[PATCH] [RFC] Introduce mmap randomization

[william.c.roberts@intel.com]

From: William Roberts <william.c.roberts@intel.com>

This patch introduces the ability randomize mmap locations where the
address is not requested, for instance when ld is allocating pages for
shared libraries. It chooses to randomize based on the current
personality for ASLR.

Currently, allocations are done sequentially within unmapped address
space gaps. This may happen top down or bottom up depending on scheme.

For instance these mmap calls produce contiguous mappings:
int size = getpagesize();
mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40026000
mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40027000

Note no gap between.

After patches:
int size = getpagesize();
mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x400b4000
mmap(NULL, size, flags, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40055000

Note gap between.

Using the test program mentioned here, that allocates fixed sized blocks
till exhaustion: https://www.linux-mips.org/archives/linux-mips/2011-05/msg00252.html,
no difference was noticed in the number of allocations. Most varied from
run to run, but were always within a few allocations of one another
between patched and un-patched runs.

Performance Measurements:
Using strace with -T option and filtering for mmap on the program
ls shows a slowdown of approximate 3.7%

Signed-off-by: William Roberts <william.c.roberts@intel.com>
---
 mm/mmap.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index de2c176..7891272 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -43,6 +43,7 @@
 #include <linux/userfaultfd_k.h>
 #include <linux/moduleparam.h>
 #include <linux/pkeys.h>
+#include <linux/random.h>
 
 #include <asm/uaccess.h>
 #include <asm/cacheflush.h>
@@ -1582,6 +1583,24 @@ unacct_error:
 	return error;
 }
 
+/*
+ * Generate a random address within a range. This differs from randomize_addr() by randomizing
+ * on len sized chunks. This helps prevent fragmentation of the virtual memory map.
+ */
+static unsigned long randomize_mmap(unsigned long start, unsigned long end, unsigned long len)
+{
+	unsigned long slots;
+
+	if ((current->personality & ADDR_NO_RANDOMIZE) || !randomize_va_space)
+		return 0;
+
+	slots = (end - start)/len;
+	if (!slots)
+		return 0;
+
+	return PAGE_ALIGN(start + ((get_random_long() % slots) * len));
+}
+
 unsigned long unmapped_area(struct vm_unmapped_area_info *info)
 {
 	/*
@@ -1676,6 +1695,8 @@ found:
 	if (gap_start < info->low_limit)
 		gap_start = info->low_limit;
 
+	gap_start = randomize_mmap(gap_start, gap_end, length) ? : gap_start;
+
 	/* Adjust gap address to the desired alignment */
 	gap_start += (info->align_offset - gap_start) & info->align_mask;
 
@@ -1775,6 +1796,9 @@ found:
 found_highest:
 	/* Compute highest gap address at the desired alignment */
 	gap_end -= info->length;
+
+	gap_end = randomize_mmap(gap_start, gap_end, length) ? : gap_end;
+
 	gap_end -= (gap_end - info->align_offset) & info->align_mask;
 
 	VM_BUG_ON(gap_end < info->low_limit);
-- 
1.9.1