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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 20 Jun 2023 17:04:50 -0700
Message-ID: <CABCJKucbBfpwPYnKoEpAx1xe58uShKsxBhX5y8JyaTRs3eWanQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/fineibt: Poison ENDBR at +0
To:     Kees Cook <keescook@chromium.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        alyssa.milburn@linux.intel.com, linux-kernel@vger.kernel.org,
        jpoimboe@kernel.org, joao@overdrivepizza.com,
        tim.c.chen@linux.intel.com
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On Tue, Jun 20, 2023 at 2:55=E2=80=AFPM Kees Cook <keescook@chromium.org> w=
rote:
>
> On Thu, Jun 15, 2023 at 09:35:48PM +0200, Peter Zijlstra wrote:
> > Alyssa noticed that when building the kernel with CFI_CLANG+IBT and
> > booting on IBT enabled hardware obtain FineIBT, the indirect functions
> > look like:
> >
> >   __cfi_foo:
> >       endbr64
> >       subl    $hash, %r10d
> >       jz      1f
> >       ud2
> >       nop
> >   1:
> >   foo:
> >       endbr64
> >
> > This is because clang currently does not supress ENDBR emission for
> > functions it provides a __cfi prologue symbol for.
>
> Should this be considered a bug in Clang?

The endbr is needed with KCFI if we have FineIBT disabled for some
reason. There's some discussion here:

https://github.com/ClangBuiltLinux/linux/issues/1735

However, since the kernel handles FineIBT patching, it might be
cleaner to let it also poison the extra endbr.

Sami