Messages in this thread | | | Date | Tue, 24 Nov 2015 16:47:37 -0800 | Subject | Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR. | From | Kees Cook <> |
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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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