Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation

[Linus Torvalds]

On Fri, Dec 2, 2016 at 10:50 AM, Borislav Petkov <bp@xxxxxxxxxx> wrote:
>
> Right, we can try to do something like invalidate_icache() or so in
> there with the JMP so that the BSP refetches modified code and see where
> it gets us.

I'd really rather rjust mark it noinline with a comment. That way the
return from the function acts as the control flow change.

> The good thing is, the early patching paths run before SMP is
> up but from looking at load_module(), for example, which does
> post_relocation()->module_finalize()->apply_alternatives(), this can
> happen late.
>
> Now there I'd like to avoid other cores walking into that address being
> patched. Or are we "safe" there in the sense that load_module() happens
> on one CPU only sequentially? (I haven't looked at that code to see
> what's going on there, actually).

'sync_core()' doesn't help for other CPU's anyway, you need to do the
cross-call IPI. So worrying about other CPU's is *not* a valid reason
to keep a "sync_core()" call.

Seriously, the only reason I can see for "sync_core()" really is:

 - some deep non-serialized MSR  access or similar (ie things like
firmware loading etc really might want it, and a mchine check might
want it)

 - writing to one virtual address, and then executing from another. We
do this for (a) user mode (of course) and (b) text_poke(), but
text_poke() is a whole different dance.

but I may have forgotten some other case.

The issues with modifying code while another CPU may be just about to
access it is a separate issue. And as noted, "sync_core()" is not
sufficient for that, you have to do a whole careful dance with
single-byte debug instruction writes and then a final cross-call.

See the whole "text_poke_bp()" and "text_poke()" for *that* whole
dance. That's a much more complex thing from the normal
apply_alternatives().

             Linus