On Mon, Nov 30, 2015 at 4:01 PM, Andrew Morton
[off-list ref] wrote:

On Mon, 30 Nov 2015 15:54:12 -0800 Andrew Morton [off-list ref] wrote:

quoted

On Thu, 26 Nov 2015 14:59:42 -0800 Daniel Cashman [off-list ref] wrote:

quoted

ASLR  only uses as few as 8 bits to generate the random offset for the
mmap base address on 32 bit architectures. This value was chosen to
prevent a poorly chosen value from dividing the address space in such
a way as to prevent large allocations. This may not be an issue on all
platforms. Allow the specification of a minimum number of bits so that
platforms desiring greater ASLR protection may determine where to place
the trade-off.

--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c

@@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
            .mode           = 0644,
            .proc_handler   = proc_doulongvec_minmax,
    },
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+   {
+           .procname       = "mmap_rnd_bits",
+           .data           = &mmap_rnd_bits,
+           .maxlen         = sizeof(mmap_rnd_bits),
+           .mode           = 0600,
+           .proc_handler   = proc_dointvec_minmax,
+           .extra1         = (void *) &mmap_rnd_bits_min,
+           .extra2         = (void *) &mmap_rnd_bits_max,

hm, why the typecasts?  They're unneeded and are omitted everywhere(?)
else in kernel/sysctl.c.

Oh.  Casting away constness.

What's the thinking here?  They can change at any time so they aren't
const so we shouldn't declare them to be const?

The _min and _max values shouldn't be changing: they're decided based
on the various CONFIG options that calculate the valid min/maxes. Only
mmap_rnd_bits itself should be changing.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security