linux-hardening - Re: [PATCH v5 3/6] mm/memfd: add MFD_NOEXEC_SEAL and MFD

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Message-ID: <202212061521.2BD762D@keescook>
Date:   Tue, 6 Dec 2022 15:24:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jeff Xu <jeffxu@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jeffxu@...omium.org,
        skhan@...uxfoundation.org, akpm@...ux-foundation.org,
        dmitry.torokhov@...il.com, dverkamp@...omium.org, hughd@...gle.com,
        jorgelo@...omium.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        jannh@...gle.com, linux-hardening@...r.kernel.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v5 3/6] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC

On Tue, Dec 06, 2022 at 09:48:55AM -0800, Jeff Xu wrote:
> On Tue, Dec 6, 2022 at 8:35 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Tue, Dec 06, 2022 at 08:26:30AM -0800, Jeff Xu wrote:
> > > On Tue, Dec 6, 2022 at 8:04 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> > > >
> > > > On Tue, Dec 06, 2022 at 03:23:55PM +0000, jeffxu@...omium.org wrote:
> > > > > --- a/kernel/pid_namespace.c
> > > > > +++ b/kernel/pid_namespace.c
> > > > > @@ -110,6 +110,11 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
> > > > >       ns->ucounts = ucounts;
> > > > >       ns->pid_allocated = PIDNS_ADDING;
> > > > >
> > > > > +#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
> > > > > +     ns->memfd_noexec_scope =
> > > > > +             task_active_pid_ns(current)->memfd_noexec_scope;
> > > > > +#endif
> > > >
> > > > .c files should never have #if in them.  Can't you put this in a .h file
> > > > properly so that this does not get really messy over time?
> > > >
> > > >
> > > Thanks for reviewing.
> > > It seems to me that checking for CONFIG_XXX is  common in c code in
> > > kernel/ path.
> >
> > Maybe, but please don't make it any worse if at all possible.  It's
> > tough to maintain code like that.
> >
> > > Do you have a sample code pattern (link/function) that I can follow?
> >
> > Any of the zillions of #if statements in .h files :)
> >
> Thanks.
> I will take the approach of having real/stub implementation in the h
> file, and the c file  using it without a compile flag.
> Please kindly let me know if this is not right.

Right; for example:

in .h:

#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
static inline void ns_copy_memfd_scope(... dst, ... src) {
	dst->memfd_noexec_scope = src->memfd_noexec_scope;
}
#else
static inline void ns_set_memfd_scope(... ns, ... scope) { }
#endif


in .c:

	ns_copy_memfd_scope(ns, task_active_pid_ns(current));


-- 
Kees Cook