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

[Kyle Huey]

On Tue, Aug 25, 2020 at 12:32 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Tue, Aug 25, 2020 at 11:50 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
> >
> > On Tue, Aug 25, 2020 at 10:31 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Aug 25, 2020 at 9:46 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Aug 25, 2020 at 9:32 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.
> >
> > Alright I reverted the previous changes and switched us to use
> > PTRACE_SETREGSET with NT_PRSTATUS[0] and our 32 bit tests pass again.
> > I assume this behavior will remain unchanged indefinitely even when
> > the fs/gsbase manipulation instructions are not available since the 32
> > bit user_regs_struct can't grow?
>
> Correct.
>
> I think it would be reasonable to add new, less erratic PTRACE_SETxyz
> modes, but the old ones will stay as is.
>
> Strictly speaking, the issue you had wasn't a ptrace change.  It was a
> context switching change.  Before the change, you were programming
> garbage into the tracee state, but the context switch code wasn't
> capable of restoring the garbage correctly and instead happened to
> restore the state you actually wanted.  With the new code, the context
> switch actually worked correctly (for some value of correctly), and
> the tracee crashed.  Not that this distinction really matters.

Yes. We've been feeding the kernel trash for years. You were just
letting us get away with it before ;)

- Kyle