Re: [PATCH v2] proc: allow restricting /proc/pid/mem writes

Kees Cook
2024-03-05T09:41:31Z

On Tue, Mar 05, 2024 at 09:59:47AM +0100, Christian Brauner wrote:
> > > Uhm, this will break the seccomp notifier, no? So you can't turn on
> > > SECURITY_PROC_MEM_RESTRICT_WRITE when you want to use the seccomp
> > > notifier to do system call interception and rewrite memory locations of
> > > the calling task, no? Which is very much relied upon in various
> > > container managers and possibly other security tools.
> > >
> > > Which means that you can't turn this on in any of the regular distros.
> >
> > FWIW, it's a run-time toggle, but yes, let's make sure this works
> > correctly.
> >
> > > So you need to either account for the calling task being a seccomp
> > > supervisor for the task whose memory it is trying to access or you need
> > > to provide a migration path by adding an api that let's caller's perform
> > > these writes through the seccomp notifier.
> >
> > How do seccomp supervisors that use USER_NOTIF do those kinds of
> > memory writes currently? I thought they were actually using ptrace?
> > Everything I'm familiar with is just using SECCOMP_IOCTL_NOTIF_ADDFD,
> > and not doing fancy memory pokes.
>
> For example, incus has a seccomp supervisor such that each container
> gets it's own goroutine that is responsible for handling system call
> interception.
>
> If a container is started the container runtime connects to an AF_UNIX
> socket to register with the seccomp supervisor. It stays connected until
> it stops. Everytime a system call is performed that is registered in the
> seccomp notifier filter the container runtime will send a AF_UNIX
> message to the seccomp supervisor. This will include the following fds:
>
> - the pidfd of the task that performed the system call (we should
> actually replace this with SO_PEERPIDFD now that we have that)
> - the fd of the task's memory to /proc/<pid>/mem
>
> The seccomp supervisor will then perform the system call interception
> including the required memory reads and writes.

Okay, so the patch would very much break that. Some questions, though:
- why not use process_vm_writev()?
- does the supervisor depend on FOLL_FORCE?

Perhaps is is sufficient to block the use of FOLL_FORCE?

I took a look at the Chrome OS exploit, and I _think_ it is depending
on the FOLL_FORCE behavior (it searches for a symbol to overwrite that
if I'm following correctly is in a read-only region), but some of the
binaries don't include source code, so I couldn't easily see what was
being injected. Mike or Adrian can you confirm this?

--
Kees Cook