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

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Message-ID: <CANaxB-xbsOMkKqfaOJ0Za7-yP2N8axO=E1XS1KufnP78H1YzsA@mail.gmail.com>
Date: Sat, 26 Jul 2025 10:12:34 -0700
From: Andrei Vagin <avagin@...il.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>, 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 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. I
understand this change might be necessary in its current form, and
that some collateral damage could be unavoidable. But if that's the
case, I'd expect a detailed explanation of why it had to be so and why
userspace breakage is unavoidable.

The original change was merged two decades ago. We need to
consider that some applications might rely on that behavior. I'm not
questioning the security aspect - that must be addressed. But for
anything else, we need to minimize the impact on user applications that
don't violate security.

We can consider a cleaner fix for the upstream kernel, but when we are
talking about stable kernels, the user-space backward compatibility
aspect should be even more critical.

Thanks,
Andrei