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

[Sasha Levin]

On Wed, Apr 22, 2020 at 04:00:16PM -0700, Andy Lutomirski wrote:
> On Tue, Apr 21, 2020 at 1:51 PM Sasha Levin <sashal@xxxxxxxxxx> 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@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.
>
> gdb and rr are certainly a good start.  If patches show up, I'll take a look.

I'm sorry, but what patches are we talking about?

I just went to gdb to check again that I'm not crazy, and the scenario
you were worried about seems to work just fine:

134			asm volatile ("mov %%gs:(%%rcx), %%rax" : : "c" (offset) : "rax");
(gdb) p printme()
Hi!
$1 = void
(gdb)

Again, please point me to a specific user we break.

--
Thanks,
Sasha