linux-kernel - Re: [PATCH 1/7] security, overlayfs: provide copy up security hook for unioned files

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Date:	Mon, 11 Jul 2016 12:54:23 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Stephen Smalley <sds@...ho.nsa.gov>
Cc:	miklos@...redi.hu, pmoore@...hat.com, casey@...aufler-ca.com,
	linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org,
	linux-security-module@...r.kernel.org, dwalsh@...hat.com,
	dhowells@...hat.com, viro@...IV.linux.org.uk,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/7] security, overlayfs: provide copy up security hook
 for unioned files

On Mon, Jul 11, 2016 at 11:24:26AM -0400, Stephen Smalley wrote:
> On 07/08/2016 12:19 PM, Vivek Goyal wrote:
> > Provide a security hook to label new file correctly when a file is copied
> > up from lower layer to upper layer of a overlay/union mount.
> > 
> > This hook can prepare a new set of creds which are suitable for new file
> > creation during copy up. Caller will use new creds to create file and then
> > revert back to old creds and release new creds.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> > ---
> >  fs/overlayfs/copy_up.c    | 18 ++++++++++++++++++
> >  include/linux/lsm_hooks.h | 11 +++++++++++
> >  include/linux/security.h  |  6 ++++++
> >  security/security.c       |  8 ++++++++
> >  4 files changed, 43 insertions(+)
> > 
> > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > index 80aa6f1..8ebea18 100644
> > --- a/fs/overlayfs/copy_up.c
> > +++ b/fs/overlayfs/copy_up.c
> > @@ -246,6 +246,8 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
> >  	struct dentry *upper = NULL;
> >  	umode_t mode = stat->mode;
> >  	int err;
> > +	const struct cred *old_creds = NULL;
> > +	struct cred *new_creds = NULL;
> >  
> >  	newdentry = ovl_lookup_temp(workdir, dentry);
> >  	err = PTR_ERR(newdentry);
> > @@ -258,10 +260,26 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
> >  	if (IS_ERR(upper))
> >  		goto out1;
> >  
> > +	err = security_inode_copy_up(dentry, &new_creds);
> > +	if (err < 0) {
> > +		if (new_creds)
> > +			put_cred(new_creds);
> 
> Why do we need a put_cred() here?

Being paranoid for the case of stacked modules. Say first module allocated
creds but second module returned error, in that case creds will have to
be freed.

I can get rid of it for now and if in future two LSMs implement this hook,
one can change it, if need be.

Thanks
Vivek