Unprivileged filesystem mounts

[Al Viro]

On Tue, Mar 11, 2025 at 12:01:48PM +0100, Christian Brauner wrote:

> The case where arbitrary devices stuck into a laptop (e.g., USB sticks)
> are mounted isn't solved by making a filesystem mountable unprivileged.
> The mounted device cannot show up in the global mount namespace
> somewhere since the user doesn't own the initial mount+user namespace.
> So it's pointless. In other words, there's filesystem level checks and
> mount namespace based checks. Circumventing that restriction means that
> any user can just mount the device at any location in the global mount
> namespace and therefore simply overmount other stuff.

Note that "untrusted contents" is not the worst thing you can run into -
it can be content changing behind your back.  I seriously doubt that
anyone fuzzes for that kind of crap (and no, it's not an invitation to
start).  I seriously doubt that there's any local filesystem that would
be resilent to that...