s390/mmap: randomize mmap base for bottom up direction

Implement mmap base randomization for the bottom up direction, so ASLR
works for both mmap layouts on s390.  See also commit df54d6fa ("x86
get_unmapped_area(): use proper mmap base for bottom-up direction").
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Cc: Radu Caragea <sinaelgl@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/s390/mm/mmap.c

@@ -64,6 +64,11 @@ static unsigned long mmap_rnd(void)
 	return (get_random_int() & 0x7ffUL) << PAGE_SHIFT;
 }
 
+static unsigned long mmap_base_legacy(void)
+{
+	return TASK_UNMAPPED_BASE + mmap_rnd();
+}
+
 static inline unsigned long mmap_base(void)
 {
 	unsigned long gap = rlimit(RLIMIT_STACK);
@@ -89,7 +94,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 	 * bit is set, or if the expected stack growth is unlimited:
 	 */
 	if (mmap_is_legacy()) {
-		mm->mmap_base = TASK_UNMAPPED_BASE;
+		mm->mmap_base = mmap_base_legacy();
 		mm->get_unmapped_area = arch_get_unmapped_area;
 	} else {
 		mm->mmap_base = mmap_base();
@@ -164,7 +169,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 	 * bit is set, or if the expected stack growth is unlimited:
 	 */
 	if (mmap_is_legacy()) {
-		mm->mmap_base = TASK_UNMAPPED_BASE;
+		mm->mmap_base = mmap_base_legacy();
 		mm->get_unmapped_area = s390_get_unmapped_area;
 	} else {
 		mm->mmap_base = mmap_base();