On Wed, Oct 28, 2020 at 6:52 PM Rich Felker <dal...@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 <dal...@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 <comm...@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...