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: I438ae2dae939ea7f311246832cb789afdd6cba4e

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 6803b6fe9c3244ec4bb9a08c70ecfb3448e69aff..d3159ff4de5b4c8698dd7ac5d02ab33dc0d2fb51 100644 (file)
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -35,6 +35,7 @@ config ARM
        select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
        select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32
        select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32
+       select HAVE_ARCH_MMAP_RND_BITS if MMU
        select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
        select HAVE_ARCH_TRACEHOOK
        select HAVE_BPF_JIT
@@ -308,6 +309,14 @@ 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.

diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index 407dc786583aec0e6077f50a8374f99002ca8233..4b4058db0781f63e307d4f5101a74d5330ddf337 100644 (file)
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -173,8 +173,7 @@ 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) - 1);
 
        return rnd << PAGE_SHIFT;
 }