Messages in this thread | | | Date | Wed, 28 Oct 2020 13:52:41 -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 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.
Rich
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