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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 20, 2023 at 02:55:10PM -0700, Kees Cook 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?

No, I don't think so. I was going to say this is perhaps insufficiently
explored space, but upon more consideration I think this is actually
correct behaviour (and I need to write a better Changelog).

The issue is that the compiler generates code for kCFI+IBT, it doesn't
know about FineIBT *at*all*. Additionally, one can inhibit patching of
FineIBT by booting with 'cfi=kcfi' on IBT enabled hardware.

And in that case (kCFI+IBT), we'll do the caller hash check and still
jump to +0, so there really must be an ENDBR there.

Only if we were to dis-allow this combination could we say the ENDBR at
+0 becomes superfluous and should find means for the compiler not emit
it.

> > Having this second ENDBR however makes it possible to elide the CFI
> > check. Therefore, we should poison this second ENDBR (if present) when
> > switching to FineIBT mode.
> > 
> > Fixes: 931ab63664f0 ("x86/ibt: Implement FineIBT")
> > Reported-by: "Milburn, Alyssa" <alyssa.milburn@intel.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> Looks like a good work-around.
> 
> Acked-by: Kees Cook <keescook@chromium.org>

Thanks!