linux-kernel - Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

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Message-ID: <1486485440.2488.15.camel@HansenPartnership.com>
Date:   Tue, 07 Feb 2017 08:37:20 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Djalal Harouni <tixxdz@...il.com>, Chris Mason <clm@...com>,
        Theodore Tso <tytso@....edu>,
        Josh Triplett <josh@...htriplett.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Seth Forshee <seth.forshee@...onical.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Dongsu Park <dongsu@...ocode.com>,
        David Herrmann <dh.herrmann@...glemail.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Alban Crequy <alban.crequy@...il.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        "Serge E. Hallyn" <serge@...lyn.com>, Phil Estes <estesp@...il.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

On Tue, 2017-02-07 at 01:19 -0800, Christoph Hellwig wrote:
> On Sat, Feb 04, 2017 at 11:19:32AM -0800, James Bottomley wrote:
> > This allows any subtree to be uid/gid shifted and bound elsewhere. 
> >  It does this by operating simlarly to overlayfs.  Its primary use 
> > is for shifting the underlying uids of filesystems used to support
> > unpriviliged (uid shifted) containers.  The usual use case here is
> > that the container is operating with an uid shifted unprivileged 
> > root but sometimes needs to make use of or work with a filesystem 
> > image that has root at real uid 0.
> > 
> > The mechanism is to allow any subordinate mount namespace to mount 
> > a shiftfs filesystem (by marking it FS_USERNS_MOUNT) but only
> > allowing it to mount marked subtrees (using the -o mark option as 
> > root).  Once mounted, the subtree is mapped via the super block 
> > user namespace so that the interior ids of the mounting user 
> > namespace are the ids written to the filesystem.
> 
> Please move this into VFS instead of a stackable fs.  We might need
> addtional parameters to getattr/setattr to specify the ID 
> translation, but that's why better than a horrible hack like this.

I would need a lot more than that: getattr controls the cosmetic
permission display to the user, but enforcement is done in the core
permission checks which are inode based.  To make this a real bind
mount, the core permission checks will have to become subtree aware
because knowledge of whether we need a uid shift in the permission
check becomes a subtree property.  Effectively inode_permission would
become dentry_permission and generic_permission would take a dentry
instead of an inode.  This will be a huge amount of VFS and underlying
filesystem churn, since the permissions calls are threaded through a
huge chunk of code.

Is this the approach that you really want?

I suppose I could see the security people linking it because all the
security hooks in the permission code become path aware.

James