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Date:   Mon, 1 Nov 2021 10:01:55 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, X86 ML <x86@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching
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On Mon, Nov 01, 2021 at 12:36:18AM +0100, Ard Biesheuvel wrote:
> On Sun, 31 Oct 2021 at 21:45, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Sun, Oct 31, 2021 at 09:21:56PM +0100, Ard Biesheuvel wrote:
> >
> > > That means we can support static calls on arm64 now without breaking
> > > Clang CFI, and work on a solution for the redundant jumps on a more
> > > relaxed schedule.
> >
> > Yes, arm64 has a 'problem' with having already merged the clang-cfi
> > stuff :/
> >
> > I'm hoping the x86 solution can be an alternative CFI scheme, I'm
> > starting to really hate this one. And I'm not at all convinced the
> > proposed scheme is the best possible scheme given the constraints of
> > kernel code. AFAICT it's a compromise made in userspace.
> 
> Your scheme only works with IBT: the value of %r11 is under the
> adversary's control so it could just point it at 'foo+0x10' if it
> wants to call foo indirectly, and circumvent the check. So without IBT
> (or BTI), I think the check fundamentally belongs in the caller, not
> in the callee.

How is that not true for the jump table approach? Like I showed earlier,
it is *trivial* to reconstruct the actual function pointer from a
jump-table entry pointer.

In any case, I really want the discussion to start at square one, and
show/explain why any chosen CFI scheme is actually good for the kernel.
Just because clang happened to have implemented it, doesn't make it the
most suitable scheme for the kernel.