[RFC PATCH 6/6] shiftfs: support nested shiftfs mounts

Amir Goldstein amir73il at gmail.com
Fri Nov 2 13:16:05 UTC 2018

On Fri, Nov 2, 2018 at 2:44 PM Seth Forshee <seth.forshee at canonical.com> wrote:
> On Fri, Nov 02, 2018 at 12:02:45PM +0200, Amir Goldstein wrote:
> > On Thu, Nov 1, 2018 at 11:49 PM Seth Forshee <seth.forshee at canonical.com> wrote:
> > >
> > > shiftfs mounts cannot be nested for two reasons -- global
> > > CAP_SYS_ADMIN is required to set up a mark mount, and a single
> > > functional shiftfs mount meets the filesystem stacking depth
> > > limit.
> > >
> > > The CAP_SYS_ADMIN requirement can be relaxed. All of the kernel
> > > ids in a mount must be within that mount's s_user_ns, so all that
> > > is needed is CAP_SYS_ADMIN within that s_user_ns.
> > >
> > > The stack depth issue can be worked around with a couple of
> > > adjustments. First, a mark mount doesn't really need to count
> > > against the stacking depth as it doesn't contribute to the call
> > > stack depth during filesystem operations. Therefore the mount
> > > over the mark mount only needs to count as one more than the
> > > lower filesystems stack depth.
> >
> > That's true, but it also highlights the point that the "mark" sb is
> > completely unneeded and it really is just a nice little hack.
> > All the information it really stores is a lower mount reference,
> > a lower root dentry and a declarative statement "I am shiftable!".
> Seems I should have saved some of the things I said in my previous
> response for this one. As you no doubt gleaned from that email, I agree
> with this.
> > Come to think about it, "container shiftable" really isn't that different from
> > NFS export with "no_root_squash" and auto mounted USB drive.
> > I mean the shifting itself is different of course, but the
> > declaration, not so much.
> > If I am allowing sudoers on another machine to mess with root owned
> > files visible
> > on my machine, I am pretty much have the same issues as container admins
> > accessing root owned files on my init_user_ns filesystem. In all those cases,
> > I'd better not be executing suid binaries from the untrusted "external" source.
> >
> > Instead of mounting a dummy filesystem to make the declaration, you could
> > get the same thing with:
> >    mount(path, path, "none", MS_BIND | MS_EXTERN | MS_NOEXEC)
> > and all you need to do is add MS_EXTERN (MS_SHIFTABLE MS_UNTRUSTED
> > or whatnot)  constant to uapi and manage to come up good man page description.
> >
> > Then users could actually mount a filesystem in init_user_ns MS_EXTERN and
> > avoid the extra bind mount step (for a full filesystem tree export).
> > Declaring a mounted image MS_EXTERN has merits on its own even without
> > containers and shitfs, for example with pluggable storage. Other LSMs could make
> > good use of that declaration.
> I'm missing how we figure out the target user ns in this scheme. We need
> a context with privileges towards the source path's s_user_ns to say
> it's okay to shift ids for the files under the source path, and then we
> need a target user ns for the id shifts. Currently the target is
> current_user_ns when the final shiftfs mount is created.
> So, how do we determine the target s_user_ns in your scheme?

Unless I am completely misunderstanding shiftfs, I think we are saying the
same thing. You said you wish to get rid of the "mark" fs and that you had
a POC of implementing the "mark" using xattr.
I'm just saying another option to implement the mark is using a super block
flag and you get the target s_user_ns from mnt_sb.
I did miss the fact that a mount flag is not enough, so that makes the bind
mount concept fail. Unless, maybe, the mount in the container is a slave
mount and the "shiftable" mark is set on the master.
I know too little about mount ns vs. user ns to tell if any of this makes sense.

Feel free to ignore MS_EXTERN idea.

Hopefully, mount API v2 will provide the proper facility to implement mark.


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