ARM: Initialize split page table locks for vector page

Without this patch, ARM can not use SPLIT_PTLOCK_CPUS if
PREEMPT_RT_FULL=y because vectors_user_mapping() creates a
VM_ALWAYSDUMP mapping of the vector page (address 0xffff0000), but no
ptl->lock has been allocated for the page.  An attempt to coredump
that page will result in a kernel NULL pointer dereference when
follow_page() attempts to lock the page.

The call tree to the NULL pointer dereference is:

   do_notify_resume()
      get_signal_to_deliver()
         do_coredump()
            elf_core_dump()
               get_dump_page()
                  __get_user_pages()
                     follow_page()
                        pte_offset_map_lock() <----- a #define
                           ...
                              rt_spin_lock()

The underlying problem is exposed by mm-shrink-the-page-frame-to-rt-size.patch.

Signed-off-by: Frank Rowand <[email protected]>
Cc: Frank <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

Author: Frank Rowand

arch/arm/kernel/process.c

@@ -324,6 +324,30 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 }
 
 #ifdef CONFIG_MMU
+/*
+ * CONFIG_SPLIT_PTLOCK_CPUS results in a page->ptl lock.  If the lock is not
+ * initialized by pgtable_page_ctor() then a coredump of the vector page will
+ * fail.
+ */
+static int __init vectors_user_mapping_init_page(void)
+{
+	struct page *page;
+	unsigned long addr = 0xffff0000;
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	pgd = pgd_offset_k(addr);
+	pud = pud_offset(pgd, addr);
+	pmd = pmd_offset(pud, addr);
+	page = pmd_page(*(pmd));
+
+	pgtable_page_ctor(page);
+
+	return 0;
+}
+late_initcall(vectors_user_mapping_init_page);
+
 #ifdef CONFIG_KUSER_HELPERS
 /*
  * The vectors page is always readable from user space for the