Re: Isolating abstract sockets

["Mickaël Salaün" <mic@digikod.net>]

On Wed, Nov 01, 2023 at 05:23:12PM +0100, Jann Horn wrote:
> On Wed, Nov 1, 2023 at 11:57 AM Mickaël Salaün <mic@digikod.net> wrote:
> > On Tue, Oct 31, 2023 at 09:40:59PM +0100, Stefan Bavendiek wrote:
> > > On Tue, Oct 24, 2023 at 11:07:14AM -0500, Serge E. Hallyn wrote:
> > > > In 2005, before namespaces were upstreamed, I posted the 'bsdjail' LSM,
> > > > which briefly made it into the -mm kernel, but was eventually rejected as
> > > > being an abuse of the LSM interface for OS level virtualization :)
> > > >
> > > > It's not 100% clear to me whether Stefan only wants isolation, or
> > > > wants something closer to virtualization.
> > > >
> > > > Stefan, would an LSM allowing you to isolate certain processes from
> > > > some abstract unix socket paths (or by label, whatever0 suffice for you?
> > > >
> > >
> > > My intention was to find a clean way to isolate abstract sockets in network
> > > applications without adding dependencies like LSMs. However the entire approach
> > > of using namespaces for this is something I have mostly abandoned. LSMs like
> > > Apparmor and SELinux would work fine for process isolation when you can control
> > > the target system, but for general deployment of sandboxed processes, I found it
> > > to be significantly easier (and more effective) to build this into the
> > > application itself by using a multi process approach with seccomp (Basically how
> > > OpenSSH did it)
> >
> > I agree that for sandbox use cases embedding such security policy into
> > the application itself makes sense. Landlock works the same way as
> > seccomp but it sandboxes applications according to the kernel semantic
> > (e.g. process, socket). The LSM framework is just a kernel
> > implementation detail. ;)
> 
> (Related, it might be nice if Landlock had a way to completely deny
> access to abstract unix sockets,

I think it would make more sense to scope access to abstract unix
sockets: https://lore.kernel.org/all/20231025.eecai4uGh5Ie@digikod.net/

A complementary approach would be to restrict socket creation according
to their properties:
https://lore.kernel.org/all/b8a2045a-e7e8-d141-7c01-bf47874c7930@digikod.net/

> and a way to restrict filesystem unix
> sockets with filesystem rules... LANDLOCK_ACCESS_FS_MAKE_SOCK exists
> for restricting bind(), but I don't think there's an analogous
> permission for connect().

I agree. It should not be too difficult to add a new LSM path hook for
connect (and sendmsg) to named unix socket with the related access
rights.  We should be careful about the impact on sendmsg calls though.

> 
> Currently, when you try to sandbox an application with Landlock, you
> have to use seccomp to completely block access to unix domain sockets,
> or alternatively use something like the seccomp_unotify feature to
> interactively filter connect() calls.
> 
> On the other hand, maybe such a feature would be a bit superfluous
> when we have seccomp_unotify already... idk.)

seccomp_unotify enables user space to emulate syscalls, which requires a
service per sandbox. seccomp is useful but will always be delicate to
use and to maintain the related filters for sandboxing use cases:
https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/

Anyway, I'd be happy to help improve Landlock with new access control
types.

FYI, TCP connect and bind access control should be part of Linux 6.7:
https://lore.kernel.org/all/20231102131354.263678-1-mic@digikod.net/