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

["Adrian Ratiu" <adrian.ratiu@collabora.com>]

On Tuesday, March 05, 2024 11:41 EET, Kees Cook <keescook@chromium.org> wrote:

> 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?

I can't speak for what is acceptable for ChromeOS security because 
I'm not part of that project, so I'll let Mike answer whether blocking
writes is mandatory for them or blocking FOLL_FORCE is enough.

From a design perspective, the question is whether to
1. block writes and allow known good exceptions 
or
2. allow writes and block known bad/exploitable exceptions. 
 
I am looking into reproducing and adding an exception for the
container syscall intercept use-case raised by Christian, because
I think it's easier to justify allowing known good exceptions from
a security perspective.

Otherwise I'm fine with both approaches.

@Mike WDYT ?