Re: do_change_type(): refuse to operate on unmounted/not ours mounts

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Jul 26, 2025 at 10:12:34AM -0700, Andrei Vagin wrote:
> On Thu, Jul 24, 2025 at 4:00 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Thu, Jul 24, 2025 at 01:02:48PM -0700, Andrei Vagin wrote:
> > > Hi Al and Christian,
> > >
> > > The commit 12f147ddd6de ("do_change_type(): refuse to operate on
> > > unmounted/not ours mounts") introduced an ABI backward compatibility
> > > break. CRIU depends on the previous behavior, and users are now
> > > reporting criu restore failures following the kernel update. This change
> > > has been propagated to stable kernels. Is this check strictly required?
> >
> > Yes.
> >
> > > Would it be possible to check only if the current process has
> > > CAP_SYS_ADMIN within the mount user namespace?
> >
> > Not enough, both in terms of permissions *and* in terms of "thou
> > shalt not bugger the kernel data structures - nobody's priveleged
> > enough for that".
> 
> Al,
> 
> I am still thinking in terms of "Thou shalt not break userspace"...
> 
> Seriously though, this original behavior has been in the kernel for 20
> years, and it hasn't triggered any corruptions in all that time.

For a very mild example of fun to be had there:
	mount("none", "/mnt", "tmpfs", 0, "");
	chdir("/mnt");
	umount2(".", MNT_DETACH);
	mount(NULL, ".", NULL, MS_SHARED, NULL);
Repeat in a loop, watch mount group id leak.  That's a trivial example
of violating the assertion ("a mount that had been through umount_tree()
is out of propagation graph and related data structures for good").

As for the "CAP_SYS_ADMIN within the mount user namespace" - which
userns do you have in mind?