Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system

[Kyle Huey <me@kylehuey.com>]

On Tue, Aug 25, 2020 at 9:46 AM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Tue, Aug 25, 2020 at 9:32 AM Kyle Huey <me@kylehuey.com> wrote:
> >
> > On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <luto@amacapital.net> wrote:
> > > I don’t like this at all. Your behavior really shouldn’t depend on
> > > whether the new instructions are available.  Also, some day I would
> > > like to change Linux to have the new behavior even if FSGSBASE
> > > instructions are not available, and this will break rr again.  (The
> > > current !FSGSBASE behavior is an ugly optimization of dubious value.
> > > I would not go so far as to describe it as correct.)
> >
> > Ok.
> >
> > > I would suggest you do one of the following things:
> > >
> > > 1. Use int $0x80 directly to load 32-bit regs into a child.  This
> > > might dramatically simplify your code and should just do the right
> > > thing.
> >
> > I don't know what that means.
>
> This is untested, but what I mean is:
>
> static int ptrace32(int req, pid_t pid, int addr, int data) {
>    int ret;
>    /* new enough kernels won't clobber r8, etc. */
>    asm volatile ("int $0x80" : "=a" (ret) : "a" (26 /* ptrace */), "b"
> (req), "c" (pid), "d" (addr), "S" (data) : "flags", "r8", "r9", "r10",
> "r11");
>    return ret;
> }
>
> with a handful of caveats:
>
>  - This won't compile with -fPIC, I think.  Instead you'll need to
> write a little bit of asm to set up and restore ebx yourself.  gcc is
> silly like this.
>
>  - Note that addr is an int.  You'll need to mmap(..., MAP_32BIT, ...)
> to get a buffer that can be pointed to with an int.
>
> The advantage is that this should work on all kernels that support
> 32-bit mode at all.
>
> >
> > > 2. Something like your patch but make it unconditional.
> > >
> > > 3. Ask for, and receive, real kernel support for setting FS and GS in
> > > the way that 32-bit code expects.
> >
> > I think the easiest way forward for us would be a PTRACE_GET/SETREGSET
> > like operation that operates on the regsets according to the
> > *tracee*'s bitness (rather than the tracer, as it works currently).
> > Does that sound workable?
> >
>
> Strictly speaking, on Linux, there is no unified concept of a task's
> bitness, so "set all these registers according to the target's
> bitness" is not well defined.  We could easily give you a
> PTRACE_SETREGS_X86_32, etc, though.

In the process of responding to this I spent some time doing code
inspection and discovered a subtlety in the ptrace API that I was
previously unaware of. PTRACE_GET/SETREGS use the regset views
corresponding to the tracer but PTRACE_GET/SETREGSET use the regset
views corresponding to the tracee. This means it is possible for us
today to set FS/GS "the old way" with a 64 bit tracer/32 bit tracee
combo, as long as we use PTRACE_SETREGSET with NT_PRSTATUS instead of
PTRACE_SETREGS.

- Kyle