Re: [PATCH v2] module: Harden STRICT_MODULE_RWX

[Jessica Yu]

+++ Ard Biesheuvel [31/08/20 16:25 +0300]:
> On Mon, 31 Aug 2020 at 13:43, Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> > On Mon, Aug 31, 2020 at 6:47 PM Jessica Yu <jeyu@xxxxxxxxxx> 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