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: I2f6c18a0060e1c21b53200ecdcfde9a8c2e3db98

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 78e54d1d111dd96ad38afef76e0258881337c583..4e96bf6a31027141ebadc8cfa728a0c33e02bb17 100644 (file)
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -24,6 +24,7 @@ config ARM
        select HARDIRQS_SW_RESEND
        select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
        select HAVE_ARCH_KGDB
+       select HAVE_ARCH_MMAP_RND_BITS if MMU
        select HAVE_ARCH_SECCOMP_FILTER
        select HAVE_ARCH_TRACEHOOK
        select HAVE_BPF_JIT
@@ -282,6 +283,14 @@ 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 984509eb44b945693c4bd6d90f806eff94c427db..f556fc75125fd5ded0ab639da55f996cc34d7f63 100644 (file)
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -173,10 +173,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 {
        unsigned long random_factor = 0UL;
 
-       /* 8 bits of randomness in 20 address space bits */
        if ((current->flags & PF_RANDOMIZE) &&
            !(current->personality & ADDR_NO_RANDOMIZE))
-               random_factor = (get_random_int() % (1 << 8)) << PAGE_SHIFT;
+               random_factor = (get_random_int() & ((1 << mmap_rnd_bits) - 1)) << PAGE_SHIFT;
 
        if (mmap_is_legacy()) {
                mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;