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Subject[PATCH 4.14 52/95] x86/bugs, KVM: Support the combination of guest and host IBRS
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

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

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

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@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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_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
@@ -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;

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -5015,8 +5015,7 @@ 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"
@@ -5128,8 +5127,7 @@ 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
@@ -9452,8 +9452,7 @@ 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(
@@ -9591,8 +9590,7 @@ 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_spec_ctrl_restore_host(vmx->spec_ctrl);

/* Eliminate branch target predictions from guest mode */
vmexit_fill_RSB();

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