x86/mm/mpx: Work around MPX erratum SKD046

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@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160512220400.3B35F1BC@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>