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

[Adrian Ratiu]

On Tuesday, March 05, 2024 20:37 EET, Kees Cook <keescook at chromium.org> wrote:

> On Tue, Mar 05, 2024 at 11:32:04AM +0100, Christian Brauner wrote:
> > On Tue, Mar 05, 2024 at 02:12:26AM -0800, Kees Cook wrote:
> > > On Tue, Mar 05, 2024 at 10:58:25AM +0100, Christian Brauner wrote:
> > > > Since the write handler for /proc/<pid>/mem does raise FOLL_FORCE
> > > > unconditionally it likely would implicitly. But I'm not familiar enough
> > > > with FOLL_FORCE to say for sure.
> > > 
> > > I should phrase the question better. :) Is the supervisor writing into
> > > read-only regions of the child process?
> > 
> > Hm... I suspect we don't. Let's take two concrete examples so you can
> > tell me.
> > 
> > Incus intercepts the sysinfo() syscall. It prepares a struct sysinfo
> > with cgroup aware values for the supervised process and then does:
> > 
> > unix.Pwrite(siov.memFd, &sysinfo, sizeof(struct sysinfo), seccomp_data.args[0]))
> > 
> > It also intercepts some bpf system calls attaching bpf programs for the
> > caller. If that fails we update the log buffer for the supervised
> > process:
> > 
> > union bpf_attr attr = {}, new_attr = {};
> > 
> > // read struct bpf_attr from mem_fd
> > ret = pread(mem_fd, &attr, attr_len, req->data.args[1]);
> > if (ret < 0)
> >         return -errno;
> > 
> > // Do stuff with attr. Stuff fails. Update log buffer for supervised process:
> > if ((new_attr.log_size) > 0 && (pwrite(mem_fd, new_attr.log_buf, new_attr.log_size, attr.log_buf) != new_attr.log_size))
> 
> This is almost certainly in writable memory (either stack or .data).

Mostly yes, but we can't be certain where it comes from, because
SECCOMP_IOCTL_NOTIF_RECV passes any addresses set by the
caller to the supervisor process.

It is a kind of "implementation defined" behavior, just like we
can't predict what the supervisor will do with the caller mem :)

> 
> > But I'm not sure if there are other use-cases that would require this.
> 
> Maybe this option needs to be per-process (like no_new_privs), and with
> a few access levels:
> 
> - as things are now
> - no FOLL_FORCE unless by ptracer
> - no writes unless by ptracer
> - no FOLL_FORCE ever
> - no writes ever
> - no reads unless by ptracer
> - no reads ever
> 
> Which feels more like 3 toggles: read, write, FOLL_FORCE. Each set to
> "DAC", "ptracer", and "none"?

I really like this approach because it provides a  mechanism
with maximum flexibility without imposing a specific policy.

What does DAC mean in this context? My mind jumps to
Digital to Analog Converter :)

Shall I give it a try in v3?

> 
> -- 
> Kees Cook