KVM: VMX: Prevent RSB underflow before vmenter

On VMX, there are some balanced returns between the time the guest's
SPEC_CTRL value is written, and the vmenter.

Balanced returns (matched by a preceding call) are usually ok, but it's
at least theoretically possible an NMI with a deep call stack could
empty the RSB before one of the returns.

For maximum paranoia, don't allow *any* returns (balanced or otherwise)
between the SPEC_CTRL write and the vmenter.

  [ bp: Fix 32-bit build. ]
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/asm-offsets.c

@@ -19,6 +19,7 @@
 #include <asm/suspend.h>
 #include <asm/tlbflush.h>
 #include <asm/tdx.h>
+#include "../kvm/vmx/vmx.h"
 
 #ifdef CONFIG_XEN
 #include <xen/interface/xen.h>
@@ -107,4 +108,9 @@ static void __used common(void)
 	OFFSET(TSS_sp0, tss_struct, x86_tss.sp0);
 	OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
 	OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
+
+	if (IS_ENABLED(CONFIG_KVM_INTEL)) {
+		BLANK();
+		OFFSET(VMX_spec_ctrl, vcpu_vmx, spec_ctrl);
+	}
 }

arch/x86/kernel/cpu/bugs.c

@@ -196,8 +196,8 @@ void __init check_bugs(void)
 }
 
 /*
- * NOTE: For VMX, this function is not called in the vmexit path.
- * It uses vmx_spec_ctrl_restore_host() instead.
+ * NOTE: This function is *only* called for SVM.  VMX spec_ctrl handling is
+ * done in vmenter.S.
  */
 void
 x86_virt_spec_ctrl(u64 guest_spec_ctrl, u64 guest_virt_spec_ctrl, bool setguest)

arch/x86/kvm/vmx/capabilities.h

@@ -4,8 +4,8 @@
 
 #include <asm/vmx.h>
 
-#include "lapic.h"
-#include "x86.h"
+#include "../lapic.h"
+#include "../x86.h"
 
 extern bool __read_mostly enable_vpid;
 extern bool __read_mostly flexpriority_enabled;

arch/x86/kvm/vmx/vmenter.S

 /* SPDX-License-Identifier: GPL-2.0 */
 #include <linux/linkage.h>
 #include <asm/asm.h>
+#include <asm/asm-offsets.h>
 #include <asm/bitsperlong.h>
 #include <asm/kvm_vcpu_regs.h>
 #include <asm/nospec-branch.h>
+#include <asm/percpu.h>
 #include <asm/segment.h>
 #include "run_flags.h"
 
@@ -73,6 +75,33 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	lea (%_ASM_SP), %_ASM_ARG2
 	call vmx_update_host_rsp
 
+	ALTERNATIVE "jmp .Lspec_ctrl_done", "", X86_FEATURE_MSR_SPEC_CTRL
+
+	/*
+	 * SPEC_CTRL handling: if the guest's SPEC_CTRL value differs from the
+	 * host's, write the MSR.
+	 *
+	 * IMPORTANT: To avoid RSB underflow attacks and any other nastiness,
+	 * there must not be any returns or indirect branches between this code
+	 * and vmentry.
+	 */
+	mov 2*WORD_SIZE(%_ASM_SP), %_ASM_DI
+	movl VMX_spec_ctrl(%_ASM_DI), %edi
+	movl PER_CPU_VAR(x86_spec_ctrl_current), %esi
+	cmp %edi, %esi
+	je .Lspec_ctrl_done
+	mov $MSR_IA32_SPEC_CTRL, %ecx
+	xor %edx, %edx
+	mov %edi, %eax
+	wrmsr
+
+.Lspec_ctrl_done:
+
+	/*
+	 * Since vmentry is serializing on affected CPUs, there's no need for
+	 * an LFENCE to stop speculation from skipping the wrmsr.
+	 */
+
 	/* Load @regs to RAX. */
 	mov (%_ASM_SP), %_ASM_AX
 

arch/x86/kvm/vmx/vmx.c

@@ -6988,14 +6988,6 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
 
 	kvm_wait_lapic_expire(vcpu);
 
-	/*
-	 * If this vCPU has touched SPEC_CTRL, restore the guest's value if
-	 * it's non-zero. Since vmentry is serialising on affected CPUs, there
-	 * is no need to worry about the conditional branch over the wrmsr
-	 * being speculatively taken.
-	 */
-	x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
-
 	/* The actual VMENTER/EXIT is in the .noinstr.text section. */
 	vmx_vcpu_enter_exit(vcpu, vmx, __vmx_vcpu_run_flags(vmx));
 

arch/x86/kvm/vmx/vmx.h

@@ -8,11 +8,11 @@
 #include <asm/intel_pt.h>
 
 #include "capabilities.h"
-#include "kvm_cache_regs.h"
+#include "../kvm_cache_regs.h"
 #include "posted_intr.h"
 #include "vmcs.h"
 #include "vmx_ops.h"
-#include "cpuid.h"
+#include "../cpuid.h"
 #include "run_flags.h"
 
 #define MSR_TYPE_R	1

arch/x86/kvm/vmx/vmx_ops.h

@@ -8,7 +8,7 @@
 
 #include "evmcs.h"
 #include "vmcs.h"
-#include "x86.h"
+#include "../x86.h"
 
 asmlinkage void vmread_error(unsigned long field, bool fault);
 __attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,