[PATCH v7 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

Daniel Cashman
2015-12-21T17:28:03Z

x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
64-bit, 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 default
values as new minimums.

Signed-off-by: Daniel Cashman <dcashman@android.com>
---
arch/x86/Kconfig | 16 ++++++++++++++++
arch/x86/mm/mmap.c | 12 ++++++------
2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f..1eeccb3 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -82,6 +82,8 @@ config X86
select HAVE_ARCH_KASAN if X86_64 && SPARSEMEM_VMEMMAP
select HAVE_ARCH_KGDB
select HAVE_ARCH_KMEMCHECK
+ select HAVE_ARCH_MMAP_RND_BITS if MMU
+ select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_SOFT_DIRTY if X86_64
select HAVE_ARCH_TRACEHOOK
@@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT
config MMU
def_bool y

+config ARCH_MMAP_RND_BITS_MIN
+ default 28 if 64BIT
+ default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+ default 32 if 64BIT
+ default 16
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+ default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+ default 16
+
config SBUS
bool

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 844b06d..96bd1e2 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
{
unsigned long rnd;

- /*
- * 8 bits of randomness in 32bit mmaps, 20 address space bits
- * 28 bits of randomness in 64bit mmaps, 40 address space bits
- */
if (mmap_is_ia32())
- rnd = (unsigned long)get_random_int() % (1<<8);
+#ifdef CONFIG_COMPAT
+ rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_compat_bits) - 1);
+#else
+ rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_bits) - 1);
+#endif
else
- rnd = (unsigned long)get_random_int() % (1<<28);