x86/mm: Avoid VLA in pgd_alloc()

x86/mm: Avoid VLA in pgd_alloc()

Arnd Bergmann reported that turning on -Wvla found a new (unintended) VLA usage:

  arch/x86/mm/pgtable.c: In function 'pgd_alloc':
  include/linux/build_bug.h:29:45: error: ISO C90 forbids variable length array 'u_pmds' [-Werror=vla]
  arch/x86/mm/pgtable.c:190:34: note: in expansion of macro 'static_cpu_has'
   #define PREALLOCATED_USER_PMDS  (static_cpu_has(X86_FEATURE_PTI) ? \
                                    ^~~~~~~~~~~~~~
  arch/x86/mm/pgtable.c:431:16: note: in expansion of macro 'PREALLOCATED_USER_PMDS'
    pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
                ^~~~~~~~~~~~~~~~~~~~~~

Use the actual size of the array that is used for X86_FEATURE_PTI,
which is known at build time, instead of the variable size.

[ mingo: Squashed original fix with followup fix to avoid bisection breakage, wrote new changelog. ]

Reported-by: Arnd Bergmann <[email protected]>
Original-written-by: Arnd Bergmann <[email protected]>
Reported-by: Borislav Petkov <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Toshi Kani <[email protected]>
Fixes: 1be3f247c288 ("x86/mm: Avoid VLA in pgd_alloc()")
Link: http://lkml.kernel.org/r/20181008235434.GA35035@beast
Signed-off-by: Ingo Molnar <[email protected]>