From: Daniel Cashman <dcashman@...>

commit e0c25d958f78acfd5c97df5776eeba3e0684101b upstream.

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.

[arnd@...: ARM: avoid ARCH_MMAP_RND_BITS for NOMMU]
Signed-off-by: Daniel Cashman <dcashman@...>
Cc: Russell King <linux@...>
Acked-by: Kees Cook <keescook@...>
Cc: Ingo Molnar <mingo@...>
Cc: Jonathan Corbet <corbet@...>
Cc: Don Zickus <dzickus@...>
Cc: Eric W. Biederman <ebiederm@...>
Cc: Heinrich Schuchardt <xypron.glpk@...>
Cc: Josh Poimboeuf <jpoimboe@...>
Cc: Kirill A. Shutemov <kirill.shutemov@...>
Cc: Naoya Horiguchi <n-horiguchi@...>
Cc: Andrea Arcangeli <aarcange@...>
Cc: Mel Gorman <mgorman@...>
Cc: Thomas Gleixner <tglx@...>
Cc: David Rientjes <rientjes@...>
Cc: Mark Salyzyn <salyzyn@...>
Cc: Jeff Vander Stoep <jeffv@...>
Cc: Nick Kralevich <nnk@...>
Cc: Catalin Marinas <catalin.marinas@...>
Cc: Will Deacon <will.deacon@...>
Cc: "H. Peter Anvin" <hpa@...>
Cc: Hector Marco-Gisbert <hecmargi@...>
Cc: Borislav Petkov <bp@...>
Cc: Ralf Baechle <ralf@...>
Cc: Heiko Carstens <heiko.carstens@...>
Cc: Martin Schwidefsky <schwidefsky@...>
Cc: Benjamin Herrenschmidt <benh@...>
Signed-off-by: Arnd Bergmann <arnd@...>
Signed-off-by: Andrew Morton <akpm@...>
Signed-off-by: Linus Torvalds <torvalds@...>
[bwh: Backported to 4.4: adjust context]
Signed-off-by: Ben Hutchings <ben.hutchings@...>
---
arch/arm/Kconfig | 9 +++++++++
arch/arm/mm/mmap.c | 3 +--
2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 34e1569a11ee..a1b8ca129a8a 100644
--- 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 407dc786583a..4b4058db0781 100644
--- 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;
}
--
2.10.2



--
Ben Hutchings
Software Developer, Codethink Ltd.