linux-kernel - Re: [PATCH 2/2] kernel: Support compiling out the prctl syscall

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Date:   Tue, 8 Nov 2016 16:47:57 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Zefan Li <lizefan@...wei.com>,
        Li Bin <huawei.libin@...wei.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Alex Thorlton <athorlton@....com>,
        Michal Hocko <mhocko@...e.com>,
        Mateusz Guzik <mguzik@...hat.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        John Stultz <john.stultz@...aro.org>,
        Al Viro <viro@...iv.linux.org.uk>, Zach Brown <zab@...hat.com>,
        Anna Schumaker <Anna.Schumaker@...app.com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] kernel: Support compiling out the prctl syscall

On Tue, Nov 08, 2016 at 04:40:02PM -0800, Kees Cook wrote:
> On Tue, Nov 8, 2016 at 4:18 PM, Josh Triplett <josh@...htriplett.org> wrote:
> > Some embedded systems can do without the prctl syscall, saving some
> > space.
> >
> > This also avoids regular increases in tinyconfig size as people add more
> > non-optional functionality to prctl (observed via the 0-day kernel
> > infrastructure).
> >
> > bloat-o-meter results:
> >
> > add/remove: 0/3 grow/shrink: 0/1 up/down: 0/-2143 (-2143)
> > function                                     old     new   delta
> > offsets                                       23      12     -11
> > prctl_set_auxv                                97       -     -97
> > sys_prctl                                    794       -    -794
> > prctl_set_mm                                1241       -   -1241
> > Total: Before=1902583, After=1900440, chg -0.11%
> >
> > Signed-off-by: Josh Triplett <josh@...htriplett.org>
> 
> I'm absolutely a fan of doing this, but I wonder how this interacts
> with the LSMs that define prctl hooks, etc. I wouldn't expect a system
> that didn't want prctl to want an LSM, but maybe the LSMs all need to
> depend on CONFIG_PRCTL now?

I did think about that (as well as SECCOMP), but I did confirm that the
kernel builds fine with allyesconfig minus CONFIG_PRCTL.  An LSM that
wants to restrict access to some prctls should be fine with no process
having any access to prctl. :)  Beyond that, anything wanting
configuration via LSM (such as SECCOMP) still exists and functions, even
if you can't access it from outside the kernel.