[PATCH 4.4 56/88] x86/bugs, KVM: Support the combination of guest and host IBRS

[Greg Kroah-Hartman]

4.4-stable 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>
[bwh: Backported to 4.4: This was partly applied before; apply just the
 missing bits]
Signed-off-by: Ben Hutchings <ben.hutchings@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/svm.c |    6 ++----
 arch/x86/kvm/vmx.c |    6 ++----
 2 files changed, 4 insertions(+), 8 deletions(-)

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -3904,8 +3904,7 @@ 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"
@@ -4017,8 +4016,7 @@ static void svm_vcpu_run(struct kvm_vcpu
 	if (!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
@@ -8658,8 +8658,7 @@ 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(
@@ -8797,8 +8796,7 @@ static void __noclone vmx_vcpu_run(struc
 	if (!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_spec_ctrl_restore_host(vmx->spec_ctrl);
 
 	/* Eliminate branch target predictions from guest mode */
 	vmexit_fill_RSB();