Re: [PATCH v2 1/4] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

[Alexander Potapenko <glider@google.com>]

From: Kees Cook <keescook@chromium.org>
Date: Thu, May 16, 2019 at 6:20 PM
To: Alexander Potapenko
Cc: <akpm@linux-foundation.org>, <cl@linux.com>,
<kernel-hardening@lists.openwall.com>, Masahiro Yamada, James Morris,
Serge E. Hallyn, Nick Desaulniers, Kostya Serebryany, Dmitry Vyukov,
Sandeep Patil, Laura Abbott, Randy Dunlap, Jann Horn, Mark Rutland,
<linux-mm@kvack.org>, <linux-security-module@vger.kernel.org>

> On Tue, May 14, 2019 at 04:35:34PM +0200, Alexander Potapenko wrote:
> > Slowdown for the new features compared to init_on_free=0,
> > init_on_alloc=0:
> >
> > hackbench, init_on_free=1:  +7.62% sys time (st.err 0.74%)
> > hackbench, init_on_alloc=1: +7.75% sys time (st.err 2.14%)
>
> I wonder if the patch series should be reorganized to introduce
> __GFP_NO_AUTOINIT first, so that when the commit with benchmarks appears,
> we get the "final" numbers...
>
> > Linux build with -j12, init_on_free=1:  +8.38% wall time (st.err 0.39%)
> > Linux build with -j12, init_on_free=1:  +24.42% sys time (st.err 0.52%)
> > Linux build with -j12, init_on_alloc=1: -0.13% wall time (st.err 0.42%)
> > Linux build with -j12, init_on_alloc=1: +0.57% sys time (st.err 0.40%)
>
> I'm working on reproducing these benchmarks. I'd really like to narrow
> down the +24% number here. But it does
I suspect the slowdown of init_on_free is bigger than that of
PAX_SANITIZE_MEMORY, as we've set the goal to have fully zeroed memory
at alloc time.
If we want a mode that only wipes the user data upon free() but
doesn't eliminate all uninit memory, then we can make it faster.
> > The slowdown for init_on_free=0, init_on_alloc=0 compared to the
> > baseline is within the standard error.
>
> I think the use of static keys here is really great: this is available
> by default for anyone that wants to turn it on.
>
> I'm thinking, given the configuable nature of this, it'd be worth adding
> a little more detail at boot time. I think maybe a separate patch could
> be added to describe the kernel's memory auto-initialization features,
> and add something like this to mm_init():
>
> +void __init report_meminit(void)
> +{
> +       const char *stack;
> +
> +       if (IS_ENABLED(CONFIG_INIT_STACK_ALL))
> +               stack = "all";
> +       else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL))
> +               stack = "byref_all";
> +       else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF))
> +               stack = "byref";
> +       else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_USER))
> +               stack = "__user";
> +       else
> +               stack = "off";
> +
> +       /* Report memory auto-initialization states for this boot. */
> +       pr_info("mem auto-init: stack:%s, heap alloc:%s, heap free:%s\n",
> +               stack, want_init_on_alloc(GFP_KERNEL) ? "on" : "off",
> +               want_init_on_free() ? "on" : "off");
> +}
>
> To get a boot line like:
>
>         mem auto-init: stack:off, heap alloc:off, heap free:on
For stack there's no binary on/off, as you can potentially build half
of the kernel with stack instrumentation and another half without it.
We could make the instrumentation insert a static global flag into
each translation unit, but this won't give us any interesting info.

> And one other thought I had was that in the init_on_free=1 case, there is
> a large pause at boot while memory is being cleared. I think it'd be handy
> to include a comment about that, just to keep people from being surprised:
>
> diff --git a/init/main.c b/init/main.c
> index cf0c3948ce0e..aea278392338 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -529,6 +529,8 @@ static void __init mm_init(void)
>          * bigger than MAX_ORDER unless SPARSEMEM.
>          */
>         page_ext_init_flatmem();
> +       if (want_init_on_free())
> +               pr_info("Clearing system memory ...\n");
>         mem_init();
>         kmem_cache_init();
>         pgtable_init();
>
> Beyond these thoughts, I think this series is in good shape.
>
> Andrew (or anyone else) do you have any concerns about this?
>
> --
> Kees Cook



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg