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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 29 Apr 2019 13:16:10 -0700
Message-ID: <CAHk-=wjyyKDv-WZLXZbVD=V05p2X7eg74z2SpR4TQTxN9JLq4Q@mail.gmail.com>
Subject: Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip
 fops invocation
To:     Andrew Lutomirski <luto@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
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On Mon, Apr 29, 2019 at 12:02 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> If nmi were to break it, it would be a cpu bug. I'm pretty sure I've
> seen the "shadow stops even nmi" documented for some uarch, but as
> mentioned it's not necessarily the only way to guarantee the shadow.

In fact, the documentation is simply the official Intel instruction
docs for "STI":

    The IF flag and the STI and CLI instructions do not prohibit the
    generation of exceptions and NMI interrupts. NMI interrupts (and
    SMIs) may be blocked for one macroinstruction following an STI.

note the "may be blocked". As mentioned, that's just one option for
not having NMI break the STI shadow guarantee, but it's clearly one
that Intel has done at times, and clearly even documents as having
done so.

There is absolutely no question that the sti shadow is real, and that
people have depended on it for _decades_. It would be a horrible
errata if the shadow can just be made to go away by randomly getting
an NMI or SMI.

              Linus