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

[Andy Lutomirski]

On 2/8/22 18:18, Edgecombe, Rick P wrote:
> On Tue, 2022-02-08 at 20:02 +0300, Cyrill Gorcunov wrote:
> > On Tue, Feb 08, 2022 at 08:21:20AM -0800, Andy Lutomirski wrote:
> > > > > But such a knob will immediately reduce the security value of
> > > > > the entire
> > > > > thing, and I don't have good ideas how to deal with it :(
> > > >
> > > > Probably a kind of latch in the task_struct which would trigger
> > > > off once
> > > > returt to a different address happened, thus we would be able to
> > > > jump inside
> > > > paratite code. Of course such trigger should be available under
> > > > proper
> > > > capability only.
> > >
> > > I'm not fully in touch with how parasite, etc works.  Are we
> > > talking about save or restore?
> >
> > We use parasite code in question during checkpoint phase as far as I
> > remember.
> > push addr/lret trick is used to run "injected" code (code injection
> > itself is
> > done via ptrace) in compat mode at least. Dima, Andrei, I didn't look
> > into this code
> > for years already, do we still need to support compat mode at all?
> >
> > > If it's restore, what exactly does CRIU need to do?  Is it just
> > > that CRIU needs to return
> > > out from its resume code into the to-be-resumed program without
> > > tripping CET?  Would it
> > > be acceptable for CRIU to require that at least one shstk slot be
> > > free at save time?
> > > Or do we need a mechanism to atomically switch to a completely full
> > > shadow stack at resume?
> > >
> > > Off the top of my head, a sigreturn (or sigreturn-like mechanism)
> > > that is intended for
> > > use for altshadowstack could safely verify a token on the
> > > altshdowstack, possibly
> > > compare to something in ucontext (or not -- this isn't clearly
> > > necessary) and switch
> > > back to the previous stack.  CRIU could use that too.  Obviously
> > > CRIU will need a way
> > > to populate the relevant stacks, but WRUSS can be used for that,
> > > and I think this
> > > is a fundamental requirement for CRIU -- CRIU restore absolutely
> > > needs a way to write
> > > the saved shadow stack data into the shadow stack.
>
> Still wrapping my head around the CRIU save and restore steps, but
> another general approach might be to give ptrace the ability to
> temporarily pause/resume/set CET enablement and SSP for a stopped
> thread. Then injected code doesn't need to jump through any hoops or
> possibly run into road blocks. I'm not sure how much this opens things
> up if the thread has to be stopped...

Hmm, that's maybe not insane.

An alternative would be to add a bona fide ptrace call-a-function 
mechanism.  I can think of two potentially usable variants:

1. Straight call.  PTRACE_CALL_FUNCTION(addr) just emulates CALL addr, 
shadow stack push and all.

2. Signal-style.  PTRACE_CALL_FUNCTION_SIGFRAME injects an actual signal 
frame just like a real signal is being delivered with the specified 
handler.  There could be a variant to opt-in to also using a specified 
altstack and altshadowstack.

2 would be more expensive but would avoid the need for much in the way 
of asm magic.  The injected code could be plain C (or Rust or Zig or 
whatever).

All of this only really handles save, not restore.  I don't understand 
restore enough to fully understand the issue.

--Andy