On Mon, Sep 07, 2020 at 07:15:52AM -0700, Andy Lutomirski wrote: > > > > On Sep 7, 2020, at 3:15 AM, Christian Brauner <christian.brauner@xxxxxxxxxx> wrote: > > > > On Fri, Sep 04, 2020 at 04:31:44PM -0400, Gabriel Krisman Bertazi wrote: > >> Syscall User Dispatch (SUD) must take precedence over seccomp, since the > >> use case is emulation (it can be invoked with a different ABI) such that > >> seccomp filtering by syscall number doesn't make sense in the first > >> place. In addition, either the syscall is dispatched back to userspace, > >> in which case there is no resource for seccomp to protect, or the > > > > Tbh, I'm torn here. I'm not a super clever attacker but it feels to me > > that this is still at least a clever way to circumvent a seccomp > > sandbox. > > If I'd be confined by a seccomp profile that would cause me to be > > SIGKILLed when I try do open() I could prctl() myself to do user > > dispatch to prevent that from happening, no? > > > > Not really, I think. The idea is that you didn’t actually do open(). > You did a SYSCALL instruction which meant something else, and the > syscall dispatch correctly prevented the kernel from misinterpreting > it as open(). Right, for the case where you're e.g. emulating windows syscalls that's true. I was thinking when you're running natively on Linux: couldn't I first load a seccomp profile "kill me if someone does an open()", then I exec() the target binary and that binary is setup to do prctl(USER_DISPATCH) first thing. I guess, it's ok because as far as I had time to read it this is a nothing or all mechanism, i.e. _all_ system calls are re-routed in contrast to e.g. seccomp where I could do this per-syscall. So for user-dispatch it wouldn't make sense to use it on Linux per se. Still makes me a little uneasy. :) Christian
