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

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Date:   Mon, 31 Aug 2020 16:25:42 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Jessica Yu <jeyu@...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

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,

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.