Messages in this thread | | | Date | Wed, 28 Oct 2020 14:35:11 -0400 | From | Rich Felker <> | Subject | Re: [seccomp] Request for a "enable on execve" mode for Seccomp filters |
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On Wed, Oct 28, 2020 at 07:25:45PM +0100, Jann Horn wrote: > On Wed, Oct 28, 2020 at 6:52 PM Rich Felker <dalias@libc.org> wrote: > > On Wed, Oct 28, 2020 at 06:34:56PM +0100, Jann Horn wrote: > > > On Wed, Oct 28, 2020 at 5:49 PM Rich Felker <dalias@libc.org> wrote: > > > > On Wed, Oct 28, 2020 at 01:42:13PM +0100, Jann Horn wrote: > > > > > On Wed, Oct 28, 2020 at 12:18 PM Camille Mougey <commial@gmail.com> wrote: > > > > > You're just focusing on execve() - I think it's important to keep in > > > > > mind what happens after execve() for normal, dynamically-linked > > > > > binaries: The next step is that the dynamic linker runs, and it will > > > > > poke around in the file system with access() and openat() and fstat(), > > > > > it will mmap() executable libraries into memory, it will mprotect() > > > > > some memory regions, it will set up thread-local storage (e.g. using > > > > > arch_prctl(); even if the process is single-threaded), and so on. > > > > > > > > > > The earlier you install the seccomp filter, the more of these steps > > > > > you have to permit in the filter. And if you want the filter to take > > > > > effect directly after execve(), the syscalls you'll be forced to > > > > > permit are sufficient to cobble something together in userspace that > > > > > effectively does almost the same thing as execve(). > > > > > > > > I would assume you use SECCOMP_RET_USER_NOTIF to implement policy for > > > > controlling these operations and allowing only the ones that are valid > > > > during dynamic linking. This also allows you to defer application of > > > > the filter until after execve. So unless I'm missing some reason why > > > > this doesn't work, I think the requested functionality is already > > > > available. > > > > > > Ah, yeah, good point. > > > > > > > If you really just want the "activate at exec" behavior, it might be > > > > possible (depending on how SECCOMP_RET_USER_NOTIF behaves when there's > > > > no notify fd open; I forget) > > > > > > syscall returns -ENOSYS. Yeah, that'd probably do the job. (Even > > > though it might be a bit nicer if userspace had control over the errno > > > there, such that it could be EPERM instead... oh well.) > > > > EPERM is a major bug in current sandbox implementations, so ENOSYS is > > at least mildly better, but indeed it should be controllable, probably > > by allowing a code path for the BPF to continue with a jump to a > > different logic path if the notify listener is missing. > > I guess we might be able to expose the listener status through a bit / > a field in the struct seccomp_data, and then filters could branch on > that. (And the kernel would run the filter twice if we raced with > filter detachment.) I don't know whether it would look pretty, but I > think it should be doable...
I was thinking the race wouldn't be salvagable, but indeed since the filter is side-effect-free you can just re-run it if the status changes between start of filter processing and the attempt at notification. This sounds like it should work.
I guess it's not possible to chain two BPF filters to do this, because that only works when the first one allows? Or am I misunderstanding the multiple-filters case entirely? (I've never gotten that far with programming it.)
Rich
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