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

Andy Lutomirski
2022-03-03T23:00:58Z

On Thu, Mar 3, 2022 at 11:43 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Mon, Feb 28, 2022 at 02:55:30PM -0800, Andy Lutomirski wrote:
> >
> >
> > On Mon, Feb 28, 2022, at 1:30 PM, Mike Rapoport wrote:
> > > On Mon, Feb 28, 2022 at 12:30:41PM -0800, Andy Lutomirski wrote:
> > >>
> > >>
> > >> On Mon, Feb 28, 2022, at 12:27 PM, Mike Rapoport wrote:
> > >> > On Wed, Feb 09, 2022 at 06:37:53PM -0800, Andy Lutomirski wrote:
> > >> >> On 2/8/22 18:18, Edgecombe, Rick P wrote:
> > >> >> > On Tue, 2022-02-08 at 20:02 +0300, Cyrill Gorcunov wrote:
> > >> >> >
> > >> >
> > >> > Even with the current shadow stack interface Rick proposed, CRIU can restore
> > >> > the victim using ptrace without any additional knobs, but we loose an
> > >> > important ability to "self-cure" the victim from the parasite in case
> > >> > anything goes wrong with criu control process.
> > >> >
> > >> > Moreover, the issue with backward compatibility is not with ptrace but with
> > >> > sigreturn and it seems that criu is not its only user.
> > >>
> > >> So we need an ability for a tracer to cause the tracee to call a function
> > >> and to return successfully. Apparently a gdb branch can already do this
> > >> with shstk, and my PTRACE_CALL_FUNCTION_SIGFRAME should also do the
> > >> trick. I don't see why we need a sigretur-but-dont-verify -- we just
> > >> need this mechanism to create a frame such that sigreturn actually works.
> > >
> > > If I understand correctly, PTRACE_CALL_FUNCTION_SIGFRAME() injects a frame
> > > into the tracee and makes the tracee call sigreturn.
> > > I.e. the tracee is stopped and this is used pretty much as PTRACE_CONT or
> > > PTRACE_SYSCALL.
> > >
> > > In such case this defeats the purpose of sigreturn in CRIU because it is
> > > called asynchronously by the tracee when the tracer is about to detach or
> > > even already detached.
> >
> > 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.