Re: [PATCH v5 3/6] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC

From: Kees Cook <keescook@chromium.org>
To: Jeff Xu <jeffxu@google.com>
Cc: Greg KH <gregkh@linuxfoundation.org>,
	jeffxu@chromium.org, skhan@linuxfoundation.org,
	akpm@linux-foundation.org, dmitry.torokhov@gmail.com,
	dverkamp@chromium.org, hughd@google.com, jorgelo@chromium.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-mm@kvack.org, jannh@google.com,
	linux-hardening@vger.kernel.org,
	kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v5 3/6] mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC
Date: Tue, 6 Dec 2022 15:24:07 -0800	[thread overview]
Message-ID: <202212061521.2BD762D@keescook> (raw)
In-Reply-To: <CALmYWFvO8kKGusv8_Jwevw5pS4LwbF9G12=RthWZeHYE++ZO3Q@mail.gmail.com>

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@linuxfoundation.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@linuxfoundation.org> wrote:
> > > >
> > > > On Tue, Dec 06, 2022 at 03:23:55PM +0000, jeffxu@chromium.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