[PATCH 2/2] s390/mmap: randomize mmap base for bottom up direction

[Heiko Carstens <heiko.carstens@de.ibm.com>]

Implement mmap base randomization for the bottom up direction, so ASLR works
for both mmap layouts on s390.
See also df54d6fa54 "x86 get_unmapped_area(): use proper mmap base for
bottom-up direction".

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---
 arch/s390/mm/mmap.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/s390/mm/mmap.c b/arch/s390/mm/mmap.c
index 4002329..7110c0f 100644
--- a/arch/s390/mm/mmap.c
+++ b/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();
@@ -172,7 +177,7 @@ 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();
-- 
1.8.3.4

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