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

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Date:   Tue, 1 Sep 2020 13:51:15 +0100
From:   Will Deacon <will@...nel.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     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 11:46:51AM +0200, Jessica Yu 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.

Acked-by: Will Deacon <will@...nel.org>

Will