[PATCH 5.10 10/11] KVM: x86/speculation: Disable Fill buffer clear within guests

Greg Kroah-Hartman
2022-06-14T18:50:06Z

From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

commit 027bbb884be006b05d9c577d6401686053aa789e upstream

The enumeration of MD_CLEAR in CPUID(EAX=7,ECX=0).EDX{bit 10} is not an
accurate indicator on all CPUs of whether the VERW instruction will
overwrite fill buffers. FB_CLEAR enumeration in
IA32_ARCH_CAPABILITIES{bit 17} covers the case of CPUs that are not
vulnerable to MDS/TAA, indicating that microcode does overwrite fill
buffers.

Guests running in VMM environments may not be aware of all the
capabilities/vulnerabilities of the host CPU. Specifically, a guest may
apply MDS/TAA mitigations when a virtual CPU is enumerated as vulnerable
to MDS/TAA even when the physical CPU is not. On CPUs that enumerate
FB_CLEAR_CTRL the VMM may set FB_CLEAR_DIS to skip overwriting of fill
buffers by the VERW instruction. This is done by setting FB_CLEAR_DIS
during VMENTER and resetting on VMEXIT. For guests that enumerate
FB_CLEAR (explicitly asking for fill buffer clear capability) the VMM
will not use FB_CLEAR_DIS.

Irrespective of guest state, host overwrites CPU buffers before VMENTER
to protect itself from an MMIO capable guest, as part of mitigation for
MMIO Stale Data vulnerabilities.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/msr-index.h | 6 ++
arch/x86/kvm/vmx/vmx.c | 69 +++++++++++++++++++++++++++++++++
arch/x86/kvm/vmx/vmx.h | 2
arch/x86/kvm/x86.c | 3 +
tools/arch/x86/include/asm/msr-index.h | 6 ++
5 files changed, 86 insertions(+)

--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -133,6 +133,11 @@
* VERW clears CPU fill buffer
* even on MDS_NO CPUs.
*/
+#define ARCH_CAP_FB_CLEAR_CTRL BIT(18) /*
+ * MSR_IA32_MCU_OPT_CTRL[FB_CLEAR_DIS]
+ * bit available to control VERW
+ * behavior.
+ */

#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*
@@ -150,6 +155,7 @@
/* SRBDS support */
#define MSR_IA32_MCU_OPT_CTRL 0x00000123
#define RNGDS_MITG_DIS BIT(0)
+#define FB_CLEAR_DIS BIT(3) /* CPU Fill buffer clear disable */

#define MSR_IA32_SYSENTER_CS 0x00000174
#define MSR_IA32_SYSENTER_ESP 0x00000175
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -226,6 +226,9 @@ static const struct {
#define L1D_CACHE_ORDER 4
static void *vmx_l1d_flush_pages;

+/* Control for disabling CPU Fill buffer clear */
+static bool __read_mostly vmx_fb_clear_ctrl_available;
+
static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
{
struct page *page;
@@ -357,6 +360,60 @@ static int vmentry_l1d_flush_get(char *s
return sprintf(s, "%s\n", vmentry_l1d_param[l1tf_vmx_mitigation].option);
}

+static void vmx_setup_fb_clear_ctrl(void)
+{
+ u64 msr;
+
+ if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES) &&
+ !boot_cpu_has_bug(X86_BUG_MDS) &&
+ !boot_cpu_has_bug(X86_BUG_TAA)) {
+ rdmsrl(MSR_IA32_ARCH_CAPABILITIES, msr);
+ if (msr & ARCH_CAP_FB_CLEAR_CTRL)
+ vmx_fb_clear_ctrl_available = true;
+ }
+}
+
+static __always_inline void vmx_disable_fb_clear(struct vcpu_vmx *vmx)
+{
+ u64 msr;
+
+ if (!vmx->disable_fb_clear)
+ return;
+
+ rdmsrl(MSR_IA32_MCU_OPT_CTRL, msr);
+ msr |= FB_CLEAR_DIS;
+ wrmsrl(MSR_IA32_MCU_OPT_CTRL, msr);
+ /* Cache the MSR value to avoid reading it later */
+ vmx->msr_ia32_mcu_opt_ctrl = msr;
+}
+
+static __always_inline void vmx_enable_fb_clear(struct vcpu_vmx *vmx)
+{
+ if (!vmx->disable_fb_clear)
+ return;
+
+ vmx->msr_ia32_mcu_opt_ctrl &= ~FB_CLEAR_DIS;
+ wrmsrl(MSR_IA32_MCU_OPT_CTRL, vmx->msr_ia32_mcu_opt_ctrl);
+}
+
+static void vmx_update_fb_clear_dis(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
+{
+ vmx->disable_fb_clear = vmx_fb_clear_ctrl_available;
+
+ /*
+ * If guest will not execute VERW, there is no need to set FB_CLEAR_DIS
+ * at VMEntry. Skip the MSR read/write when a guest has no use case to
+ * execute VERW.
+ */
+ if ((vcpu->arch.arch_capabilities & ARCH_CAP_FB_CLEAR) ||
+ ((vcpu->arch.arch_capabilities & ARCH_CAP_MDS_NO) &&
+ (vcpu->arch.arch_capabilities & ARCH_CAP_TAA_NO) &&
+ (vcpu->arch.arch_capabilities & ARCH_CAP_PSDP_NO) &&
+ (vcpu->arch.arch_capabilities & ARCH_CAP_FBSDP_NO) &&
+ (vcpu->arch.arch_capabilities & ARCH_CAP_SBDR_SSDP_NO)))
+ vmx->disable_fb_clear = false;
+}
+
static const struct kernel_param_ops vmentry_l1d_flush_ops = {
.set = vmentry_l1d_flush_set,
.get = vmentry_l1d_flush_get,
@@ -2211,6 +2268,10 @@ static int vmx_set_msr(struct kvm_vcpu *
ret = kvm_set_msr_common(vcpu, msr_info);
}

+ /* FB_CLEAR may have changed, also update the FB_CLEAR_DIS behavior */
+ if (msr_index == MSR_IA32_ARCH_CAPABILITIES)
+ vmx_update_fb_clear_dis(vcpu, vmx);
+
return ret;
}

@@ -4483,6 +4544,8 @@ static void vmx_vcpu_reset(struct kvm_vc
vpid_sync_context(vmx->vpid);
if (init_event)
vmx_clear_hlt(vcpu);
+
+ vmx_update_fb_clear_dis(vcpu, vmx);
}

static void enable_irq_window(struct kvm_vcpu *vcpu)
@@ -6658,6 +6721,8 @@ static noinstr void vmx_vcpu_enter_exit(
kvm_arch_has_assigned_device(vcpu->kvm))
mds_clear_cpu_buffers();

+ vmx_disable_fb_clear(vmx);
+
if (vcpu->arch.cr2 != native_read_cr2())
native_write_cr2(vcpu->arch.cr2);

@@ -6666,6 +6731,8 @@ static noinstr void vmx_vcpu_enter_exit(

vcpu->arch.cr2 = native_read_cr2();

+ vmx_enable_fb_clear(vmx);
+
/*
* VMEXIT disables interrupts (host state), but tracing and lockdep
* have them in state 'on' as recorded before entering guest mode.
@@ -8050,6 +8117,8 @@ static int __init vmx_init(void)
return r;
}

+ vmx_setup_fb_clear_ctrl();
+
for_each_possible_cpu(cpu) {
INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));

--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -300,6 +300,8 @@ struct vcpu_vmx {
u64 msr_ia32_feature_control;
u64 msr_ia32_feature_control_valid_bits;
u64 ept_pointer;
+ u64 msr_ia32_mcu_opt_ctrl;
+ bool disable_fb_clear;

struct pt_desc pt_desc;

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1415,6 +1415,9 @@ static u64 kvm_get_arch_capabilities(voi
*/
}

+ /* Guests don't need to know "Fill buffer clear control" exists */
+ data &= ~ARCH_CAP_FB_CLEAR_CTRL;
+
return data;
}

--- a/tools/arch/x86/include/asm/msr-index.h
+++ b/tools/arch/x86/include/asm/msr-index.h
@@ -133,6 +133,11 @@
* VERW clears CPU fill buffer
* even on MDS_NO CPUs.
*/
+#define ARCH_CAP_FB_CLEAR_CTRL BIT(18) /*
+ * MSR_IA32_MCU_OPT_CTRL[FB_CLEAR_DIS]
+ * bit available to control VERW
+ * behavior.
+ */

#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*
@@ -150,6 +155,7 @@
/* SRBDS support */
#define MSR_IA32_MCU_OPT_CTRL 0x00000123
#define RNGDS_MITG_DIS BIT(0)
+#define FB_CLEAR_DIS BIT(3) /* CPU Fill buffer clear disable */

#define MSR_IA32_SYSENTER_CS 0x00000174
#define MSR_IA32_SYSENTER_ESP 0x00000175