FROMLIST: arm: mm: support ARCH_MMAP_RND_BITS.

(cherry picked from commit https://lkml.org/lkml/2015/12/21/341)

arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the
random offset for the mmap base address.  This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this compromise. Keep 8 as the minimum acceptable value.

Bug: 24047224
Signed-off-by: Daniel Cashman <dcashman@android.com>
Signed-off-by: Daniel Cashman <dcashman@google.com>
Change-Id: I2f6c18a0060e1c21b53200ecdcfde9a8c2e3db98
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>

arch/arm/Kconfig

@@ -16,6 +16,7 @@ config ARM
 	select HAVE_KPROBES if !XIP_KERNEL
 	select HAVE_KRETPROBES if (HAVE_KPROBES)
 	select HAVE_FUNCTION_TRACER if (!XIP_KERNEL)
+	select HAVE_ARCH_MMAP_RND_BITS if MMU
 	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)
 	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL)
 	select HAVE_FUNCTION_GRAPH_TRACER if (!THUMB2_KERNEL)
@@ -312,6 +313,14 @@ config MMU
 	  Select if you want MMU-based virtualised addressing space
 	  support by paged memory management. If unsure, say 'Y'.
 
+config ARCH_MMAP_RND_BITS_MIN
+	default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+	default 14 if PAGE_OFFSET=0x40000000
+	default 15 if PAGE_OFFSET=0x80000000
+	default 16
+
 #
 # The "ARM system type" choice list is ordered alphabetically by option
 # text.  Please add new entries in the option alphabetic order.

arch/arm/mm/mmap.c

@@ -280,10 +280,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 {
 	unsigned long random_factor = 0UL;
 
-	/* 8 bits of randomness in 20 address space bits */
 	if ((current->flags & PF_RANDOMIZE) &&
 	    !(current->personality & ADDR_NO_RANDOMIZE))
-		random_factor = (get_random_int() % (1 << 8)) << PAGE_SHIFT;
+		random_factor = (get_random_int() & ((1 << mmap_rnd_bits) - 1)) << PAGE_SHIFT;
 
 	if (mmap_is_legacy()) {
 		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;