linux-hardening - Re: [PATCH 06/17] kthread: cfi: disable callback pointer check with modules

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Date:   Wed, 17 Mar 2021 09:05:44 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        bpf@...r.kernel.org, linux-hardening@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/17] kthread: cfi: disable callback pointer check with modules

On Thu, Mar 11, 2021 at 10:13 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Thu, Mar 11, 2021 at 04:49:08PM -0800, Sami Tolvanen wrote:
> > With CONFIG_CFI_CLANG, a callback function passed to
> > __kthread_queue_delayed_work from a module points to a jump table
> > entry defined in the module instead of the one used in the core
> > kernel, which breaks function address equality in this check:
> >
> >   WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);
> >
> > Disable the warning when CFI and modules are enabled.
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > ---
> >  kernel/kthread.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index 1578973c5740..af5fee350586 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -963,7 +963,13 @@ static void __kthread_queue_delayed_work(struct kthread_worker *worker,
> >       struct timer_list *timer = &dwork->timer;
> >       struct kthread_work *work = &dwork->work;
> >
> > -     WARN_ON_ONCE(timer->function != kthread_delayed_work_timer_fn);
> > +     /*
> > +      * With CFI, timer->function can point to a jump table entry in a module,
>
> you keep spewing this comment line that has exactly 81 characters and
> thus badly messes up read it with a normal termina everywhere.
>
> Maybe instead of fixing that in ever duplication hide the whole check in
> a well documented helper (which would have to be a macro due to the
> typing involved).

Sure, that sounds cleaner. I'll add a helper macro in v2.

Sami