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

[Sasha Levin]

On Tue, Apr 21, 2020 at 01:21:39PM -0700, Andy Lutomirski wrote:
> > On Apr 21, 2020, at 12:56 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> >
> > ï
> > > 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, Iâm 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]
>
> Thatâs 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.

--
Thanks,
Sasha