linux-kernel - Re: [PATCH v2 0/4] Static calls

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Date:   Wed, 5 Dec 2018 15:36:10 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>, julia@...com, jeyu@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 0/4] Static calls

>> On Dec 5, 2018, at 7:04 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
>
>> Anyway, I have a new objection to Josh’s create_gap proposal: what on
>> Earth will kernel CET do to it?  Maybe my longjmp-like hack is
>> actually better.
>
> Does CET even care about iret?  I assumed it didn't.  If it does, your
> proposal would have the same problem, no?

I think it doesn’t, but it doesn’t really matter.  The shadow stack looks like:

retaddr of function being poked
call do_int3 + 5

And, to emulate a call, you need to stick a new frame right in the
middle.  At least with a longjmp-like approach, you can clobber the
“call do_int3 + 5” part and then INCSSP on the way out.  To be fair, I
think this also sucks.

PeterZ, can we abuse NMI to make this problem go away?  I don't
suppose that we have some rule that NMI handlers never wait for other
CPUs to finish doing anything?