Re: [PACTH v3 1/3] mm, proc: Implement /proc/<pid>/totmaps

[Robert Foss]

On 2016-08-16 02:18 PM, Jann Horn wrote:
> On Tue, Aug 16, 2016 at 01:34:14PM -0400, robert.foss@xxxxxxxxxxxxx wrote:
> > From: Robert Foss <robert.foss@xxxxxxxxxxxxx>
> >
> > This is based on earlier work by Thiago Goncales. It implements a new
> > per process proc file which summarizes the contents of the smaps file
> > but doesn't display any addresses.  It gives more detailed information
> > than statm like the PSS (proprotional set size).  It differs from the
> > original implementation in that it doesn't use the full blown set of
> > seq operations, uses a different termination condition, and doesn't
> > displayed "Locked" as that was broken on the original implemenation.
> >
> > This new proc file provides information faster than parsing the potentially
> > huge smaps file.
> >
> > Tested-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
> > Signed-off-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
> >
> > Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> > ---
> [...]
> > +static int totmaps_open(struct inode *inode, struct file *file)
> > +{
> > +	struct proc_maps_private *priv = NULL;
> > +	struct seq_file *seq;
> > +	int ret;
> > +
> > +	ret = do_maps_open(inode, file, &proc_totmaps_op);
> > +	if (ret)
> > +		goto error;
> > +
> > +	/*
> > +	 * We need to grab references to the task_struct
> > +	 * at open time, because there's a potential information
> > +	 * leak where the totmaps file is opened and held open
> > +	 * while the underlying pid to task mapping changes
> > +	 * underneath it
> > +	 */
> > +	seq = file->private_data;
> > +	priv = seq->private;
> > +	priv->task = get_proc_task(inode);
> > +	if (!priv->task) {
> > +		ret = -ESRCH;
> > +		goto error;
>
> I see that you removed the proc_map_release() call for the upper
> error case as I recommended. However, for the second error case,
> you do have to call it because do_maps_open() succeeded.
>
> You could fix this by turning the first "goto error;" into
> "return;" and adding the proc_map_release() call back in after
> the "error:" label. This would be fine - if an error branch just
> needs to return an error code, it's okay to do so directly
> without jumping to an error label.
>
> Alternatively, you could add a second label
> in front of the existing "error:" label, jump to the new label
> for the second error case, and call proc_map_release() between
> the new label and the old one.

Ah, naturally. Thanks for the patience and advice!

> > +	}
> > +
> > +	return 0;
> > +
> > +error:
> > +	return ret;
> > +}
> > +
> [...]
> > +const struct file_operations proc_totmaps_operations = {
> > +	.open		= totmaps_open,
> > +	.read		= seq_read,
> > +	.llseek		= seq_lseek,
> > +	.release	= proc_map_release,
> > +};
>
> As I said regarding v2 already:
> This won't release priv->task, causing a memory leak (exploitable
> through a reference counter overflow of the task_struct usage
> counter).

Sorry about dropping the ball on that one, what's correct way to release 
priv->task?