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Subject[PATCH 07/19] x86, mpx: boot-time disable
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From: Dave Hansen <dave.hansen@linux.intel.com>

MPX has the _potential_ to cause some issues. Say part of your init
system tried to protect one of its components from buffer overflows
with MPX. If there were a false positive, it's possible that MPX
could keep a system from booting.

MPX could also potentially cause performance issues since it is
present in hot paths like the unmap path.

Allow it to be disabled at boot time.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de
---

b/Documentation/kernel-parameters.txt | 4 ++++
b/arch/x86/kernel/cpu/common.c | 16 ++++++++++++++++
2 files changed, 20 insertions(+)

diff -puN arch/x86/kernel/cpu/common.c~x86-mpx-disable-boot-time arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~x86-mpx-disable-boot-time 2015-06-01 10:24:05.614793473 -0700
+++ b/arch/x86/kernel/cpu/common.c 2015-06-01 10:24:05.620793744 -0700
@@ -144,6 +144,22 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_p
} };
EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);

+static int __init x86_mpx_setup(char *s)
+{
+ /* require an exact match without trailing characters */
+ if (strlen(s))
+ return 0;
+
+ /* do not emit a message if the feature is not present */
+ if (!boot_cpu_has(X86_FEATURE_MPX))
+ return 1;
+
+ setup_clear_cpu_cap(X86_FEATURE_MPX);
+ pr_info("nompx: Intel Memory Protection Extensions (MPX) disabled\n");
+ return 1;
+}
+__setup("nompx", x86_mpx_setup);
+
#ifdef CONFIG_X86_32
static int cachesize_override = -1;
static int disable_x86_serial_nr = 1;
diff -puN Documentation/kernel-parameters.txt~x86-mpx-disable-boot-time Documentation/kernel-parameters.txt
--- a/Documentation/kernel-parameters.txt~x86-mpx-disable-boot-time 2015-06-01 10:24:05.616793563 -0700
+++ b/Documentation/kernel-parameters.txt 2015-06-01 10:24:05.621793789 -0700
@@ -937,6 +937,10 @@ bytes respectively. Such letter suffixes
Enable debug messages at boot time. See
Documentation/dynamic-debug-howto.txt for details.

+ nompx [X86] Disables Intel Memory Protection Extensions.
+ See Documentation/x86/intel_mpx.txt for more
+ information about the feature.
+
eagerfpu= [X86]
on enable eager fpu restore
off disable eager fpu restore
_

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