linux-kernel - Re: [PATCH v9 00/17] Enable FSGSBASE instructions

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Date:   Wed, 22 Apr 2020 16:00:16 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sasha Levin <sashal@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "hpa@...or.com" <hpa@...or.com>, "bp@...en8.de" <bp@...en8.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Pedro Alves <palves@...hat.com>,
        Simon Marchi <simark@...ark.ca>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 00/17] Enable FSGSBASE instructions

On Tue, Apr 21, 2020 at 1:51 PM Sasha Levin <sashal@...nel.org> wrote:
>
> On Tue, Apr 21, 2020 at 01:21:39PM -0700, Andy Lutomirski wrote:
> >
> >
> >> On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> >>
> >> 
> >>>
> >>> Andi's point is that there is no known user it breaks, and the Intel
> >>> folks did some digging into potential users who might be affected by
> >>> this, including 'rr' brought up by Andy, and concluded that there won't
> >>> be breakage as a result of this patchset:
> >>>
> >>>    https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html
> >>>
> >>> Sure, if you poke at it you could see a behavior change, but is there
> >>> an actual user that will be affected by it? I suspect not.
> >>
> >> Actually we don't know of any behavior changes caused by the kernel
> >> with selectors.
> >>
> >> The application can change itself of course, but only if it uses the
> >> new instructions, which no current application does.
> >
> >If you use ptrace to change the gs selector, the behavior is different on a patched kernel.
> >
> >Again, I’m not saying that the change is problematic. But I will say that the fact that anyone involved in this series keeps ignoring this fact makes me quite uncomfortable with the patch set.
>
> That's what I referred to with "poke at it". While the behavior may be
> different, I fail to find anyone who cares.
>
> >>
> >> [This was different in the original patch kit long ago which could
> >> change behavior on context switch for programs with out of sync selectors,
> >> but this has been long fixed]
> >
> >That’s the issue I was referring to.
> >
> >>
> >> A debugger can also change behavior, but we're not aware of any case
> >> that it would break.
> >
> >How hard did you look?
>
> Come on, how does one respond to this?
>
> Is there a real use case affected by this? If so, point it out and I'll
> be happy to go test it. This was already done (per your previous
> request) for gdb and rr.
>

gdb and rr are certainly a good start.  If patches show up, I'll take a look.