powerpc: Ensure random space between stack and mmaps

get_random_int() returns the same value within a 1 jiffy interval. This means
that the mmap and stack regions will almost always end up the same distance
apart, making a relative offset based attack possible.

To fix this, shift the randomness we use for the mmap region by 1 bit.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mmap.c

@@ -46,6 +46,14 @@ static inline int mmap_is_legacy(void)
 	return sysctl_legacy_va_layout;
 }
 
+/*
+ * Since get_random_int() returns the same value within a 1 jiffy window,
+ * we will almost always get the same randomisation for the stack and mmap
+ * region. This will mean the relative distance between stack and mmap will
+ * be the same.
+ *
+ * To avoid this we can shift the randomness by 1 bit.
+ */
 static unsigned long mmap_rnd(void)
 {
 	unsigned long rnd = 0;
@@ -53,11 +61,11 @@ static unsigned long mmap_rnd(void)
 	if (current->flags & PF_RANDOMIZE) {
 		/* 8MB for 32bit, 1GB for 64bit */
 		if (is_32bit_task())
-			rnd = (long)(get_random_int() % (1<<(23-PAGE_SHIFT)));
+			rnd = (long)(get_random_int() % (1<<(22-PAGE_SHIFT)));
 		else
-			rnd = (long)(get_random_int() % (1<<(30-PAGE_SHIFT)));
+			rnd = (long)(get_random_int() % (1<<(29-PAGE_SHIFT)));
 	}
-	return rnd << PAGE_SHIFT;
+	return (rnd << PAGE_SHIFT) * 2;
 }
 
 static inline unsigned long mmap_base(void)