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Subject[PATCH 4.9 078/107] x86/bugs, kvm: Introduce boot-time control of L1TF mitigations
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

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

From: Jiri Kosina <jkosina@suse.cz>

commit d90a7a0ec83fb86622cd7dae23255d3c50a99ec8 upstream

Introduce the 'l1tf=' kernel command line option to allow for boot-time
switching of mitigation that is used on processors affected by L1TF.

The possible values are:

full
Provides all available mitigations for the L1TF vulnerability. Disables
SMT and enables all mitigations in the hypervisors. SMT control via
/sys/devices/system/cpu/smt/control is still possible after boot.
Hypervisors will issue a warning when the first VM is started in
a potentially insecure configuration, i.e. SMT enabled or L1D flush
disabled.

full,force
Same as 'full', but disables SMT control. Implies the 'nosmt=force'
command line option. sysfs control of SMT and the hypervisor flush
control is disabled.

flush
Leaves SMT enabled and enables the conditional hypervisor mitigation.
Hypervisors will issue a warning when the first VM is started in a
potentially insecure configuration, i.e. SMT enabled or L1D flush
disabled.

flush,nosmt
Disables SMT and enables the conditional hypervisor mitigation. SMT
control via /sys/devices/system/cpu/smt/control is still possible
after boot. If SMT is reenabled or flushing disabled at runtime
hypervisors will issue a warning.

flush,nowarn
Same as 'flush', but hypervisors will not warn when
a VM is started in a potentially insecure configuration.

off
Disables hypervisor mitigations and doesn't emit any warnings.

Default is 'flush'.

Let KVM adhere to these semantics, which means:

- 'lt1f=full,force' : Performe L1D flushes. No runtime control
possible.

- 'l1tf=full'
- 'l1tf-flush'
- 'l1tf=flush,nosmt' : Perform L1D flushes and warn on VM start if
SMT has been runtime enabled or L1D flushing
has been run-time enabled

- 'l1tf=flush,nowarn' : Perform L1D flushes and no warnings are emitted.

- 'l1tf=off' : L1D flushes are not performed and no warnings
are emitted.

KVM can always override the L1D flushing behavior using its 'vmentry_l1d_flush'
module parameter except when lt1f=full,force is set.

This makes KVM's private 'nosmt' option redundant, and as it is a bit
non-systematic anyway (this is something to control globally, not on
hypervisor level), remove that option.

Add the missing Documentation entry for the l1tf vulnerability sysfs file
while at it.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20180713142323.202758176@linutronix.de
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/ABI/testing/sysfs-devices-system-cpu | 4 +
Documentation/kernel-parameters.txt | 68 +++++++++++++++++++--
arch/x86/include/asm/processor.h | 12 +++
arch/x86/kernel/cpu/bugs.c | 44 +++++++++++++
arch/x86/kvm/vmx.c | 56 +++++++++++++----
5 files changed, 165 insertions(+), 19 deletions(-)

--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -356,6 +356,7 @@ What: /sys/devices/system/cpu/vulnerabi
/sys/devices/system/cpu/vulnerabilities/spectre_v1
/sys/devices/system/cpu/vulnerabilities/spectre_v2
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass
+ /sys/devices/system/cpu/vulnerabilities/l1tf
Date: January 2018
Contact: Linux kernel mailing list <linux-kernel@vger.kernel.org>
Description: Information about CPU vulnerabilities
@@ -368,6 +369,9 @@ Description: Information about CPU vulne
"Vulnerable" CPU is affected and no mitigation in effect
"Mitigation: $M" CPU is affected and mitigation $M is in effect

+ Details about the l1tf file can be found in
+ Documentation/admin-guide/l1tf.rst
+
What: /sys/devices/system/cpu/smt
/sys/devices/system/cpu/smt/active
/sys/devices/system/cpu/smt/control
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1989,12 +1989,6 @@ bytes respectively. Such letter suffixes
for all guests.
Default is 1 (enabled) if in 64-bit or 32-bit PAE mode.

- kvm-intel.nosmt=[KVM,Intel] If the L1TF CPU bug is present (CVE-2018-3620)
- and the system has SMT (aka Hyper-Threading) enabled then
- don't allow guests to be created.
-
- Default is 0 (allow guests to be created).
-
kvm-intel.ept= [KVM,Intel] Disable extended page tables
(virtualized MMU) support on capable Intel chips.
Default is 1 (enabled)
@@ -2032,6 +2026,68 @@ bytes respectively. Such letter suffixes
feature (tagged TLBs) on capable Intel chips.
Default is 1 (enabled)

+ l1tf= [X86] Control mitigation of the L1TF vulnerability on
+ affected CPUs
+
+ The kernel PTE inversion protection is unconditionally
+ enabled and cannot be disabled.
+
+ full
+ Provides all available mitigations for the
+ L1TF vulnerability. Disables SMT and
+ enables all mitigations in the
+ hypervisors, i.e. unconditional L1D flush.
+
+ SMT control and L1D flush control via the
+ sysfs interface is still possible after
+ boot. Hypervisors will issue a warning
+ when the first VM is started in a
+ potentially insecure configuration,
+ i.e. SMT enabled or L1D flush disabled.
+
+ full,force
+ Same as 'full', but disables SMT and L1D
+ flush runtime control. Implies the
+ 'nosmt=force' command line option.
+ (i.e. sysfs control of SMT is disabled.)
+
+ flush
+ Leaves SMT enabled and enables the default
+ hypervisor mitigation, i.e. conditional
+ L1D flush.
+
+ SMT control and L1D flush control via the
+ sysfs interface is still possible after
+ boot. Hypervisors will issue a warning
+ when the first VM is started in a
+ potentially insecure configuration,
+ i.e. SMT enabled or L1D flush disabled.
+
+ flush,nosmt
+
+ Disables SMT and enables the default
+ hypervisor mitigation.
+
+ SMT control and L1D flush control via the
+ sysfs interface is still possible after
+ boot. Hypervisors will issue a warning
+ when the first VM is started in a
+ potentially insecure configuration,
+ i.e. SMT enabled or L1D flush disabled.
+
+ flush,nowarn
+ Same as 'flush', but hypervisors will not
+ warn when a VM is started in a potentially
+ insecure configuration.
+
+ off
+ Disables hypervisor mitigations and doesn't
+ emit any warnings.
+
+ Default is 'flush'.
+
+ For details see: Documentation/admin-guide/l1tf.rst
+
l2cr= [PPC]

l3cr= [PPC]
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -860,4 +860,16 @@ bool xen_set_default_idle(void);

void stop_this_cpu(void *dummy);
void df_debug(struct pt_regs *regs, long error_code);
+
+enum l1tf_mitigations {
+ L1TF_MITIGATION_OFF,
+ L1TF_MITIGATION_FLUSH_NOWARN,
+ L1TF_MITIGATION_FLUSH,
+ L1TF_MITIGATION_FLUSH_NOSMT,
+ L1TF_MITIGATION_FULL,
+ L1TF_MITIGATION_FULL_FORCE
+};
+
+extern enum l1tf_mitigations l1tf_mitigation;
+
#endif /* _ASM_X86_PROCESSOR_H */
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -643,7 +643,11 @@ void x86_spec_ctrl_setup_ap(void)
#undef pr_fmt
#define pr_fmt(fmt) "L1TF: " fmt

+/* Default mitigation for L1TF-affected CPUs */
+enum l1tf_mitigations l1tf_mitigation __ro_after_init = L1TF_MITIGATION_FLUSH;
#if IS_ENABLED(CONFIG_KVM_INTEL)
+EXPORT_SYMBOL_GPL(l1tf_mitigation);
+
enum vmx_l1d_flush_state l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_AUTO;
EXPORT_SYMBOL_GPL(l1tf_vmx_mitigation);
#endif
@@ -655,6 +659,20 @@ static void __init l1tf_select_mitigatio
if (!boot_cpu_has_bug(X86_BUG_L1TF))
return;

+ switch (l1tf_mitigation) {
+ case L1TF_MITIGATION_OFF:
+ case L1TF_MITIGATION_FLUSH_NOWARN:
+ case L1TF_MITIGATION_FLUSH:
+ break;
+ case L1TF_MITIGATION_FLUSH_NOSMT:
+ case L1TF_MITIGATION_FULL:
+ cpu_smt_disable(false);
+ break;
+ case L1TF_MITIGATION_FULL_FORCE:
+ cpu_smt_disable(true);
+ break;
+ }
+
#if CONFIG_PGTABLE_LEVELS == 2
pr_warn("Kernel not compiled for PAE. No mitigation for L1TF\n");
return;
@@ -673,6 +691,32 @@ static void __init l1tf_select_mitigatio

setup_force_cpu_cap(X86_FEATURE_L1TF_PTEINV);
}
+
+static int __init l1tf_cmdline(char *str)
+{
+ if (!boot_cpu_has_bug(X86_BUG_L1TF))
+ return 0;
+
+ if (!str)
+ return -EINVAL;
+
+ if (!strcmp(str, "off"))
+ l1tf_mitigation = L1TF_MITIGATION_OFF;
+ else if (!strcmp(str, "flush,nowarn"))
+ l1tf_mitigation = L1TF_MITIGATION_FLUSH_NOWARN;
+ else if (!strcmp(str, "flush"))
+ l1tf_mitigation = L1TF_MITIGATION_FLUSH;
+ else if (!strcmp(str, "flush,nosmt"))
+ l1tf_mitigation = L1TF_MITIGATION_FLUSH_NOSMT;
+ else if (!strcmp(str, "full"))
+ l1tf_mitigation = L1TF_MITIGATION_FULL;
+ else if (!strcmp(str, "full,force"))
+ l1tf_mitigation = L1TF_MITIGATION_FULL_FORCE;
+
+ return 0;
+}
+early_param("l1tf", l1tf_cmdline);
+
#undef pr_fmt

#ifdef CONFIG_SYSFS
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -69,9 +69,6 @@ static const struct x86_cpu_id vmx_cpu_i
};
MODULE_DEVICE_TABLE(x86cpu, vmx_cpu_id);

-static bool __read_mostly nosmt;
-module_param(nosmt, bool, S_IRUGO);
-
static bool __read_mostly enable_vpid = 1;
module_param_named(vpid, enable_vpid, bool, 0444);

@@ -216,15 +213,31 @@ static int vmx_setup_l1d_flush(enum vmx_
{
struct page *page;

- /* If set to 'auto' select 'cond' */
- if (l1tf == VMENTER_L1D_FLUSH_AUTO)
- l1tf = VMENTER_L1D_FLUSH_COND;
-
if (!enable_ept) {
l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_EPT_DISABLED;
return 0;
}

+ /* If set to auto use the default l1tf mitigation method */
+ if (l1tf == VMENTER_L1D_FLUSH_AUTO) {
+ switch (l1tf_mitigation) {
+ case L1TF_MITIGATION_OFF:
+ l1tf = VMENTER_L1D_FLUSH_NEVER;
+ break;
+ case L1TF_MITIGATION_FLUSH_NOWARN:
+ case L1TF_MITIGATION_FLUSH:
+ case L1TF_MITIGATION_FLUSH_NOSMT:
+ l1tf = VMENTER_L1D_FLUSH_COND;
+ break;
+ case L1TF_MITIGATION_FULL:
+ case L1TF_MITIGATION_FULL_FORCE:
+ l1tf = VMENTER_L1D_FLUSH_ALWAYS;
+ break;
+ }
+ } else if (l1tf_mitigation == L1TF_MITIGATION_FULL_FORCE) {
+ l1tf = VMENTER_L1D_FLUSH_ALWAYS;
+ }
+
if (l1tf != VMENTER_L1D_FLUSH_NEVER && !vmx_l1d_flush_pages &&
!boot_cpu_has(X86_FEATURE_FLUSH_L1D)) {
page = alloc_pages(GFP_KERNEL, L1D_CACHE_ORDER);
@@ -9500,16 +9513,33 @@ free_vcpu:
return ERR_PTR(err);
}

-#define L1TF_MSG "SMT enabled with L1TF CPU bug present. Refer to CVE-2018-3620 for details.\n"
+#define L1TF_MSG_SMT "L1TF CPU bug present and SMT on, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html for details.\n"
+#define L1TF_MSG_L1D "L1TF CPU bug present and virtualization mitigation disabled, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html for details.\n"

static int vmx_vm_init(struct kvm *kvm)
{
- if (boot_cpu_has(X86_BUG_L1TF) && cpu_smt_control == CPU_SMT_ENABLED) {
- if (nosmt) {
- pr_err(L1TF_MSG);
- return -EOPNOTSUPP;
+ if (boot_cpu_has(X86_BUG_L1TF) && enable_ept) {
+ switch (l1tf_mitigation) {
+ case L1TF_MITIGATION_OFF:
+ case L1TF_MITIGATION_FLUSH_NOWARN:
+ /* 'I explicitly don't care' is set */
+ break;
+ case L1TF_MITIGATION_FLUSH:
+ case L1TF_MITIGATION_FLUSH_NOSMT:
+ case L1TF_MITIGATION_FULL:
+ /*
+ * Warn upon starting the first VM in a potentially
+ * insecure environment.
+ */
+ if (cpu_smt_control == CPU_SMT_ENABLED)
+ pr_warn_once(L1TF_MSG_SMT);
+ if (l1tf_vmx_mitigation == VMENTER_L1D_FLUSH_NEVER)
+ pr_warn_once(L1TF_MSG_L1D);
+ break;
+ case L1TF_MITIGATION_FULL_FORCE:
+ /* Flush is enforced */
+ break;
}
- pr_warn(L1TF_MSG);
}
return 0;
}

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