[PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

[dcashman@android.com (Daniel Cashman)]

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..12768c4 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
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if 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..647fecf 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);
+#else
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
+#endif
 	else
-		rnd = (unsigned long)get_random_int() % (1<<28);
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
 
 	return rnd << PAGE_SHIFT;
 }
-- 
2.6.0.rc2.230.g3dd15c0