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Date:   Wed, 21 Jun 2023 10:51:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Kees Cook <keescook@chromium.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
Subject: Re: [PATCH 2/2] x86/fineibt: Poison ENDBR at +0
Message-ID: <20230621085104.GH2046280@hirez.programming.kicks-ass.net>
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On Tue, Jun 20, 2023 at 05:04:50PM -0700, Sami Tolvanen wrote:
> On Tue, Jun 20, 2023 at 2:55 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > 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.

That's what I get for replying before reading all replies. Anyway, we're
in agreement.