x86/bugs, KVM: Support the combination of guest and host IBRS

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@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 9ec3d4d..d1c2630 100644 (file)
--- 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_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[];
 

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6ed84f5..38a8626 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -123,6 +123,24 @@ 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;
 

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 1fc05e4..8bfc8fe 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -5557,8 +5557,7 @@ static void svm_vcpu_run(struct kvm_vcpu *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"
@@ -5670,8 +5669,7 @@ static void svm_vcpu_run(struct kvm_vcpu *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();

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c766880..3091dc9 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9720,8 +9720,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
         * 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;
 
@@ -9869,8 +9868,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
        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_spec_ctrl_restore_host(vmx->spec_ctrl);
 
        /* Eliminate branch target predictions from guest mode */
        vmexit_fill_RSB();