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

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Date:   Fri, 17 Feb 2017 02:29:18 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
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>,
        "Serge E. Hallyn" <serge@...lyn.com>, Phil Estes <estesp@...il.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

On Sat, Feb 04, 2017 at 11:19:32AM -0800, James Bottomley wrote:

> +static const struct dentry_operations shiftfs_dentry_ops = {
> +	.d_release	= shiftfs_d_release,
> +	.d_real		= shiftfs_d_real,
> +};

In other words, those dentries are *never* revalidated.  Nevermind that
underlying fs might be mounted elsewhere and be actively modified under
you.

> +static struct dentry *shiftfs_lookup(struct inode *dir, struct dentry *dentry,
> +				     unsigned int flags)
> +{
> +	struct dentry *real = dir->i_private, *new;
> +	struct inode *reali = real->d_inode, *newi;
> +	const struct cred *oldcred, *newcred;
> +
> +	inode_lock(reali);
> +	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
> +	new = lookup_one_len(dentry->d_name.name, real, dentry->d_name.len);
> +	shiftfs_old_creds(oldcred, &newcred);
> +	inode_unlock(reali);
> +
> +	if (IS_ERR(new))
> +		return new;
> +
> +	dentry->d_fsdata = new;
> +
> +	if (!new->d_inode)
> +		return NULL;

What happens when somebody comes along and creates the damn thing on the
underlying fs?  _Not_ via your code, that is - using the underlying fs
mounted elsewhere.