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