> On Jul 15, 2020, at 4:07 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > Earlier Andy Lutomirski wrote: >> Let’s add some seccomp folks. We probably also want to be able to run >> seccomp-like filters on io_uring requests. So maybe io_uring should call into >> seccomp-and-tracing code for each action. > > Okay, I'm finally able to spend time looking at this. And thank you to > the many people that CCed me into this and earlier discussions (at least > Jann, Christian, and Andy). > > It *seems* like there is a really clean mapping of SQE OPs to syscalls. > To that end, yes, it should be trivial to add ptrace and seccomp support > (sort of). The trouble comes for doing _interception_, which is how both > ptrace and seccomp are designed. > > In the basic case of seccomp, various syscalls are just being checked > for accept/reject. It seems like that would be easy to wire up. For the > more ptrace-y things (SECCOMP_RET_TRAP, SECCOMP_RET_USER_NOTIF, etc), > I think any such results would need to be "upgraded" to "reject". Things > are a bit complex in that seccomp's form of "reject" can be "return > errno" (easy) or it can be "kill thread (or thread_group)" which ... > becomes less clear. (More on this later.) My intuition is not to do this kind of creative reinterpretation of return values. Instead let’s have a new type of seccomp filter specifically for io_uring. So we can have SECCOMP_IO_URING_ACCEPT, ERRNO, and eventually other things. We probably will want a user notifier feature for io_uring, but I'd be a bit surprised if it ends up ABI-compatible with current users of user notifiers. > - There appear to be three classes of desired restrictions: > - opcodes for io_uring_register() (which can be enforced entirely with > seccomp right now). Agreed. > - opcodes from SQEs (this _could_ be intercepted by seccomp, but is > not currently written) As above, I think this should be intercepted by seccomp, but in a new mode. I think that existing seccomp filters should not intercept it. > - opcodes of the types of restrictions to restrict... for making sure > things can't be changed after being set? seccomp already enforces > that kind of "can only be made stricter" Agreed. > > - How does no_new_privs play a role in the existing io_uring credential > management? Using _any_ kind of syscall-effective filtering, whether > it's seccomp or Stefano's existing proposal, needs to address the > potential inheritable restrictions across privilege boundaries (which is > what no_new_privs tries to eliminate). In regular syscall land, this is > an issue when a filter follows a process through setuid via execve() > and it gains privileges that now the filter-creator can trick into > doing weird stuff -- io_uring has a concept of alternative credentials > so I have to ask about it. (I don't *think* there would be a path to > install a filter before gaining privilege, but I likely just > need to do my homework on the io_uring internals. Regardless, > use of seccomp by io_uring would need to have this issue "solved" > in the sense that it must be "safe" to filter io_uring OPs, from a > privilege-boundary-crossing perspective. > > - From which task perspective should filters be applied? It seems like it > needs to follow the io_uring personalities, as that contains the > credentials. (This email is a brain-dump so far -- I haven't gone to > look to see if that means io_uring is literally getting a reference to > struct cred; I assume so.) Seccomp filters are attached to task_struct. > However, for v5.9, seccomp will gain a more generalized get/put system > for having filters attached to the SECCOMP_RET_USER_NOTIF fd. Adding > more get/put-ers for some part of the io_uring context shouldn't > be hard. Let's ignore personalities for a moment (and see below). Thinking through the possibilities: A: io_uring seccomp filters are attached to tasks. When an io_uring is created, it inherits an immutable copy of its creating task's filter, and that's the filter set that applies to that io_uring instance. This could have somewhat bizarre consequences if the fd gets passed around, but io_uring already has odd security effects if fds are passed around. It has the annoying property that, if a library creates an io_uring and then a seccomp filter is loaded, the io_uring bypasses the library. B: The same, but the io_uring references the creating task so new filters on the task apply to the io_uring, too. This allows loading and then sandboxing. Is this too bizarre overall? C: io_uring filters are attached directly to io_urings. This has the problem where an io_uring created before a task sandboxes itself isn't sandboxed. It also would require that a filter be able to hook io_uring creation to sandbox it. Does anyone actually pass io_urings around with SCM_RIGHTS? It would be really nice if we could make the default be that io_urings are bound to their creating mm and can't be used outside it. Then creating an mm-crossing io_uring could, itself, be restricted. In any case, my inclination is to go for choice B. Choice C could also be supported if there's a use case.
