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Date:   Fri, 21 Aug 2020 20:03:16 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Robert O'Callahan" <rocallahan@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on
 a 64 bit system

On Fri, Aug 21, 2020 at 7:53 PM Andy Lutomirski <luto@...capital.net> wrote:
>
>
>
> > On Aug 21, 2020, at 2:33 PM, Kyle Huey <me@...ehuey.com> wrote:
> >
> > On Fri, Aug 21, 2020 at 1:08 PM Bae, Chang Seok
> > <chang.seok.bae@...el.com> wrote:
> >>
> >>
> >>>> On Aug 20, 2020, at 21:41, Kyle Huey <me@...ehuey.com> wrote:
> >>>
> >>> On the x86-64 5.9-rc1 TLS is completely broken in 32 bit tracees when
> >>> running under rr[0]. Booting the kernel with `nofsgsbase` fixes it and
> >>> I bisected to https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.8&id=673903495c85137791d5820d690229efe09c8f7b.
> >>>
> >>> STR:
> >>> 1. Build rr from source by
> >>> a. git clone https://github.com/mozilla/rr
> >>> b. mkdir rr/obj
> >>> c. cd rr/obj
> >>> d. cmake ..
> >>> e. make -j16
> >>> 2. Run the simple 32 bit tracee outside of rr with `./bin/simple_32`.
> >>> It should print a message and exit cleanly.
> >>> 3. Run it under rr with `./bin/rr ./bin/simple_32`.
> >>>
> >>> It should behave the same way, but with fsgsbase enabled it will
> >>> segfault. The `simple_32` binary is a simple "hello world" type
> >>> program but it does link to pthreads, so pre-main code attempts to
> >>> access TLS variables.
> >>>
> >>> The interplay between 32 bit and 64 bit TLS is dark magic to me
> >>> unfortunately so this is all the useful information I have.
> >>
> >> As I run it and collect the ptrace logs, it starts to set FSBASE with
> >> some numbers, e.g. 140632147826496, and then later attempts to set GS
> >> with 99 through putreg(), not putreg32().
> >>
> >> With FSGSBASE, the FS/GS base is decoupled from the selector. Andy
> >> made putreg32() to retain the old behavior, fetching FS/GS base
> >> according to the index, but the putreg() does not do. So, rr probably
> >> relies on the old behavior as observed to setting the GS index only.
> >> But it was previously considered to be okay with this comment [1]:
> >>
> >>   "Our modifications to fs/gs/fs_base/gs_base are always either a)
> >>    setting values that the kernel set during recording to make them
> >>    happen during replay or b) emulating PTRACE_SET_REGS that a tracee
> >>    ptracer tried to set on another tracee. Either way I think the
> >>    effects are going to be the same as what would happen if the
> >>    program were run without rr."
> >>
> >> It is not straightforward to go into the rr internal to me. Robert,
> >> any thought?
> >
> > Hmm. When we are running a 32 bit tracee in a 64 bit build of rr we
> > internally convert between the 32 bit and 64 bit user_regs_structs[0]
> > at the ptrace boundary. This does not preserve the fs/gsbase (because
> > there is no fs/gsbase in the 32 bit user_regs_struct, of course).
> >
> > 40c45904f818c1f6555294ca27afc5fda4f09e68 added magic for a 32 bit
> > tracer tracing a 32 bit tracee on a 64 bit kernel, but it looks like a
> > 64 bit tracer tracing a 32 bit tracee on a 64 bit kernel *is* now
> > expected to preserve the fs/gsbase values (or die, in our case).
> >
> > Is that correct?
>
> I was certainly not expecting rr to do this, and I thought I had asked in advance.  What exact ptrace() calls are you doing here?  Is this POKEUSER or something else?  Breaking rr is at least impolite, and I’d like to fix this.

I believe we are PTRACE_GETREGSing and later PTRACE_SETREGSing, but
doing the latter with garbage for fs/gs_base for a 32 bit tracee. That
didn't used to matter (because those values were completely ignored
for a 32 bit tracee) but now it does.

There's a good case that that's our fault and I'm happy to spend my
"don't break userspace" points somewhere else ;)

- Kyle

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