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

Willgerodt, Felix
2022-02-10T13:52:52Z

> -----Original Message-----
> From: H.J. Lu <hjl.tools@gmail.com>
> Sent: Donnerstag, 10. Februar 2022 03:54
> To: Lutomirski, Andy <luto@kernel.org>; Willgerodt, Felix
> <felix.willgerodt@intel.com>
> Cc: Edgecombe, Rick P <rick.p.edgecombe@intel.com>; gorcunov@gmail.com;
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> Subject: Re: [PATCH 00/35] Shadow stacks for userspace
>
> 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.

I don't know much about CRIU or kernel code, so I will stick to explaining
what our GDB patches for CET (not upstream yet) currently do.

GDB does inferior calls by setting the PC to the function it wants to call
and by manipulating the return address. It basically creates a dummy
frame and runs that on top of where it currently is.

To enable this for CET, our GDB CET patches push onto the shstk of the
inferior by writing to the inferiors memory, if it isn't out of range,
and by incrementing the SSP (using NT_X86_CET), both via ptrace.

(GDB also has a command called 'return', which basically returns early from
a function. Here GDB just decrements the SSP via ptrace.)

This was based on earlier versions of the kernel patches.
If the interface needs to change or if new interfaces would be available to
do this better, that is fine from our pov. We didn't upstream this yet.

Also, if you have any concerns with this approach please let me know.

Regards,
Felix

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