[PATCH] x86_64: access of some bad address

x86_64 has a large sparse gate area between VSYSCALL_START and
VSYSCALL_END, not all of it presently backed by pmds.  Alexander Nyberg has
found that in some circumstances gdb may try to ptrace here, and hit
get_user_pages BUG_ON.  It seems odd that gdb should be accessing here, but
it certainly shouldn't crash in this way: relax BUG_ON to -EFAULT.  Fixes
kernel bugzilla #4801.
Signed-off-by: NHugh Dickins <hugh@veritas.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/memory.c

@@ -910,9 +910,13 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 			pud = pud_offset(pgd, pg);
 			BUG_ON(pud_none(*pud));
 			pmd = pmd_offset(pud, pg);
-			BUG_ON(pmd_none(*pmd));
+			if (pmd_none(*pmd))
+				return i ? : -EFAULT;
 			pte = pte_offset_map(pmd, pg);
-			BUG_ON(pte_none(*pte));
+			if (pte_none(*pte)) {
+				pte_unmap(pte);
+				return i ? : -EFAULT;
+			}
 			if (pages) {
 				pages[i] = pte_page(*pte);
 				get_page(pages[i]);