[PATCH 2/2] arm: mm: support ARCH_MMAP_RND_BITS.

Daniel Cashman
2015-10-28T21:26:36Z

From: dcashman <dcashman@google.com>

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.

Signed-off-by: Daniel Cashman <dcashman@google.com>
---
arch/arm/Kconfig | 24 ++++++++++++++++++++++++
arch/arm/mm/mmap.c | 7 +++++--
2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 639411f..d61e7e2 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -306,6 +306,30 @@ 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
+ int
+ default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+ int
+ default 14 if MMU && PAGE_OFFSET=0x40000000
+ default 15 if MMU && PAGE_OFFSET=0x80000000
+ default 16 if MMU
+ default 8
+
+config ARCH_MMAP_RND_BITS
+ int "Number of bits to use for ASLR of mmap base address" if EXPERT
+ range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
+ default ARCH_MMAP_RND_BITS_MIN
+ help
+ This value can be used to select the number of bits to use to
+ determine the random offset to the base address of vma regions
+ resulting from mmap allocations. This value will be bounded
+ by the architecture's minimum and maximum supported values.
+
+ This value can be changed after boot using the
+ /proc/sys/kernel/mmap_rnd_bits tunable
+
#
# The "ARM system type" choice list is ordered alphabetically by option
# text. Please add new entries in the option alphabetic order.
diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index 407dc78..73ca3a7 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -11,6 +11,10 @@
#include <linux/random.h>
#include <asm/cachetype.h>

+int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
+int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
+int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
+
#define COLOUR_ALIGN(addr,pgoff) \
((((addr)+SHMLBA-1)&~(SHMLBA-1)) + \
(((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
@@ -173,8 +177,7 @@ unsigned long arch_mmap_rnd(void)
{
unsigned long rnd;

- /* 8 bits of randomness in 20 address space bits */
- rnd = (unsigned long)get_random_int() % (1 << 8);
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);

return rnd << PAGE_SHIFT;
}
--
2.6.0.rc2.230.g3dd15c0