Re: [PATCH v2 seccomp 1/6] seccomp: Move config option SECCOMP to arch/Kconfig

[Geert Uytterhoeven]

Hi Kees,

On Wed, Oct 28, 2020 at 1:06 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, Oct 27, 2020 at 10:52:39AM +0100, Geert Uytterhoeven wrote:
> > On Thu, Sep 24, 2020 at 2:48 PM YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote:
> > > From: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>
> > >
> > > In order to make adding configurable features into seccomp
> > > easier, it's better to have the options at one single location,
> > > considering easpecially that the bulk of seccomp code is
> > > arch-independent. An quick look also show that many SECCOMP
> > > descriptions are outdated; they talk about /proc rather than
> > > prctl.
> > >
> > > As a result of moving the config option and keeping it default
> > > on, architectures arm, arm64, csky, riscv, sh, and xtensa
> > > did not have SECCOMP on by default prior to this and SECCOMP will
> > > be default in this change.
> > >
> > > Architectures microblaze, mips, powerpc, s390, sh, and sparc
> > > have an outdated depend on PROC_FS and this dependency is removed
> > > in this change.
> > >
> > > Suggested-by: Jann Horn <jannh@xxxxxxxxxx>
> > > Link: https://lore.kernel.org/lkml/CAG48ez1YWz9cnp08UZgeieYRhHdqh-ch7aNwc4JRBnGyrmgfMg@xxxxxxxxxxxxxx/
> > > Signed-off-by: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>
> >
> > Thanks for your patch. which is now commit 282a181b1a0d66de ("seccomp:
> > Move config option SECCOMP to arch/Kconfig") in v5.10-rc1.
> >
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -458,6 +462,23 @@ config HAVE_ARCH_SECCOMP_FILTER
> > >             results in the system call being skipped immediately.
> > >           - seccomp syscall wired up
> > >
> > > +config SECCOMP
> > > +       def_bool y
> > > +       depends on HAVE_ARCH_SECCOMP
> > > +       prompt "Enable seccomp to safely compute untrusted bytecode"
> > > +       help
> > > +         This kernel feature is useful for number crunching applications
> > > +         that may need to compute untrusted bytecode during their
> > > +         execution. By using pipes or other transports made available to
> > > +         the process as file descriptors supporting the read/write
> > > +         syscalls, it's possible to isolate those applications in
> > > +         their own address space using seccomp. Once seccomp is
> > > +         enabled via prctl(PR_SET_SECCOMP), it cannot be disabled
> > > +         and the task is only allowed to execute a few safe syscalls
> > > +         defined by each seccomp mode.
> > > +
> > > +         If unsure, say Y. Only embedded should say N here.
> > > +
> >
> > Please tell me why SECCOMP is special, and deserves to default to be
> > enabled.  Is it really that critical, given only 13.5 (half of sparc
> > ;-) out of 24
> > architectures implement support for it?
>
> That's an excellent point; I missed this in my review as I saw several
> Kconfig already marked "def_bool y" but failed to note it wasn't _all_
> of them. Okay, checking before this patch, these had them effectively
> enabled:
>
> via Kconfig:
>
> parisc
> s390
> um
> x86

Mostly "server" and "desktop" platforms.

> via defconfig, roughly speaking:
>
> arm
> arm64
> sh

Note that these defconfigs are example configs, not meant for production.
E.g. arm/multi_v7_defconfig and arm64/defconfig enable about everything
for compile coverage.

> How about making the default depend on HAVE_ARCH_SECCOMP_FILTER?
>
> These have SECCOMP_FILTER support:
>
> arch/arm/Kconfig:       select HAVE_ARCH_SECCOMP_FILTER if AEABI && !OABI_COMPAT
> arch/arm64/Kconfig:     select HAVE_ARCH_SECCOMP_FILTER
> arch/csky/Kconfig:      select HAVE_ARCH_SECCOMP_FILTER
> arch/mips/Kconfig:      select HAVE_ARCH_SECCOMP_FILTER
> arch/parisc/Kconfig:    select HAVE_ARCH_SECCOMP_FILTER
> arch/powerpc/Kconfig:   select HAVE_ARCH_SECCOMP_FILTER
> arch/riscv/Kconfig:     select HAVE_ARCH_SECCOMP_FILTER
> arch/s390/Kconfig:      select HAVE_ARCH_SECCOMP_FILTER
> arch/sh/Kconfig:        select HAVE_ARCH_SECCOMP_FILTER
> arch/um/Kconfig:        select HAVE_ARCH_SECCOMP_FILTER
> arch/x86/Kconfig:       select HAVE_ARCH_SECCOMP_FILTER
> arch/xtensa/Kconfig:    select HAVE_ARCH_SECCOMP_FILTER
>
> So the "new" promotions would be:
>
> csky
> mips
> powerpc
> riscv
> xtensa
>
> Which would leave only these two:
>
> arch/microblaze/Kconfig:        select HAVE_ARCH_SECCOMP
> arch/sparc/Kconfig:     select HAVE_ARCH_SECCOMP if SPARC64
>
> At this point, given the ubiquity of seccomp usage (e.g. systemd), I
> guess it's not unreasonable to make it def_bool y?

Having support does not necessarily imply you want it enabled.
If systemd needs it (does it? I have Debian nfsroots with systemd,
without SECCOMP), you can enable it in the defconfig.
"Default y" is for things you cannot do without, unless you know
better.

Bloat-o-meter says enabling SECCOMP consumes only ca. 8 KiB
(on arm32), so perhaps "default y if !EXPERT"?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds