linux-kernel - Re: [PATCH 12/13] ovl: switch to __inode

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Date:	Wed, 15 Aug 2012 18:07:58 +0100
From:	Andy Whitcroft <apw@...onical.com>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, viro@...IV.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	hch@...radead.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, nbd@...nwrt.org, neilb@...e.de,
	hramrach@...trum.cz, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	ricwheeler@...il.com, dhowells@...hat.com, hpj@...la.net,
	sedat.dilek@...glemail.com, penberg@...nel.org,
	goran.cetusic@...il.com, romain@...bokech.com, mszeredi@...e.cz
Subject: Re: [PATCH 12/13] ovl: switch to __inode_permission()

On Wed, Aug 15, 2012 at 09:59:51AM -0700, Casey Schaufler wrote:
> On 8/15/2012 8:48 AM, Miklos Szeredi wrote:
> > From: Andy Whitcroft <apw@...onical.com>
> >
> > When checking permissions on an overlayfs inode we do not take into
> > account either device cgroup restrictions nor security permissions.
> > This allows a user to mount an overlayfs layer over a restricted device
> > directory and by pass those permissions to open otherwise restricted
> > files.
> 
> Why is this a good idea? Either you're not including enough context
> about the conditions under which this can occur, or you're suggesting
> the introduction of a trivial mechanism for bypassing all file access
> controls. This does not seem right.

It is stating that the unprotected case is how things was before this
patch switches us over to __inode_permisssions.  The patch is closing
the hole indicated.

-apw
> >
> > Switch over to __inode_permissions.
> >
> > Signed-off-by: Andy Whitcroft <apw@...onical.com>
> > Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
> > ---
> >  fs/overlayfs/inode.c |   12 +-----------
> >  1 files changed, 1 insertions(+), 11 deletions(-)
> >
> > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > index e854720..f3a534f 100644
> > --- a/fs/overlayfs/inode.c
> > +++ b/fs/overlayfs/inode.c
> > @@ -100,19 +100,9 @@ int ovl_permission(struct inode *inode, int mask)
> >  		if (is_upper && !IS_RDONLY(inode) && IS_RDONLY(realinode) &&
> >  		    (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
> >  			goto out_dput;
> > -
> > -		/*
> > -		 * Nobody gets write access to an immutable file.
> > -		 */
> > -		err = -EACCES;
> > -		if (IS_IMMUTABLE(realinode))
> > -			goto out_dput;
> >  	}
> >  
> > -	if (realinode->i_op->permission)
> > -		err = realinode->i_op->permission(realinode, mask);
> > -	else
> > -		err = generic_permission(realinode, mask);
> > +	err = __inode_permission(realinode, mask);
> >  out_dput:
> >  	dput(alias);
> >  	return err;
> 
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