linux-hardening - Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules

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Message-ID: <CAK7LNAS+EG9doX3qUmu4M3=mRNmdybSv4180Xnuubiwmsq0Agw@mail.gmail.com>
Date:   Tue, 26 Jan 2021 06:44:35 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-hardening@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Justin Forbes <jforbes@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules

On Tue, Jan 26, 2021 at 6:28 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Tue, Jan 26, 2021 at 06:16:01AM +0900, Masahiro Yamada wrote:
> > On Tue, Jan 26, 2021 at 5:42 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > > When building out-of-tree kernel modules, the build system doesn't
> > > require the GCC version to match the version used to build the original
> > > kernel.  That's probably [1] fine.
> > >
> > > In fact, for many distros, the version of GCC used to build the latest
> > > kernel doesn't necessarily match the latest released GCC, so a GCC
> > > mismatch turns out to be pretty common.  And with CONFIG_MODVERSIONS
> > > it's probably more common.
> > >
> > > So a lot of users have come to rely on being able to use a different
> > > version of GCC when building OOT modules.
> > >
> > > But with GCC plugins enabled, that's no longer allowed:
> > >
> > >   cc1: error: incompatible gcc/plugin versions
> > >   cc1: error: failed to initialize plugin ./scripts/gcc-plugins/structleak_plugin.so
> > >
> > > That error comes from the plugin's call to
> > > plugin_default_version_check(), which strictly enforces the GCC version.
> > > The strict check makes sense, because there's nothing to prevent the GCC
> > > plugin ABI from changing -- and it often does.
> > >
> > > But failing the build isn't necessary.  For most plugins, OOT modules
> > > will otherwise work just fine without the plugin instrumentation.
> > >
> > > When a GCC version mismatch is detected, print a warning and disable the
> > > plugin.  The only exception is the RANDSTRUCT plugin which needs all
> > > code to see the same struct layouts.  In that case print an error.
> > >
> > > [1] Ignoring, for the moment, that the kernel now has
> > >     toolchain-dependent kconfig options, which can silently disable
> > >     features and cause havoc when compiler versions differ, or even when
> > >     certain libraries are missing.  This is a separate problem which
> > >     also needs to be addressed.
> > >
> > > Reported-by: Ondrej Mosnacek <omosnace@...hat.com>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > ---
> >
> >
> > We are based on the assumption that we use the same
> > compiler for in-tree and out-of-tree.
>
> Sorry, but that assumption isn't based in reality.  And it's not
> enforced.
>
> > If people use a different compiler, they must be
> > prepared for any possible problem.
> >
> > Using different compiler flags for in-tree and out-of-tree
> > is even more dangerous.
> >
> > For example, CONFIG_GCC_PLUGIN_RANDSTRUCT is enabled
> > for in-tree build, and then disabled for out-of-tree modules,
> > the struct layout will mismatch, won't it?
>
> If you read the patch you'll notice that it handles that case, when it's
> caused by GCC mismatch.
>
> However, as alluded to in the [1] footnote, it doesn't handle the case
> where the OOT build system doesn't have gcc-plugin-devel installed.
> Then CONFIG_GCC_PLUGIN_RANDSTRUCT gets silently disabled and the build
> succeeds!  That happens even without a GCC mismatch.


Ah, sorry.

I responded too early before reading the patch fully.

But, I do not like to make RANDSTRUCT a special case.

I'd rather want to stop building for any plugin.







> > This patch is ugly, and not doing the right thing.
>
> Maybe, but I doubt the solution is to make assumptions.
>
> --
> Josh
>


-- 
Best Regards
Masahiro Yamada