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Date:   Mon, 8 Feb 2021 11:33:30 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bp@suse.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, x86-ml <x86@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [GIT PULL] x86/urgent for v5.11-rc7
Message-ID: <YCETejMWLKW9Se6p@hirez.programming.kicks-ass.net>
References: <661b9809-2c6a-5fc8-163b-a159b84c9ab8@intel.com>
 <2BFAADF3-EBAA-46D6-B1F6-7A41CB85DA1C@amacapital.net>
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On Sun, Feb 07, 2021 at 10:31:32AM -0800, Andy Lutomirski wrote:
> 
> > On Feb 7, 2021, at 10:19 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> > 
> > ﻿On 2/7/21 9:58 AM, Borislav Petkov wrote:
> >>> On Sun, Feb 07, 2021 at 09:49:18AM -0800, Linus Torvalds wrote:
> >>> On Sun, Feb 7, 2021 at 2:40 AM Borislav Petkov <bp@suse.de> wrote:
> >>>> - Disable CET instrumentation in the kernel so that gcc doesn't add
> >>>> ENDBR64 to kernel code and thus confuse tracing.
> >>> So this is clearly the right thing to do for now, but I wonder if
> >>> people have a plan for actually enabling CET and endbr at cpl0 at some
> >>> point?
> >> It probably is an item on some Intel manager's to-enable list. So far,
> >> the CET enablement concentrates only on userspace but dhansen might know
> >> more about future plans. CCed.
> > 
> > It's definitely on our radar to look at after CET userspace.
> > 
> > The only question for me is whether it will be worth doing with the
> > exiting kernel entry/exit architecture.
> 
> I assume you mean: is anyone sufficiently inspired to try to handle
> NMI correctly?  I have a whole pile of nacks saved up for incorrect
> implementations, although I will try to wrap them in polite
> explanations of precisely what is wrong :)

Yeah, the IST stack recursion possibilities are 'fun'. But IIRC CET-SS
has far more problems than just NMI. It will also run into all the ROP
tricks we pull for return tracing, CALL emulation and other lovely
things.