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Subject: [PATCH 07/19] x86, mpx: boot-time disable
To: linux-kernel@vger.kernel.org
Cc: x86@kernel.org, tglx@linutronix.de, Dave Hansen <dave@sr71.net>,
        dave.hansen@linux.intel.com
From: Dave Hansen <dave@sr71.net>
Date: Sun, 07 Jun 2015 11:37:02 -0700
References: <20150607183700.7E78D631@viggo.jf.intel.com>
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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
_