linux-kernel - Re: file capabilities: clear fcaps on inode change (v3)

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Message-ID: <20070807231452.GA27198@vino.hallyn.com>
Date:	Tue, 7 Aug 2007 18:14:52 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	"Serge E. Hallyn" <sergeh@...ibm.com>,
	Andrew Morgan <morgan@...nel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Andrew Morgan <agm@...gle.com>, casey@...aufler-ca.com,
	Andrew Morton <akpm@...gle.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	KaiGai Kohei <kaigai@...gai.gr.jp>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: file capabilities: clear fcaps on inode change (v3)

Quoting Trond Myklebust (trond.myklebust@....uio.no):
> On Tue, 2007-08-07 at 17:17 -0500, Serge E. Hallyn wrote:
> 
> > diff --git a/fs/splice.c b/fs/splice.c
> > index e36c003..2df95f3 100644
> > --- a/fs/splice.c
> > +++ b/fs/splice.c
> > @@ -827,6 +827,12 @@ generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
> >  	ssize_t ret;
> >  	int err;
> >  
> > +	mutex_lock(&inode->i_mutex);
> > +	err = security_inode_killpriv(out->f_path.dentry);
> > +	mutex_unlock(&inode->i_mutex);
> > +	if (err)
> > +		return err;
> 
> You are unconditionally taking the i_mutex whether or not you actually
> have any capabilities to remove. Normally, removing capabilities due to
> a write is something which occurs once every blue moon. Can't you
> introduce a heuristic along the lines of should_remove_suid() in order
> to optimise away the common case?

Yeah, I did that in v1, but didn't want to add two new security_ hooks.
But I'll send a v4 doing that.

> In addition, if you need to remove both the capabilities and the suid
> bits, then it should be unnecessary to take the i_mutex twice.

Good point, I'll consolidate those.

> > +
> >  	err = should_remove_suid(out->f_path.dentry);
> >  	if (unlikely(err)) {
> >  		mutex_lock(&inode->i_mutex);
> 
> Trond

thanks,
-serge
-
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