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

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Date:	Tue, 16 Aug 2016 20:18:40 +0200
From:	Jann Horn <jann@...jh.net>
To:	robert.foss@...labora.com
Cc:	corbet@....net, akpm@...ux-foundation.org, vbabka@...e.cz,
	mhocko@...e.com, koct9i@...il.com, hughd@...gle.com,
	n-horiguchi@...jp.nec.com, john.stultz@...aro.org,
	minchan@...nel.org, ross.zwisler@...ux.intel.com,
	jmarchan@...hat.com, hannes@...xchg.org, mingo@...nel.org,
	keescook@...omium.org, viro@...iv.linux.org.uk,
	gorcunov@...nvz.org, sonnyrao@...omium.org,
	plaguedbypenguins@...il.com, eric.engestrom@...tec.com,
	rientjes@...gle.com, jdanis@...gle.com, calvinowens@...com,
	adobriyan@...il.com, kirill.shutemov@...ux.intel.com,
	ldufour@...ux.vnet.ibm.com, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ben Zhang <benzh@...omium.org>,
	Bryan Freed <bfreed@...omium.org>,
	Filipe Brandenburger <filbranden@...omium.org>,
	Mateusz Guzik <mguzik@...hat.com>,
	Michal Hocko <mhocko@...nel.org>, linux-api@...r.kernel.org
Subject: Re: [PACTH v3 1/3] mm, proc: Implement /proc/<pid>/totmaps

On Tue, Aug 16, 2016 at 01:34:14PM -0400, robert.foss@...labora.com wrote:
> From: Robert Foss <robert.foss@...labora.com>
> 
> 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@...labora.com>
> Signed-off-by: Robert Foss <robert.foss@...labora.com>
> 
> Signed-off-by: Sonny Rao <sonnyrao@...omium.org>
> ---
[...]
> +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.


> +	}
> +
> +	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).

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