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

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Date:   Mon, 31 Aug 2020 17:31:19 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...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 [31/08/20 16:25 +0300]:
>On Mon, 31 Aug 2020 at 13:43, Masahiro Yamada <masahiroy@...nel.org> wrote:
>>
>> On Mon, Aug 31, 2020 at 6:47 PM Jessica Yu <jeyu@...nel.org> wrote:
>> >
>> > +++ Will Deacon [21/08/20 13:30 +0100]:
>> > [snipped]
>> > >> > > > 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.
>> >
>> > Unfortunately I've been getting more reports about this issue, so let's just
>> > get the aforementioned workaround merged first. Does the following look OK?
>> >
>> > diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
>> > index 0ce3a28e3347..2e224435c024 100644
>> > --- a/arch/arm64/kernel/module-plts.c
>> > +++ b/arch/arm64/kernel/module-plts.c
>> > @@ -305,8 +305,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>> >                         mod->arch.core.plt_shndx = i;
>> >                 else if (!strcmp(secstrings + sechdrs[i].sh_name, ".init.plt"))
>> >                         mod->arch.init.plt_shndx = i;
>> > -               else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
>> > -                        !strcmp(secstrings + sechdrs[i].sh_name,
>> > +               else if (!strcmp(secstrings + sechdrs[i].sh_name,
>> >                                  ".text.ftrace_trampoline"))
>> >                         tramp = sechdrs + i;
>> >                 else if (sechdrs[i].sh_type == SHT_SYMTAB)
>> >
>> > If so I'll turn it into a formal patch and we can get that merged in the next -rc.
>> >
>> > Thanks,
>> >
>> > Jessica
>>
>>
>>
>> Sorry for the delay.
>>
>> Please try the attached patch.
>>
>
>Thanks Masahiro,

Yes, thanks Masahiro for looking into this! And no worries about
the delay. I will be able to test the patch tomorrow.

>I'll leave it to the maintainers to make the final call, but this does
>look rather intrusive to me, especially for an -rc. Renaming
>scripts/module-common.lds to scripts/module.lds means that the distros
>may have to update their scripts to generate the linux-headers
>packages etc, so if we do this at all, we'd better do it between
>releases.

Yes, agreed - I was suggesting dropping the IS_ENABLED() check for
the next -rc so that the bug reports about this module loading issue
stop cropping up, and the "proper" fix of supporting module.lds.S ->
.lds would be suitable for the next release instead.

Jessica