[PATCH 3.16 005/131] x86/bugs, KVM: Support the combination of guest and host IBRS

[Ben Hutchings]

3.16.59-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

commit 5cf687548705412da47c9cec342fd952d71ed3d5 upstream.

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.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 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
@@ -183,6 +183,16 @@ enum spectre_v2_mitigation {
 extern void x86_spec_ctrl_set(u64);
 extern u64 x86_spec_ctrl_get_default(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_spec_ctrl_set_guest(u64);
+extern void x86_spec_ctrl_restore_host(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
@@ -185,6 +185,24 @@ u64 x86_spec_ctrl_get_default(void)
 }
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_get_default);
 
+void x86_spec_ctrl_set_guest(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_spec_ctrl_set_guest);
+
+void x86_spec_ctrl_restore_host(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_spec_ctrl_restore_host);
+
 #ifdef RETPOLINE
 static bool spectre_v2_bad_module;
 
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -3967,8 +3967,7 @@ 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_spec_ctrl_set_guest(svm->spec_ctrl);
 
 	asm volatile (
 		"push %%" _ASM_BP "; \n\t"
@@ -4080,8 +4079,7 @@ 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_spec_ctrl_restore_host(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
@@ -7540,8 +7540,7 @@ 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_spec_ctrl_set_guest(vmx->spec_ctrl);
 
 	vmx->__launched = vmx->loaded_vmcs->launched;
 	asm(
@@ -7674,8 +7673,7 @@ static void __noclone vmx_vcpu_run(struc
 	if (unlikely(!msr_write_intercepted_l01(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_spec_ctrl_restore_host(vmx->spec_ctrl);
 
 	/* Eliminate branch target predictions from guest mode */
 	vmexit_fill_RSB();