linux-kernel - Re: [RFC v2 1/1] shiftfs: uid/gid shifting bind mount

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Date:   Wed, 22 Feb 2017 11:41:18 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
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>,
        Al Viro <viro@...IV.linux.org.uk>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Phil Estes <estesp@...il.com>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [RFC v2 1/1] shiftfs: uid/gid shifting bind mount

On Tue, Feb 21, 2017 at 05:23:26PM -0500, James Bottomley wrote:
> On Tue, 2017-02-21 at 11:06 -0500, Vivek Goyal wrote:
> > On Mon, Feb 20, 2017 at 04:47:05PM -0800, James Bottomley wrote:
> > 
> > [..]
> > > +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;
> > 
> > newi needs to be initialized to NULL. Otherwise if a file does not
> > exist, we try to call d_splice_alias() with uninitialized value of
> > newi.
> > 
> > This crashes for the new file creation case.
> 
> I think you've still got an older version.  The code around that region
> should read:
> 
> 	
> 
> 	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;
> 
> 	newi = NULL;
> 	if (!new->d_inode)
> 		goto out;
> 
> 	newi = shiftfs_new_inode(dentry->d_sb, new->d_inode->i_mode, new);
> 	if (!newi) {
> 		dput(new);
> 		return ERR_PTR(-ENOMEM);
> 	}
> 
>  out:
> 	return d_splice_alias(newi, dentry);
> 
> So newi is set to NULL before out is taken.  I checked: this hunk did make it to the v2 patch.

Aha.., you are right. I did not pay attention of newi=NULL in V2 and went
on to provide feeback based on my V1 testing.

Vivek