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

From: Zeng Guang
Date: Wed Jun 07 2023 - 02:29:31 EST


On 6/5/2023 11:31 AM, Binbin Wu wrote:

On 6/1/2023 10:23 PM, Zeng Guang wrote:
Intel introduces LASS (Linear Address Separation) feature providing
                      ^
 missing "Space" here
Thanks.
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.
In some cases , cpu mode check already exists before invoking LASS violation
detect, e.g. vmx instruction emulation. So it's designed to make vmx_check_lass()
focusing on LASS violation alone, and leave it to caller taking care of cpu mode
in advance.

The purpose is to avoid duplicating cpu mode check though the impact seems not
significant. :)
+
+ 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.

OK, will add comments on specific treatment in compatibility mode.
+
+ /*
+ * 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)
{