Re: RFC: Introduce -fhardened to enable security-related flags

[Marek Polacek <polacek@redhat.com>]

On Wed, Aug 30, 2023 at 03:08:46PM +0200, Richard Biener wrote:
> On Wed, Aug 30, 2023 at 12:51 PM Jakub Jelinek via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > On Tue, Aug 29, 2023 at 03:42:27PM -0400, Marek Polacek via Gcc-patches wrote:
> > > +       if (UNLIKELY (flag_hardened)
> > > +           && (opt->code == OPT_D || opt->code == OPT_U))
> > > +         {
> > > +           if (!fortify_seen_p)
> > > +             fortify_seen_p = !strncmp (opt->arg, "_FORTIFY_SOURCE", 15);
> >
> > Perhaps this should check that the char after it is either '\0' or '=', we
> > shouldn't care if user defines or undefines _FORTIFY_SOURCE_WHATEVER macro.
> >
> > > +           if (!cxx_assert_seen_p)
> > > +             cxx_assert_seen_p = !strcmp (opt->arg, "_GLIBCXX_ASSERTIONS");
> >
> > Like we don't care in this case about -D_GLIBCXX_ASSERTIONS42
> >
> > > +         }
> > > +     }
> > > +
> > > +      if (flag_hardened)
> > > +     {
> > > +       if (!fortify_seen_p && optimize > 0)
> > > +         {
> > > +           if (TARGET_GLIBC_MAJOR == 2 && TARGET_GLIBC_MINOR >= 35)
> > > +             cpp_define (parse_in, "_FORTIFY_SOURCE=3");
> > > +           else
> > > +             cpp_define (parse_in, "_FORTIFY_SOURCE=2");
> >
> > I wonder if it wouldn't be better to enable _FORTIFY_SOURCE=2 by default for
> > -fhardened only for targets which actually have such a support in the C
> > library.  There is some poor man's _FORTIFY_SOURCE support in libssp,
> > but e.g. one has to link with -lssp in that case and compile with
> > -isystem `gcc -print-include-filename=include`/ssp .
> > For glibc that is >= 2.3.4, https://maskray.me/blog/2022-11-06-fortify-source
> > mentions NetBSD support since 2006, newlib since 2017, some Darwin libc,
> > bionic (but seems they have only some clang support and dropped GCC
> > support) and some third party reimplementation of libssp.
> > Or do we just enable it and hope that either it works well or isn't
> > supported at all quietly?  E.g. it would certainly break the ssp case
> > where -isystem finds ssp headers but -lssp isn't linked in.
> >
> > > @@ -4976,6 +4993,22 @@ process_command (unsigned int decoded_options_count,
> > >  #endif
> > >      }
> > >
> > > +  /* TODO: check if -static -pie works and maybe use it.  */
> > > +  if (flag_hardened && !any_link_options_p && !static_p)
> > > +    {
> > > +      save_switch ("-pie", 0, NULL, /*validated=*/true, /*known=*/false);
> > > +      /* TODO: check if BIND_NOW/RELRO is supported.  */
> > > +      if (true)
> > > +     {
> > > +       /* These are passed straight down to collect2 so we have to break
> > > +          it up like this.  */
> > > +       add_infile ("-z", "*");
> > > +       add_infile ("now", "*");
> > > +       add_infile ("-z", "*");
> > > +       add_infile ("relro", "*");
> >
> > As the TODO comment says, to do that we need to check at configure time that
> > linker supports -z now and -z relro options.
> >
> > > @@ -1117,9 +1121,12 @@ finish_options (struct gcc_options *opts, struct gcc_options *opts_set,
> > >      }
> > >
> > >    /* We initialize opts->x_flag_stack_protect to -1 so that targets
> > > -     can set a default value.  */
> > > +     can set a default value.  With --enable-default-ssp or -fhardened
> > > +     the default is -fstack-protector-strong.  */
> > >    if (opts->x_flag_stack_protect == -1)
> > > -    opts->x_flag_stack_protect = DEFAULT_FLAG_SSP;
> > > +    opts->x_flag_stack_protect = (opts->x_flag_hardened
> > > +                               ? SPCT_FLAG_STRONG
> > > +                               : DEFAULT_FLAG_SSP);
> >
> > This needs to be careful, -fstack-protector isn't supported on all targets
> > (e.g. ia64) and we don't want toplev.cc warning:
> >   /* Targets must be able to place spill slots at lower addresses.  If the
> >      target already uses a soft frame pointer, the transition is trivial.  */
> >   if (!FRAME_GROWS_DOWNWARD && flag_stack_protect)
> >     {
> >       warning_at (UNKNOWN_LOCATION, 0,
> >                   "%<-fstack-protector%> not supported for this target");
> >       flag_stack_protect = 0;
> >     }
> > to be emitted whenever using -fhardened, it should not be enabled there
> > silently (for ia64 Fedora/RHEL gcc actually had a short patch to make it
> > work, turn the target into FRAME_GROWS_DOWNWARD one if -fstack-protect* was
> > enabled and otherwise keep it !FRAME_GROWS_DOWNWARD).
> 
> I'll note that with selectively enabling parts of -fhardening it can
> also give a false
> sensation of safety when under the hood we ignore half of the option due to
> one or another reason ...

I suppose you're right :/.
 
> How does -fhardening reflect into -[gf]record-gcc-switches?  Is it at
> least possible
> to verify the actually enabled bits?

In DW_AT_producer it simply shows "-fhardened".  It doesn't expand to what
it actually enabled.  I imagine gen_producer_string could be tweaked to
print the options enabled by -fhardened.  But, DW_AT_producer doesn't seem
to show -D options, or linked options like -Wl,-z,now.  So I think we need
something better.  Maybe add a line in gcc -v?  I would not like to add
a new option just for this.

(It would have to be careful not to show options that the target doesn't
support, like -fstack-protector on BPF.)

Marek