Re: [RFC PATCH 4/7] x86: Add x86_64 Kernel Control Flow Integrity implementation

[Kees Cook <kees@kernel.org>]

On Thu, Aug 21, 2025 at 11:46:17AM -0700, Kees Cook wrote:
> On Thu, Aug 21, 2025 at 11:29:35AM +0200, Peter Zijlstra wrote:
> > On Thu, Aug 21, 2025 at 12:26:37AM -0700, Kees Cook wrote:
> > > Build and run tested on x86_64 Linux kernel with various CPU errata
> > > handling alternatives and FineIBT.

Turns out my configs were broken -- I only tested non-retpoline.

> > 
> > I'm a little confused, does this force r11 to be the indirect call
> > register like clang does? The code seems to suggest it is possible it
> > uses another register.
> > 
> > The current kernel FineIBT code hard assumes r11 for now.
> 
> Oh, it looked like it wasn't always r11. Does clang force the call
> register to be r11? I only do that here if the call expression isn't a
> register (similar to -mindirect-branch-register). Looking at the retpoline
> implementation, I see __x86_indirect_thunk_* being generated for all the
> general registers. Hm, but in looking now I see all the hard-coded r11 use
> in the fineibt alternatives. I wonder if my boot testing is somehow not
> triggering the FineIBT alternatives patching? I will investigate more...

I've found my Kconfig problem now. Confirmed that this RFC does _not_
work with retpoline (much less FineIBT). I will get that fixed for the
next version.

-- 
Kees Cook