linux-kernel - Re: [PATCH RFC v9 0/7] Introduce the STACKLEAK feature and a test for it

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Date:   Mon, 5 Mar 2018 12:02:53 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Alexander Popov <alex.popov@...ux.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        PaX Team <pageexec@...email.hu>,
        Brad Spengler <spender@...ecurity.net>,
        Ingo Molnar <mingo@...nel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Richard Sandiford <richard.sandiford@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Emese Revfy <re.emese@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Josef Bacik <jbacik@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>,
        Ding Tianhong <dingtianhong@...wei.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Juergen Gross <jgross@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Mathias Krause <minipli@...glemail.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Kyle Huey <me@...ehuey.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 0/7] Introduce the STACKLEAK feature and a test for it

On Mon, Mar 5, 2018 at 11:42 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> On 03/05/2018 11:34 AM, Kees Cook wrote:
>> Boris, Andy, and Dave (Hansen), you've all looked at this; would you
>> be willing to give an Ack on the x86 parts? (Though I do now see a new
>> comment from Dave was just sent.) And if not, what changes would you
>> like to see?
>
> I think it could definitely use another cleanup and de-#ifdef'ing pass.
> It seems to have inherited the style from the original code and it's a
> bit more than we're used to in mainline.

There are a few places it could be minimized, that's true. It looked
like it might not be worth it, but the places I see are:

include/linux/compiler.h:
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+/* Poison value points to the unused hole in the virtual memory map */
+# define STACKLEAK_POISON -0xBEEF
+# define STACKLEAK_POISON_CHECK_DEPTH 128
+#endif

This doesn't need an #ifdef wrapper...


arch/x86/kernel/process_64.c and arch/x86/kernel/process_32.c:
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+       p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
+                                               2 * sizeof(unsigned long);
+#endif

This could be made into a helper function, maybe, in processor.h? Like:

#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
# define record_lowest_stack(p) do { \
        p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
                                                  2 * sizeof(unsigned long);
    } while (0)
#else
# define save_lowest_stack(p) do { } while (0)
#endif

And the uses in process_*.c would be:

    save_lowest_stack(p);

?


And "fs/proc: Show STACKLEAK metrics in the /proc file system" could
maybe be adjusted too?

It doesn't seem like a lot of savings, but what do you think?

One new thing did pop out at me in this review, track_stack() likely
shouldn't live in fs/exec.c. It has nothing to do with exec(). There
aren't a lot of good places, but maybe a better place would be
mm/util.c. (A whole new source file seems like overkill.)

-Kees

-- 
Kees Cook
Pixel Security