Intel introduce LASS (Linear Address Separation) feature providing
/s/introduce/introduces

> 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 data access
>      1. from user mode to supervisor-mode address space
>      2. from supervisor mode to user-mode address space
> and generates LASS violation fault accordingly.
IMO, the description of the point 2 may be misleading that LASS stops
any data access from supervisor mode to user mode address space,
although the description following adds the conditions.


>
> 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 rule of LASS violation check on the linear address(LA).
/s/rule/rules

> 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@intel.com>
> ---
>   arch/x86/include/asm/kvm-x86-ops.h |  1 +
>   arch/x86/include/asm/kvm_host.h    |  5 +++
>   arch/x86/kvm/vmx/vmx.c             | 55 ++++++++++++++++++++++++++++++
>   arch/x86/kvm/vmx/vmx.h             |  2 ++
>   4 files changed, 63 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index abccd51dcfca..f76c07f2674b 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -131,6 +131,7 @@ 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(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 8ff89a52ef66..31fb8699a1ff 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -69,6 +69,9 @@
>   #define KVM_X86_NOTIFY_VMEXIT_VALID_BITS	(KVM_X86_NOTIFY_VMEXIT_ENABLED | \
>   						 KVM_X86_NOTIFY_VMEXIT_USER)
>   
> +/* x86-specific emulation flags */
> +#define KVM_X86_EMULFLAG_SKIP_LASS	_BITULL(1)
Do you use the flag outside of emulator?
For LAM patch, it's planned to move the flags inside emulator.

> +
>   /* x86-specific vcpu->requests bit members */
>   #define KVM_REQ_MIGRATE_TIMER		KVM_ARCH_REQ(0)
>   #define KVM_REQ_REPORT_TPR_ACCESS	KVM_ARCH_REQ(1)
> @@ -1706,6 +1709,8 @@ 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, u64 flags);
The flags may be dropped if the caller knows to skip it or not.

>   };
>   
>   struct kvm_x86_nested_ops {
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c923d7599d71..581327ede66a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8070,6 +8070,59 @@ 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 VM
Should be 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, u64 flags)
> +{
> +	bool user_mode, user_as, rflags_ac;
> +
> +	if (!!(flags & KVM_X86_EMULFLAG_SKIP_LASS) ||
> +	    !kvm_is_cr4_bit_set(vcpu, X86_CR4_LASS))
> +		return false;
> +
> +	WARN_ON_ONCE(!is_long_mode(vcpu));
> +
> +	user_as = !(la >> 63);
> +
> +	/*
> +	 * 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) {
> +		/*
> +		 * 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;
> +		} else {
> +			return true;
> +		}
> +	}
> +
> +	return false;
> +}
> +
> +static bool vmx_check_lass(struct kvm_vcpu *vcpu, u64 access, u64 la, u64 flags)
> +{
> +	return is_long_mode(vcpu) && __vmx_check_lass(vcpu, access, la, flags);
> +}
> +
>   static struct kvm_x86_ops vmx_x86_ops __initdata = {
>   	.name = "kvm_intel",
>   
> @@ -8207,6 +8260,8 @@ 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 a3da84f4ea45..6569385a5978 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, u64 flags);
> +
>   static inline void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr,
>   					     int type, bool value)
>   {