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

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Message-ID: <CAK7LNARc1vjAV5ib1D0LaQA+rNGcE7YwGnp-MrshGD34_7V4WQ@mail.gmail.com>
Date:   Mon, 31 Aug 2020 19:42:30 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...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, 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.


-- 
Best Regards
Masahiro Yamada

View attachment "0001-kbuild-preprocess-module-linker-script.patch" of type "text/x-patch" (10817 bytes)