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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 29 Apr 2019 15:22:09 -0700
Message-ID: <CAHk-=whpq2=f2LdB-nc52Rd=iZkUH-N-r8OTqEfo+4UaJc7piA@mail.gmail.com>
Subject: Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip
 fops invocation
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicolai Stange <nstange@suse.de>,
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        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>,
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On Mon, Apr 29, 2019 at 3:08 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> FWIW, Lakemont (Quark) doesn't block NMI/SMI in the STI shadow, but I'm
> not sure that counters the "horrible errata" statement ;-).  SMI+RSM saves
> and restores STI blocking in that case, but AFAICT NMI has no such
> protection and will effectively break the shadow on its IRET.

Ugh. I can't say I care deeply about Quark (ie never seemed to go
anywhere), but it's odd. I thought it was based on a Pentium core (or
i486+?). Are you saying those didn't do it either?

I have this dim memory about talking about this with some (AMD?)
engineer, and having an alternative approach for the sti shadow wrt
NMI - basically not checking interrupts in the instruction you return
to with 'iret'. I don't think it was even conditional on the "iret
from NMI", I think it was basically any iret also did the sti shadow
thing.

But I can find no actual paper to back that up, so this may be me just
making sh*t up.

> KVM is generally ok with respect to STI blocking, but ancient versions
> didn't migrate STI blocking and there's currently a hole where
> single-stepping a guest (from host userspace) could drop STI_BLOCKING
> if a different VM-Exit occurs between the single-step #DB VM-Exit and the
> instruction in the shadow.  Though "don't do that" may be a reasonable
> answer in that case.

I thought the sti shadow blocked the single-step exception too? I know
"mov->ss" does block debug interrupts too.

Or are you saying that it's some "single step by emulation" that just
miss setting the STI_BLOCKING flag?

                   Linus