[PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On 11/24/2015 04:47 PM, Kees Cook wrote:

On Tue, Nov 24, 2015 at 4:40 PM, Andrew Morton
[off-list ref] wrote:

quoted

On Wed, 18 Nov 2015 15:20:05 -0800 Daniel Cashman [off-list ref] wrote:

quoted

--- 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           = 0644,

Is there any harm in permitting the attacker to read these values?

And is there any benefit in permitting non-attackers to read them?

I'm on the fence. Things like kernel/randomize_va_space is 644. But
since I don't see a benefit in exposing them, let's make them all 600
instead -- it's a new interface, better to keep it narrower now.

Is there any harm in allowing the attacker to read these values? Nothing
immediately comes to mind.  It is a form of information leakage, and I
guess a local attacker could use this information to calibrate an attack
or decide whether or not brute-forcing is a worthy approach, but this
easily could be leaked in other ways as well.

Is there a benefit to allowing non-attackers to read them?  Possibly
could be used in tests seeking to verify the system environment, but
again, this could be discovered in other ways.

I like Kees' suggestion of starting narrow and granting if need arises.

quoted

quoted

+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
+int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
+int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
+int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
+int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;

These could be __read_mostly.

If one believes in such things.  One effect of __read_mostly is to
clump the write-often stuff into the same cachelines and I've never
been convinced that one outweighs the other...

The _min and _max values should be const, actually, since they're
build-time selected. The _bits could easily be __read_mostly, yeah.

Yes, one would generally expect these to never be touched, and even if
they were, the threshold of __read_mostly would certainly be crossed.

-Dan