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

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Message-ID: <20250731-masten-resolut-89aca1e3454f@brauner>
Date: Thu, 31 Jul 2025 09:53:25 +0200
From: Christian Brauner <brauner@...nel.org>
To: Pavel Tikhomirov <snorcht@...il.com>
Cc: Andrei Vagin <avagin@...gle.com>, Al Viro <viro@...iv.linux.org.uk>, 
	Andrei Vagin <avagin@...il.com>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, criu@...ts.linux.dev, Linux API <linux-api@...r.kernel.org>, 
	stable <stable@...r.kernel.org>
Subject: Re: do_change_type(): refuse to operate on unmounted/not ours mounts

On Thu, Jul 31, 2025 at 10:40:40AM +0800, Pavel Tikhomirov wrote:
> If detached mounts are our only concern, it looks like the check instead of:
> 
> if (!check_mnt(mnt)) {
>         err = -EINVAL;
>         goto out_unlock;
> }
> 
> could've been a more relaxed one:
> 
> if (mnt_detached(mnt)) {
>         err = -EINVAL;
>         goto out_unlock;
> }
> 
> bool mnt_detached(struct mount *mnt)
> {
>         return !mnt->mnt_ns;
> }
> 
> not to allow propagation change only on detached mounts. (As
> umount_tree sets mnt_ns to NULL.)

Changing propagation settings on detached mounts is fine and shoud work?
Changing propagation settings on unmounted mounts not so much...

> 
> Also in do_mount_setattr we have a more relaxed check too:
> 
> if ((mnt_has_parent(mnt) || !is_anon_ns(mnt->mnt_ns)) && !check_mnt(mnt))
>         goto out;
> 
> Best Regards, Tikhomirov Pavel.
> 
> On Sun, Jul 27, 2025 at 5:01 AM Andrei Vagin <avagin@...gle.com> wrote:
> >
> > On Sat, Jul 26, 2025 at 10:53 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > 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@...iv.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").
> >
> > I wasn't referring to detached mounts. CRIU modifies mounts from
> > non-current namespaces.
> >
> > >
> > > As for the "CAP_SYS_ADMIN within the mount user namespace" - which
> > > userns do you have in mind?
> > >
> >
> > The user namespace of the target mount:
> > ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN)
> >