[PATCH] [v4] x86/mm/mpx: Work around MPX erratum SKD046

[Dave Hansen]

Changes from v1:
 * Unconditionally enable workaround on all CPUs with MPX despite
   whether we know it to be affected or not
 * Add a pr_warn() when the workaround is active

changes from v2:
 * fix build breakage from #ifdefs in bug.h

changes from v3:
 * fix x86 defconfig warning about cpuinfo_x86 being undefined

--

From: Dave Hansen <dave@xxxxxxxx>

This erratum essentially causes the CPU to forget which privilege
level it is operating on (kernel vs. user) for the purposes of MPX.

This erratum can only be triggered when a system is not using
Supervisor Mode Execution Prevention (SMEP).  Our workaround for
the erratum is to ensure that MPX can only be used in cases where
SMEP is present in the processor and is enabled.

This erratum only affects Core processors.  Atom is unaffected.
But, there is no architectural way to determine Atom vs. Core.
So, we just apply this workaround to all processors.  It's
possible that it will mistakenly disable MPX on some Atom
processsors or future unaffected Core processors.  There are
currently no processors that have MPX and not SMEP.  It would
take something akin to a hypervisor masking SMEP out on an Atom
processor for this to present itself on current hardware.

More details can be found at:

  http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/desktop-6th-gen-core-family-spec-update.pdf

"
  SKD046 Branch Instructions May Initialize MPX Bound Registers Incorrectly

  Problem:

  Depending on the current Intel MPX (Memory Protection
  Extensions) configuration, execution of certain branch
  instructions (near CALL, near RET, near JMP, and Jcc
  instructions) without a BND prefix (F2H) initialize the MPX bound
  registers. Due to this erratum, such a branch instruction that is
  executed both with CPL = 3 and with CPL < 3 may not use the
  correct MPX configuration register (BNDCFGU or BNDCFGS,
  respectively) for determining whether to initialize the bound
  registers; it may thus initialize the bound registers when it
  should not, or fail to initialize them when it should.

  Implication:

  A branch instruction that has executed both in user mode and in
  supervisor mode (from the same linear address) may cause a #BR
  (bound range fault) when it should not have or may not cause a
  #BR when it should have.  Workaround An operating system can
  avoid this erratum by setting CR4.SMEP[bit 20] to enable
  supervisor-mode execution prevention (SMEP). When SMEP is
  enabled, no code can be executed both with CPL = 3 and with CPL < 3.
"

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave@xxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20160504205359.19DB7812@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---

 b/arch/x86/include/asm/bugs.h  |    8 ++++++++
 b/arch/x86/kernel/cpu/common.c |    3 +++
 b/arch/x86/kernel/cpu/intel.c  |   37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 48 insertions(+)

diff -puN arch/x86/include/asm/bugs.h~x86-mm-mpx-Work-around-MPX-erratum-SKD046 arch/x86/include/asm/bugs.h
--- a/arch/x86/include/asm/bugs.h~x86-mm-mpx-Work-around-MPX-erratum-SKD046	2016-05-12 08:39:22.433049393 -0700
+++ b/arch/x86/include/asm/bugs.h	2016-05-12 08:52:13.395696312 -0700
@@ -1,8 +1,16 @@
 #ifndef _ASM_X86_BUGS_H
 #define _ASM_X86_BUGS_H
 
+#include <asm/processor.h>
+
 extern void check_bugs(void);
 
+#if defined(CONFIG_CPU_SUP_INTEL)
+void check_mpx_erratum(struct cpuinfo_x86 *c);
+#else
+static inline void check_mpx_erratum(struct cpuinfo_x86 *c) {}
+#endif
+
 #if defined(CONFIG_CPU_SUP_INTEL) && defined(CONFIG_X86_32)
 int ppro_with_ram_bug(void);
 #else
diff -puN arch/x86/kernel/cpu/common.c~x86-mm-mpx-Work-around-MPX-erratum-SKD046 arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~x86-mm-mpx-Work-around-MPX-erratum-SKD046	2016-05-12 08:39:22.435049483 -0700
+++ b/arch/x86/kernel/cpu/common.c	2016-05-12 08:39:22.448050067 -0700
@@ -37,6 +37,7 @@
 #include <asm/mtrr.h>
 #include <linux/numa.h>
 #include <asm/asm.h>
+#include <asm/bugs.h>
 #include <asm/cpu.h>
 #include <asm/mce.h>
 #include <asm/msr.h>
@@ -270,6 +271,8 @@ static inline void squash_the_stupid_ser
 static __init int setup_disable_smep(char *arg)
 {
 	setup_clear_cpu_cap(X86_FEATURE_SMEP);
+	/* Check for things that depend on SMEP being enabled: */
+	check_mpx_erratum(&boot_cpu_data);
 	return 1;
 }
 __setup("nosmep", setup_disable_smep);
diff -puN arch/x86/kernel/cpu/intel.c~x86-mm-mpx-Work-around-MPX-erratum-SKD046 arch/x86/kernel/cpu/intel.c
--- a/arch/x86/kernel/cpu/intel.c~x86-mm-mpx-Work-around-MPX-erratum-SKD046	2016-05-12 08:39:22.436049528 -0700
+++ b/arch/x86/kernel/cpu/intel.c	2016-05-12 08:39:22.449050112 -0700
@@ -25,6 +25,41 @@
 #include <asm/apic.h>
 #endif
 
+/*
+ * Just in case our CPU detection goes bad, or you have a weird system,
+ * allow a way to override the automatic disabling of MPX.
+ */
+static int forcempx;
+
+static int __init forcempx_setup(char *__unused)
+{
+	forcempx = 1;
+
+	return 1;
+}
+__setup("intel-skd-046-workaround=disable", forcempx_setup);
+
+void check_mpx_erratum(struct cpuinfo_x86 *c)
+{
+	if (forcempx)
+		return;
+	/*
+	 * Turn off the MPX feature on CPUs where SMEP is not
+	 * available or disabled.
+	 *
+	 * Works around Intel Erratum SKD046: "Branch Instructions
+	 * May Initialize MPX Bound Registers Incorrectly".
+	 *
+	 * This might falsely disable MPX on systems without
+	 * SMEP, like Atom processors without SMEP.  But there
+	 * is no such hardware known at the moment.
+	 */
+	if (cpu_has(c, X86_FEATURE_MPX) && !cpu_has(c, X86_FEATURE_SMEP)) {
+		setup_clear_cpu_cap(X86_FEATURE_MPX);
+		pr_warn("x86/mpx: Disabling MPX since SMEP not present\n");
+	}
+}
+
 static void early_init_intel(struct cpuinfo_x86 *c)
 {
 	u64 misc_enable;
@@ -173,6 +208,8 @@ static void early_init_intel(struct cpui
 		if (edx & (1U << 28))
 			c->x86_coreid_bits = get_count_order((ebx >> 16) & 0xff);
 	}
+
+	check_mpx_erratum(c);
 }
 
 #ifdef CONFIG_X86_32
_