On Fri, Jul 25, 2014 at 10:38 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Fri, Jul 25, 2014 at 10:18 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> [cc: Eric Biederman] >> >> On Fri, Jul 25, 2014 at 10:10 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >>> Julien had been wanting something like this too (though he'd suggested >>> it via prctl): limit the signal functions to "self" only. I wonder if >>> adding a prctl like done for O_BENEATH could work for signal sending? >>> >> >> >> Can we do one better and add a flag to prevent any non-self pid >> lookups? This might actually be easy on top of the pid namespace work >> (e.g. we could change the way that find_task_by_vpid works). > > Ooh, that would be extremely interesting, yes. Kind of an extreme form > of pid namespace without actually being a namespace. > >> It's far from just being signals. There's access_process_vm, ptrace, >> all the signal functions, clock_gettime (see CPUCLOCK_PID -- yes, this >> is ridiculous), and probably some others that I've forgotten about or >> never noticed in the first place. > > Yeah, that would be very interesting. Yes, this would be incredibly useful. 1. For Chromium [1], I dislike relying on seccomp purely for "access-control" (to other processes or files). Because it's really hard to think about everything (things like CPUCLOCK_PID bite, seehttps://crbug.com/374479). Se we have a first layer of sandboxing (using PID + NET namespaces and chroot) for "access-control" and a second layer for kernel attack surface reduction and a few other things using seccomp-bpf. The first layer isn't currently very good; it's heavyweight and complex (you need an init(1) per namespace and that init cannot be multi-purposed as a useful process because pid = 1 can never receive signals). One PID namespace per process isn't something that scales well. (Also before USER_NS it required a setuid root program). 2. Even with a safe pure seccomp-bpf sandbox that prevents sending signals to other process / ptrace() et al and that restrict clock_gettime(2) properly, things become quickly very tedious because as far as the kernel is concerned, the process under this BPF program can still pass ptrace_may_access() to other processes. This means for instance that no matter what you do, a model where open() is allowed can't work if /proc is available. We need a mode that says "ptrace_may_access()" will never pass. So yes, I really would like: - a prctl that says: "I'm dropping privileges and I now can't interact with other thread groups (via signals, ptrace, etc..)". - Something to drop access to the file system. It could be an unprivileged way to chroot() to an empty directory (unprivileged namespaces work for that, - except if you're already in a chroot -). This is a little tricky without allowing chroot escapes, so I suspect we would want to express it in terms of mount namespace, or something else, rather than chroot. Then we have the primitives we need to build sandboxes in a simple way and we can add seccomp-bpf on top to do things such as open() hooking (via SECCOMP_RET_TRAP) and to restrict the kernel attack surface. Julien [1] https://code.google.com/p/chromium/wiki/LinuxSandboxing -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
