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Subject[PATCH 4.9 08/31] x86/cpu: Add a "tsx=" cmdline option with TSX disabled by default
Date
From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>

commit 95c5824f75f3ba4c9e8e5a4b1a623c95390ac266 upstream.

Add a kernel cmdline parameter "tsx" to control the Transactional
Synchronization Extensions (TSX) feature. On CPUs that support TSX
control, use "tsx=on|off" to enable or disable TSX. Not specifying this
option is equivalent to "tsx=off". This is because on certain processors
TSX may be used as a part of a speculative side channel attack.

Carve out the TSX controlling functionality into a separate compilation
unit because TSX is a CPU feature while the TSX async abort control
machinery will go to cpu/bugs.c.

[ bp: - Massage, shorten and clear the arg buffer.
- Clarifications of the tsx= possible options - Josh.
- Expand on TSX_CTRL availability - Pawan. ]

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>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
[bwh: Backported to 4.9: adjust filenames, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/kernel-parameters.txt | 26 +++++++
arch/x86/kernel/cpu/Makefile | 2
arch/x86/kernel/cpu/common.c | 2
arch/x86/kernel/cpu/cpu.h | 16 ++++
arch/x86/kernel/cpu/intel.c | 5 +
arch/x86/kernel/cpu/tsx.c | 125 ++++++++++++++++++++++++++++++++++++
6 files changed, 175 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/kernel/cpu/tsx.c

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -4516,6 +4516,32 @@ bytes respectively. Such letter suffixes
platforms where RDTSC is slow and this accounting
can add overhead.

+ tsx= [X86] Control Transactional Synchronization
+ Extensions (TSX) feature in Intel processors that
+ support TSX control.
+
+ This parameter controls the TSX feature. The options are:
+
+ on - Enable TSX on the system. Although there are
+ mitigations for all known security vulnerabilities,
+ TSX has been known to be an accelerator for
+ several previous speculation-related CVEs, and
+ so there may be unknown security risks associated
+ with leaving it enabled.
+
+ off - Disable TSX on the system. (Note that this
+ option takes effect only on newer CPUs which are
+ not vulnerable to MDS, i.e., have
+ MSR_IA32_ARCH_CAPABILITIES.MDS_NO=1 and which get
+ the new IA32_TSX_CTRL MSR through a microcode
+ update. This new MSR allows for the reliable
+ deactivation of the TSX functionality.)
+
+ Not specifying this option is equivalent to tsx=off.
+
+ See Documentation/hw-vuln/tsx_async_abort.rst
+ for more details.
+
turbografx.map[2|3]= [HW,JOY]
TurboGraFX parallel port interface
Format:
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -25,7 +25,7 @@ obj-y += bugs.o
obj-$(CONFIG_PROC_FS) += proc.o
obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o

-obj-$(CONFIG_CPU_SUP_INTEL) += intel.o
+obj-$(CONFIG_CPU_SUP_INTEL) += intel.o tsx.o
obj-$(CONFIG_CPU_SUP_AMD) += amd.o
obj-$(CONFIG_CPU_SUP_CYRIX_32) += cyrix.o
obj-$(CONFIG_CPU_SUP_CENTAUR) += centaur.o
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1416,6 +1416,8 @@ void __init identify_boot_cpu(void)
enable_sep_cpu();
#endif
cpu_detect_tlb(&boot_cpu_data);
+
+ tsx_init();
}

void identify_secondary_cpu(struct cpuinfo_x86 *c)
--- a/arch/x86/kernel/cpu/cpu.h
+++ b/arch/x86/kernel/cpu/cpu.h
@@ -44,6 +44,22 @@ struct _tlb_table {
extern const struct cpu_dev *const __x86_cpu_dev_start[],
*const __x86_cpu_dev_end[];

+#ifdef CONFIG_CPU_SUP_INTEL
+enum tsx_ctrl_states {
+ TSX_CTRL_ENABLE,
+ TSX_CTRL_DISABLE,
+ TSX_CTRL_NOT_SUPPORTED,
+};
+
+extern __ro_after_init enum tsx_ctrl_states tsx_ctrl_state;
+
+extern void __init tsx_init(void);
+extern void tsx_enable(void);
+extern void tsx_disable(void);
+#else
+static inline void tsx_init(void) { }
+#endif /* CONFIG_CPU_SUP_INTEL */
+
extern void get_cpu_cap(struct cpuinfo_x86 *c);
extern void cpu_detect_cache_sizes(struct cpuinfo_x86 *c);
extern int detect_extended_topology_early(struct cpuinfo_x86 *c);
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -642,6 +642,11 @@ static void init_intel(struct cpuinfo_x8
detect_vmx_virtcap(c);

init_intel_energy_perf(c);
+
+ if (tsx_ctrl_state == TSX_CTRL_ENABLE)
+ tsx_enable();
+ if (tsx_ctrl_state == TSX_CTRL_DISABLE)
+ tsx_disable();
}

#ifdef CONFIG_X86_32
--- /dev/null
+++ b/arch/x86/kernel/cpu/tsx.c
@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Intel Transactional Synchronization Extensions (TSX) control.
+ *
+ * Copyright (C) 2019 Intel Corporation
+ *
+ * Author:
+ * Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
+ */
+
+#include <linux/cpufeature.h>
+
+#include <asm/cmdline.h>
+
+#include "cpu.h"
+
+enum tsx_ctrl_states tsx_ctrl_state __ro_after_init = TSX_CTRL_NOT_SUPPORTED;
+
+void tsx_disable(void)
+{
+ u64 tsx;
+
+ rdmsrl(MSR_IA32_TSX_CTRL, tsx);
+
+ /* Force all transactions to immediately abort */
+ tsx |= TSX_CTRL_RTM_DISABLE;
+
+ /*
+ * Ensure TSX support is not enumerated in CPUID.
+ * This is visible to userspace and will ensure they
+ * do not waste resources trying TSX transactions that
+ * will always abort.
+ */
+ tsx |= TSX_CTRL_CPUID_CLEAR;
+
+ wrmsrl(MSR_IA32_TSX_CTRL, tsx);
+}
+
+void tsx_enable(void)
+{
+ u64 tsx;
+
+ rdmsrl(MSR_IA32_TSX_CTRL, tsx);
+
+ /* Enable the RTM feature in the cpu */
+ tsx &= ~TSX_CTRL_RTM_DISABLE;
+
+ /*
+ * Ensure TSX support is enumerated in CPUID.
+ * This is visible to userspace and will ensure they
+ * can enumerate and use the TSX feature.
+ */
+ tsx &= ~TSX_CTRL_CPUID_CLEAR;
+
+ wrmsrl(MSR_IA32_TSX_CTRL, tsx);
+}
+
+static bool __init tsx_ctrl_is_supported(void)
+{
+ u64 ia32_cap = x86_read_arch_cap_msr();
+
+ /*
+ * TSX is controlled via MSR_IA32_TSX_CTRL. However, support for this
+ * MSR is enumerated by ARCH_CAP_TSX_MSR bit in MSR_IA32_ARCH_CAPABILITIES.
+ *
+ * TSX control (aka MSR_IA32_TSX_CTRL) is only available after a
+ * microcode update on CPUs that have their MSR_IA32_ARCH_CAPABILITIES
+ * bit MDS_NO=1. CPUs with MDS_NO=0 are not planned to get
+ * MSR_IA32_TSX_CTRL support even after a microcode update. Thus,
+ * tsx= cmdline requests will do nothing on CPUs without
+ * MSR_IA32_TSX_CTRL support.
+ */
+ return !!(ia32_cap & ARCH_CAP_TSX_CTRL_MSR);
+}
+
+void __init tsx_init(void)
+{
+ char arg[4] = {};
+ int ret;
+
+ if (!tsx_ctrl_is_supported())
+ return;
+
+ ret = cmdline_find_option(boot_command_line, "tsx", arg, sizeof(arg));
+ if (ret >= 0) {
+ if (!strcmp(arg, "on")) {
+ tsx_ctrl_state = TSX_CTRL_ENABLE;
+ } else if (!strcmp(arg, "off")) {
+ tsx_ctrl_state = TSX_CTRL_DISABLE;
+ } else {
+ tsx_ctrl_state = TSX_CTRL_DISABLE;
+ pr_err("tsx: invalid option, defaulting to off\n");
+ }
+ } else {
+ /* tsx= not provided, defaulting to off */
+ tsx_ctrl_state = TSX_CTRL_DISABLE;
+ }
+
+ if (tsx_ctrl_state == TSX_CTRL_DISABLE) {
+ tsx_disable();
+
+ /*
+ * tsx_disable() will change the state of the
+ * RTM CPUID bit. Clear it here since it is now
+ * expected to be not set.
+ */
+ setup_clear_cpu_cap(X86_FEATURE_RTM);
+ } else if (tsx_ctrl_state == TSX_CTRL_ENABLE) {
+
+ /*
+ * HW defaults TSX to be enabled at bootup.
+ * We may still need the TSX enable support
+ * during init for special cases like
+ * kexec after TSX is disabled.
+ */
+ tsx_enable();
+
+ /*
+ * tsx_enable() will change the state of the
+ * RTM CPUID bit. Force it here since it is now
+ * expected to be set.
+ */
+ setup_force_cpu_cap(X86_FEATURE_RTM);
+ }
+}

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