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

[Kees Cook]

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).

> 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"?

-- 
Kees Cook