linux-kernel - Re: [PATCH v2 2/2] kernel/trace: Remove function callback casts

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Date:   Fri, 24 Jul 2020 19:14:18 +0200
From:   Oscar Carter <oscar.carter@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Oscar Carter <oscar.carter@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 2/2] kernel/trace: Remove function callback casts

On Fri, Jul 24, 2020 at 12:35:28PM -0400, Steven Rostedt wrote:
> On Fri, 24 Jul 2020 18:19:21 +0200
> Oscar Carter <oscar.carter@....com> wrote:
>
> > > The linker trick is far less intrusive, and I believe less error prone.
> >
> > If we use the linker trick, the warning -Wcast-function-type dissapears,
> > but in a way that makes impossible to the compiler to get the necessary
> > info about function prototypes to insert the commented check. As far I
> > know, this linker trick (redirection of a function) is hidden for the
> > CFI build.
> >
> > So, in my opinion, the linker trick is not suitable if we want to protect
> > the function pointers of the ftrace subsystem against an attack that
> > modifiy the normal flow of the kernel.
>
> The linker trick should only affect architectures that don't implement
> the needed features. I can make it so the linker trick is only applied
> to those archs, and other archs that want more protection only need to
> add these features to their architectures.
>
> It's much less intrusive than this patch.

Sorry, but I don't understand your proposal. What features an arch need to
add if want the CFI protection?

>
> -- Steve

Thanks,
Oscar Carter