On Thu, Jul 16, 2020 at 08:12:35AM -0700, Kees Cook wrote: > On Thu, Jul 16, 2020 at 03:14:04PM +0200, Stefano Garzarella wrote: > > On Wed, Jul 15, 2020 at 04:07:00PM -0700, Kees Cook wrote: > > [...] > > > > > Speaking to Stefano's proposal[1]: > > > > > > - There appear to be three classes of desired restrictions: > > > - opcodes for io_uring_register() (which can be enforced entirely with > > > seccomp right now). > > > - opcodes from SQEs (this _could_ be intercepted by seccomp, but is > > > not currently written) > > > - 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" > > > > In addition we want to limit the SQEs to use only the registered fd and > > buffers. > > Hmm, good point. Yeah, since it's an "extra" mapping (ioring file number > vs fd number) this doesn't really map well to seccomp. (And frankly, > there's some difficulty here mapping many of the ioring-syscalls to > seccomp because it's happening "deeper" than the syscall layer (i.e. > some of the arguments have already been resolved into kernel object > pointers, etc). > > > Do you think it's better to have everything in seccomp instead of adding > > the restrictions in io_uring (the patch isn't very big)? > > I'm still trying to understand how io_uring will be used, and it seems > odd to me that it's effectively a seccomp bypass. (Though from what I > can tell it is not an LSM bypass, which is good -- though I'm worried > there might be some embedded assumptions in LSMs about creds vs current > and LSMs may try to reason (or report) on actions with the kthread in > mind, but afaict everything important is checked against creds. > > > With seccomp, would it be possible to have different restrictions for two > > instances of io_uring in the same process? > > For me, this is the most compelling reason to have the restrictions NOT > implemented via seccomp. Trying to make "which instance" choice in > seccomp would be extremely clumsy. > > So at this point, I think it makes sense for the restriction series to > carry on -- it is io_uring-specific and solves some problems that > seccomp is not in good position to reason about.
Thanks for the feedback, then I'll continue in this direction! > > All this said, I'd still like a way to apply seccomp to io_uring > because it's a rather giant syscall filter bypass mechanism, and gaining Agree. > access (IIUC) is possible without actually calling any of the io_uring > syscalls. Is that correct? A process would receive an fd (via SCM_RIGHTS, > pidfd_getfd, or soon seccomp addfd), and then call mmap() on it to gain > access to the SQ and CQ, and off it goes? (The only glitch I see is > waking up the worker thread?) It is true only if the io_uring istance is created with SQPOLL flag (not the default behaviour and it requires CAP_SYS_ADMIN). In this case the kthread is created and you can also set an higher idle time for it, so also the waking up syscall can be avoided. > > What appears to be the worst bit about adding seccomp to io_uring is the > almost complete disassociation of process hierarchy from syscall action. > Only a cred is used for io_uring, and seccomp filters are associated with > task structs. I'm not sure if there is a way to solve this disconnect > without a major internal refactoring of seccomp to attach to creds and > then make every filter attachment create a new cred... *head explody* > Sorry but I don't know seccomp that well :-( I'm learning a lot about it these days. I'll keep your concern in mind. Thanks, Stefano
