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

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Date:	Tue, 07 Aug 2007 18:38:05 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	"Serge E. Hallyn" <sergeh@...ibm.com>
Cc:	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)

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?

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.

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

Trond

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