Re: [PATCH 2/2] x86/fineibt: Poison ENDBR at +0

[Sami Tolvanen]

On Tue, Jun 20, 2023 at 2:55 PM Kees Cook <keescook@xxxxxxxxxxxx> 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.

Sami