Re: [PATCH] x86/Kconfig: make CFI_AUTO_DEFAULT depend on !RUST

[Alice Ryhl <aliceryhl@google.com>]

On Fri, May 09, 2025 at 11:11:48AM +0200, Paweł Anikiel wrote:
> On Fri, May 9, 2025 at 11:04 AM Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.com> wrote:
> >
> > On Fri, May 9, 2025 at 10:46 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > >
> > > Actually ... I don't think putting it on CFI_AUTO_DEFAULT the right
> > > approach.
> > >
> > > Shouldn't the depends on clause go on `config FINEIBT` instead? After
> > > all, the current patch just means that you can't make FineIBT the
> > > default option. But you can still pass kcfi=fineibt on boot to enble
> > > FineIBT which would result in the same crash.
> >
> > I thought the intention was to avoid disabling FineIBT at config time:
> > https://lore.kernel.org/all/202504161442.66CE2596@keescook/
> 
> I think you mean enabling FineIBT - yes, this patch does exactly that,
> it's still possible to enable it by hand with cfi=
> 
> > If another approach is needed, then we can change it on top if that is
> > OK (I am about to submit the PR).
> 
> Putting it on CONFIG_FINEIBT was my initial idea, but it turns out
> that this creates a dependency cycle in Kconfig:
> 
> error: recursive dependency detected!
>         symbol RUST depends on CALL_PADDING
>         symbol CALL_PADDING is selected by FINEIBT
>         symbol FINEIBT depends on RUST
> 
> Not sure if it's possible to make it work.

To fix that, we probably have to do one of:

Option 1: Upgrade the rustc MSRV past 1.81 and drop the depends on
          CALL_PADDING.

Option 2: Split `config FINEIBT` into two options:

config HAVE_FINEIBT
	def_bool y
	depends on X86_KERNEL_IBT && CFI_CLANG && MITIGATION_RETPOLINE
	depends on !RUST || RUSTC_VERSION >= 108800

config FINEIBT
	bool "Accept cfi=fineibt option at boot"
	depends on HAVE_FINEIBT
	select CALL_PADDING

Alice