linux-kernel - Re: [PATCH v2] module: Harden STRICT_MODULE

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Date:   Mon, 24 Aug 2020 17:24:00 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Mark Rutland <mark.rutland@....com>, nd <nd@....com>
Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

+++ Ard Biesheuvel [22/08/20 15:47 +0200]:
>(+ Masahiro)
>
>On Fri, 21 Aug 2020 at 14:30, Will Deacon <will@...nel.org> 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@...nel.org> 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@...nel.org> wrote:
>> > > > >
>> > > > > +++ Ard Biesheuvel [13/08/20 10:36 +0200]:
>> > > > > >On Wed, 12 Aug 2020 at 22:00, Peter Zijlstra <peterz@...radead.org> 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