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    SubjectRe: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.
    From
    On Tue, Nov 24, 2015 at 4:40 PM, Andrew Morton
    <akpm@linux-foundation.org> wrote:
    > On Wed, 18 Nov 2015 15:20:05 -0800 Daniel Cashman <dcashman@android.com> wrote:
    >
    >> --- 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.

    >
    >> + .proc_handler = proc_dointvec_minmax,
    >> + .extra1 = &mmap_rnd_bits_min,
    >> + .extra2 = &mmap_rnd_bits_max,
    >> + },
    >> +#endif
    >> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
    >> + {
    >> + .procname = "mmap_rnd_compat_bits",
    >> + .data = &mmap_rnd_compat_bits,
    >> + .maxlen = sizeof(mmap_rnd_compat_bits),
    >> + .mode = 0644,
    >> + .proc_handler = proc_dointvec_minmax,
    >> + .extra1 = &mmap_rnd_compat_bits_min,
    >> + .extra2 = &mmap_rnd_compat_bits_max,
    >> + },
    >> +#endif
    >>
    >> ...
    >>
    >> +#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.

    -Kees

    --
    Kees Cook
    Chrome OS & Brillo Security


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