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

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Message-ID: <f8b3bfc6-dbee-dbd1-482b-cfce2c5766d3@collabora.com>
Date:	Tue, 16 Aug 2016 14:34:15 -0400
From:	Robert Foss <robert.foss@...labora.com>
To:	Jann Horn <jann@...jh.net>
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 2016-08-16 02:18 PM, Jann Horn wrote:
> 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.

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?