Re: [PATCH 00/35] Shadow stacks for userspace

H.J. Lu
2022-03-07T19:07:41Z

On Mon, Mar 7, 2022 at 10:57 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Fri, Mar 04, 2022 at 11:13:19AM -0800, Andy Lutomirski wrote:
> > On 3/3/22 17:30, Edgecombe, Rick P wrote:
> > > On Thu, 2022-03-03 at 15:00 -0800, Andy Lutomirski wrote:
> > > > > > The intent of PTRACE_CALL_FUNCTION_SIGFRAME is push a signal
> > > > > > frame onto
> > > > > > the stack and call a function. That function should then be able
> > > > > > to call
> > > > > > sigreturn just like any normal signal handler.
> > > > >
> > > > > Ok, let me reiterate.
> > > > >
> > > > > We have a seized and stopped tracee, use
> > > > > PTRACE_CALL_FUNCTION_SIGFRAME
> > > > > to push a signal frame onto the tracee's stack so that sigreturn
> > > > > could use
> > > > > that frame, then set the tracee %rip to the function we'd like to
> > > > > call and
> > > > > then we PTRACE_CONT the tracee. Tracee continues to execute the
> > > > > parasite
> > > > > code that calls sigreturn to clean up and restore the tracee
> > > > > process.
> > > > >
> > > > > PTRACE_CALL_FUNCTION_SIGFRAME also pushes a restore token to the
> > > > > shadow
> > > > > stack, just like setup_rt_frame() does, so that sys_rt_sigreturn()
> > > > > won't
> > > > > bail out at restore_signal_shadow_stack().
> > > >
> > > > That is the intent.
> > > >
> > > > >
> > > > > The only thing that CRIU actually needs is to push a restore token
> > > > > to the
> > > > > shadow stack, so for us a ptrace call that does that would be
> > > > > ideal.
> > > > >
> > > >
> > > > That seems fine too. The main benefit of the SIGFRAME approach is
> > > > that, AIUI, CRIU eventually constructs a signal frame anyway, and
> > > > getting one ready-made seems plausibly helpful. But if it's not
> > > > actually that useful, then there's no need to do it.
> > >
> > > I guess pushing a token to the shadow stack could be done like GDB does
> > > calls, with just the basic CET ptrace support. So do we even need a
> > > specific push token operation?
>
> I've tried to follow gdb CET push implementation, but got lost.
> What is "basic CET ptrace support"? I don't see any ptrace changes in this
> series.

Here is the CET ptrace patch on CET 5.16 kernel branch:

https://github.com/hjl-tools/linux/commit/3a43ec29ddac56f87807161b5aeafa80f632363d

> > > I suppose if CRIU already used some kernel encapsulation of a seized
> > > call/return operation it would have been easier to make CRIU work with
> > > the introduction of CET. But the design of CRIU seems to be to have the
> > > kernel expose just enough and then tie it all together in userspace.
> > >
> > > Andy, did you have any other usages for PTRACE_CALL_FUNCTION in mind? I
> > > couldn't find any other CRIU-like users of sigreturn in the debian
> > > source search (but didn't read all 819 pages that come up with
> > > "sigreturn"). It seemed to be mostly seccomp sandbox references.
> >
> > I don't see a benefit compelling enough to justify the added complexity,
> > given that existing mechanisms can do it.
> >
> > The sigframe thing, OTOH, seems genuinely useful if CRIU would actually use
> > it to save the full register state. Generating a signal frame from scratch
> > is a pain. That being said, if CRIU isn't excited, then don't bother.
>
> CRIU is excited :)
>
> I just was looking for the minimal possible interface that will allow us to
> call sigreturn. Rick is right and CRIU does try to expose as little as
> possible and handle the pain in the userspace.
>
> The SIGFRAME approach is indeed very helpful, especially if we can make it
> work on other architectures eventually.
>
> --
> Sincerely yours,
> Mike.

--
H.J.