On Tue, Apr 30, 2024 at 02:15:53PM -0700, Kees Cook wrote:
> On Tue, Apr 30, 2024 at 10:48:36AM -0700, Kees Cook wrote:
> > On Tue, Apr 30, 2024 at 11:21:40AM +0200, Peter Zijlstra wrote:
> > > On Fri, Apr 26, 2024 at 03:29:44PM -0700, Kees Cook wrote:
> > >
> > > > - CONFIG_CFI_CLANG=y for x86 and arm64. (And disable FINEIBT since
> > > > it isn't as secure as straight KCFI.)
> > >
> > > Oi ?
> >
> > Same objection I always had[1]: moving the check into the destination
> > means attacks with control over executable memory contents can just omit
> > the check.
> >
> > But now that I went to go look I see 0c3e806ec0f9 ("x86/cfi: Add boot
> > time hash randomization") is only enabled under FINEIBT... seems better
> > if that were always enabled...
>
> And FINEIBT actually can't be disabled... :|
>
> And as it turns out CFI_CLANG doesn't work at all on v6.9...
>
> [ 0.587220] no CFI hash found at: __call_sites+0x339a8/0x34450
> ffffffffac20cef8 00 00 00 00 00
> [ 0.588226] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:1182
> __apply_fineibt+0x7a9/0x820
> ...
> [ 0.619220] SMP alternatives: Something went horribly wrong trying to
> rewrite the CFI implementation.
> *hang*
Welcome to my TED talk.
I'll keep debugging this: boots fine on v6.8, but booting v6.9 with
defconfig+CFI_CLANG _does_ work, so something in my .config broke it...
--
Kees Cook
