[patch V7 04/15] SBB 4

[Thomas Gleixner <tglx@linutronix.de>]

A guest may modify the SPEC_CTRL MSR from the value used by the
kernel. Since the kernel doesn't use IBRS, this means a value of zero is
what is needed in the host.

But the 336996-Speculative-Execution-Side-Channel-Mitigations.pdf refers to
the other bits as reserved so the kernel should respect the boot time
SPEC_CTRL value and use that.

This allows to deal with future extensions to the SPEC_CTRL interface if
any at all.

Note: This uses wrmsrl instead of native_wrmsl. I does not make any
difference as paravirt will over-write the callq *0xfff.. with the wrmsrl
assembler code.

A copy of this document is available at
https://bugzilla.kernel.org/show_bug.cgi?id=199511

[ tglx: Added a paranoia check for IBRS into the functions and simplified
  	them ]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>

---
v2: New patch
v3: Use the two accessory functions instead of poking at the global variable.
v4: Use x86_get_spec_ctrl instead of global variable.
v5: Use x86_get_default_spec_ctrl instead of x86_get_spec_ctrl
---
 arch/x86/include/asm/nospec-branch.h |   10 ++++++++++
 arch/x86/kernel/cpu/bugs.c           |   18 ++++++++++++++++++
 arch/x86/kvm/svm.c                   |    6 ++----
 arch/x86/kvm/vmx.c                   |    6 ++----
 4 files changed, 32 insertions(+), 8 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -228,6 +228,16 @@ enum spectre_v2_mitigation {
 extern void x86_set_spec_ctrl(u64);
 extern u64 x86_get_default_spec_ctrl(void);
 
+/*
+ * On VMENTER we must preserve whatever view of the SPEC_CTRL MSR
+ * the guest has, while on VMEXIT we restore the host view. This
+ * would be easier if SPEC_CTRL were architecturally maskable or
+ * shadowable for guests but this is not (currently) the case.
+ * Takes the guest view of SPEC_CTRL MSR as a parameter.
+ */
+extern void x86_set_guest_spec_ctrl(u64);
+extern void x86_restore_host_spec_ctrl(u64);
+
 extern char __indirect_thunk_start[];
 extern char __indirect_thunk_end[];
 
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -123,6 +123,24 @@ u64 x86_get_default_spec_ctrl(void)
 }
 EXPORT_SYMBOL_GPL(x86_get_default_spec_ctrl);
 
+void x86_set_guest_spec_ctrl(u64 guest_spec_ctrl)
+{
+	if (!boot_cpu_has(X86_FEATURE_IBRS))
+		return;
+	if (x86_spec_ctrl_base != guest_spec_ctrl)
+		wrmsrl(MSR_IA32_SPEC_CTRL, guest_spec_ctrl);
+}
+EXPORT_SYMBOL_GPL(x86_set_guest_spec_ctrl);
+
+void x86_restore_host_spec_ctrl(u64 guest_spec_ctrl)
+{
+	if (!boot_cpu_has(X86_FEATURE_IBRS))
+		return;
+	if (x86_spec_ctrl_base != guest_spec_ctrl)
+		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+}
+EXPORT_SYMBOL_GPL(x86_restore_host_spec_ctrl);
+
 #ifdef RETPOLINE
 static bool spectre_v2_bad_module;
 
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -5557,8 +5557,7 @@ static void svm_vcpu_run(struct kvm_vcpu
 	 * is no need to worry about the conditional branch over the wrmsr
 	 * being speculatively taken.
 	 */
-	if (svm->spec_ctrl)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
+	x86_set_guest_spec_ctrl(svm->spec_ctrl);
 
 	asm volatile (
 		"push %%" _ASM_BP "; \n\t"
@@ -5670,8 +5669,7 @@ static void svm_vcpu_run(struct kvm_vcpu
 	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
 		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
 
-	if (svm->spec_ctrl)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+	x86_restore_host_spec_ctrl(svm->spec_ctrl);
 
 	/* Eliminate branch target predictions from guest mode */
 	vmexit_fill_RSB();
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9726,8 +9726,7 @@ static void __noclone vmx_vcpu_run(struc
 	 * is no need to worry about the conditional branch over the wrmsr
 	 * being speculatively taken.
 	 */
-	if (vmx->spec_ctrl)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
+	x86_set_guest_spec_ctrl(vmx->spec_ctrl);
 
 	vmx->__launched = vmx->loaded_vmcs->launched;
 
@@ -9875,8 +9874,7 @@ static void __noclone vmx_vcpu_run(struc
 	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
 		vmx->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
 
-	if (vmx->spec_ctrl)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+	x86_restore_host_spec_ctrl(vmx->spec_ctrl);
 
 	/* Eliminate branch target predictions from guest mode */
 	vmexit_fill_RSB();