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

H.J. Lu
2022-02-10T02:54:16Z

On Wed, Feb 9, 2022 at 6:37 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> 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.

FWIW, CET enabled GDB can call a function in a CET enabled process.
Adding Felix who may know more about it.

--
H.J.