linux-kernel - Re: [PATCH v3 0/6] Composefs: an opportunistically sharing verified image filesystem

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Date:   Wed, 25 Jan 2023 22:23:08 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Giuseppe Scrivano <gscrivan@...hat.com>
Cc:     Dave Chinner <david@...morbit.com>,
        Alexander Larsson <alexl@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        brauner@...nel.org, viro@...iv.linux.org.uk,
        Vivek Goyal <vgoyal@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH v3 0/6] Composefs: an opportunistically sharing verified
 image filesystem

On Wed, Jan 25, 2023 at 9:45 PM Giuseppe Scrivano <gscrivan@...hat.com> wrote:
>
> Amir Goldstein <amir73il@...il.com> writes:
>
> >> >> I previously mentioned my wish of using it from a user namespace, the
> >> >> goal seems more challenging with EROFS or any other block devices.  I
> >> >> don't know about the difficulty of getting overlay metacopy working in a
> >> >> user namespace, even though it would be helpful for other use cases as
> >> >> well.
> >> >>
> >> >
> >> > There is no restriction of metacopy in user namespace.
> >> > overlayfs needs to be mounted with -o userxattr and the overlay
> >> > xattrs needs to use user.overlay. prefix.
> >>
> >> if I specify both userxattr and metacopy=on then the mount ends up in
> >> the following check:
> >>
> >> if (config->userxattr) {
> >>         [...]
> >>         if (config->metacopy && metacopy_opt) {
> >>                 pr_err("conflicting options: userxattr,metacopy=on\n");
> >>                 return -EINVAL;
> >>         }
> >> }
> >>
> >
> > Right, my bad.
> >
> >> to me it looks like it was done on purpose to prevent metacopy from a
> >> user namespace, but I don't know the reason for sure.
> >>
> >
> > With hand crafted metacopy, an unpriv user can chmod
> > any files to anything by layering another file with different
> > mode on top of it....
>
> I might be missing something obvious about metacopy, so please correct
> me if I am wrong, but I don't see how it is any different than just
> copying the file and chowning it.  Of course, as long as overlay uses
> the same security model so that a file that wasn't originally possible
> to access must be still blocked, even if referenced through metacopy.
>

You're right.
The reason for mutual exclusion maybe related to the
comment in ovl_check_metacopy_xattr() about EACCES.
Need to check with Vivek or Miklos.

But get this - you do not need metacopy=on to follow lower inode.
It should work without metacopy=on.
metacopy=on only instructs overlayfs whether to copy up data
or only metadata when changing metadata of lower object, so it is
not relevant for readonly mount.

Thanks,
Amir.