Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

[Jessica Yu]

+++ Ard Biesheuvel [22/08/20 15:47 +0200]:
> (+ Masahiro)
>
> On Fri, 21 Aug 2020 at 14:30, Will Deacon <will@xxxxxxxxxx> wrote:
> > On Fri, Aug 21, 2020 at 02:27:05PM +0200, Ard Biesheuvel wrote:
> > > On Fri, 21 Aug 2020 at 14:20, Will Deacon <will@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Aug 13, 2020 at 03:07:13PM +0200, Ard Biesheuvel wrote:
> > > > > On Thu, 13 Aug 2020 at 15:04, Jessica Yu <jeyu@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > +++ Ard Biesheuvel [13/08/20 10:36 +0200]:
> > > > > > >On Wed, 12 Aug 2020 at 22:00, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > > > > >>
> > > > > > >> On Wed, Aug 12, 2020 at 06:37:57PM +0200, Ard Biesheuvel wrote:
> > > > > > >> > I know there is little we can do at this point, apart from ignoring
> > > > > > >> > the permissions - perhaps we should just defer the w^x check until
> > > > > > >> > after calling module_frob_arch_sections()?
> > > > > > >>
> > > > > > >> My earlier suggestion was to ignore it for 0-sized sections.
> > > > > > >
> > > > > > >Only they are 1 byte sections in this case.
> > > > > > >
> > > > > > >We override the sh_type and sh_flags explicitly for these sections at
> > > > > > >module load time, so deferring the check seems like a reasonable
> > > > > > >alternative to me.
> > > > > >
> > > > > > So module_enforce_rwx_sections() is already called after
> > > > > > module_frob_arch_sections() - which really baffled me at first, since
> > > > > > sh_type and sh_flags should have been set already in
> > > > > > module_frob_arch_sections().
> > > > > >
> > > > > > I added some debug prints to see which section the module code was
> > > > > > tripping on, and it was .text.ftrace_trampoline. See this snippet from
> > > > > > arm64's module_frob_arch_sections():
> > > > > >
> > > > > >                 else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
> > > > > >                          !strcmp(secstrings + sechdrs[i].sh_name,
> > > > > >                                  ".text.ftrace_trampoline"))
> > > > > >                         tramp = sechdrs + i;
> > > > > >
> > > > > > Since Mauro's config doesn't have CONFIG_DYNAMIC_FTRACE enabled, tramp
> > > > > > is never set here and the if (tramp) check at the end of the function
> > > > > > fails, so its section flags are never set, so they remain WAX and fail
> > > > > > the rwx check.
> > > > >
> > > > > Right. Our module.lds does not go through the preprocessor, so we
> > > > > cannot add the #ifdef check there currently. So we should either drop
> > > > > the IS_ENABLED() check here, or simply rename the section, dropping
> > > > > the .text prefix (which doesn't seem to have any significance outside
> > > > > this context)
> > > > >
> > > > > I'll leave it to Will to make the final call here.
> > > >
> > > > Why don't we just preprocess the linker script, like we do for the main
> > > > kernel?
> > > >
> > >
> > > That should work as well, I just haven't checked how straight-forward
> > > it is to change that.
> >
> > Ok, if it's _not_ straightforward, then let's just drop the IS_ENABLED()
> > altogether.
>
> I played around with this for a while, but failed to get Kbuild to
> instantiate $(objtree)/arch/arm64/kernel/module.lds based on
> $(srctree)/arch/arm64/kernel/module.lds.S and the cpp_lds_S rule.
> Perhaps Masahiro has any suggestions here? Otherwise, let's just drop
> the IS_ENABLED() check for now.

I also tinkered around a bit and was able to generate
$(objtree)/arch/arm64/kernel/module.lds based on
$(srctree)/arch/arm64/kernel/module.lds.S only if I specified the
former as the make target directly. Correct me if I'm wrong, but I
guess this might be because the single build targets would utilize
scripts/Makefile.build (where the cpp_lds_S rule is defined) while the
module-related Makefiles don't seem to support .lds.S -> .lds in
general.. Masahiro, how easy would it be to extend .lds.S -> .lds
support to module linker scripts as well?

Thanks,

Jessica