On Wed, Aug 05, 2020 at 07:08:08AM +0200, Borislav Petkov wrote:
> On Tue, Aug 04, 2020 at 09:58:25PM -0700, h...@zytor.com wrote:
> > Because why use an alternative to jump over one instruction?
> >
> > I personally would prefer to have the IRET put out of line
>
> Can't yet - SERIALIZE CPUs are a minority at the moment.
>
> > and have the call/jmp replaced by SERIALIZE inline.
>
> Well, we could do:
>
> alternative_io("... IRET bunch", __ASM_SERIALIZE,
> X86_FEATURE_SERIALIZE, ...);
>
> and avoid all kinds of jumping. Alternatives get padded so there
> would be a couple of NOPs following when SERIALIZE gets patched in
> but it shouldn't be a problem. I guess one needs to look at what gcc
> generates...
But the IRET-TO-SELF code has instruction which modify the stack. This
would violate stack invariance in alternatives as enforced in commit
7117f16bf460 ("objtool: Fix ORC vs alternatives"). As a result, objtool
gives warnings as follows:
arch/x86/kernel/alternative.o: warning: objtool: do_sync_core()+0xe:
alternative modifies stack
Perhaps in this specific case it does not matter as the changes in the
stack will be undone by IRET. However, using alternative_io would require
adding the macro STACK_FRAME_NON_STANDARD to functions using sync_core().
IMHO, it wouldn't look good.
So maybe the best approach is to implement as you suggested using
static_cpu_has()?
Thanks and BR,
Ricardo
