On Wed, Mar 17, 2021 at 10:18:00AM +0100, Ingo Molnar wrote:

* Yu, Yu-cheng [off-list ref] wrote:

quoted

On 3/16/2021 2:15 PM, Peter Zijlstra wrote:

quoted

On Tue, Mar 16, 2021 at 08:10:26AM -0700, Yu-cheng Yu wrote:

quoted

Control-flow Enforcement (CET) is a new Intel processor feature that blocks
return/jump-oriented programming attacks.  Details are in "Intel 64 and
IA-32 Architectures Software Developer's Manual" [1].

CET can protect applications and the kernel.  This series enables only
application-level protection, and has three parts:

   - Shadow stack [2],
   - Indirect branch tracking [3], and
   - Selftests [4].

CET is marketing; afaict SS and IBT are 100% independent and there's no
reason what so ever to have them share any code, let alone a Kconfig
knob.

We used to have shadow stack and ibt under separate Kconfig options, but in
a few places they actually share same code path, such as the XSAVES
supervisor states and ELF header for example.  Anyways I will be happy to
make changes again if there is agreement.

I was look at:

  x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states

didn't see any IBT logic - it's essentially all shadow stack state.

Which is not surprising, forward call edge integrity protection (IBT) 
requires very little state, does it?

They hid the IBT enable bit in the U_CET MSR, which is in the XSAVE
thing.