linux-kernel - Re: [PACTH v2 1/3] mm, proc: Implement /proc/<pid>/totmaps

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Date:	Sat, 13 Aug 2016 16:39:44 +0200
From:	Jann Horn <jann@...jh.net>
To:	robert.foss@...labora.com
Cc:	corbet@....net, akpm@...ux-foundation.org, vbabka@...e.cz,
	koct9i@...il.com, mhocko@...e.com, hughd@...gle.com,
	n-horiguchi@...jp.nec.com, minchan@...nel.org,
	john.stultz@...aro.org, ross.zwisler@...ux.intel.com,
	jmarchan@...hat.com, hannes@...xchg.org, keescook@...omium.org,
	viro@...iv.linux.org.uk, gorcunov@...nvz.org,
	plaguedbypenguins@...il.com, rientjes@...gle.com,
	eric.engestrom@...tec.com, jdanis@...gle.com, calvinowens@...com,
	adobriyan@...il.com, sonnyrao@...omium.org,
	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>
Subject: Re: [PACTH v2 1/3] mm, proc: Implement /proc/<pid>/totmaps

On Fri, Aug 12, 2016 at 06:04:20PM -0400, robert.foss@...labora.com wrote:
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index aa27810..c55e1fe 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -281,6 +281,7 @@ struct proc_maps_private {
>  	struct mm_struct *mm;
>  #ifdef CONFIG_MMU
>  	struct vm_area_struct *tail_vma;
> +	struct mem_size_stats *mss;

This is unused now, right?

> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 4648c7f..b7612e9 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -246,6 +246,9 @@ static int proc_map_release(struct inode *inode, struct file *file)
>  	struct seq_file *seq = file->private_data;
>  	struct proc_maps_private *priv = seq->private;
>  
> +	if (!priv)
> +		return 0;
> +

You might want to get rid of this, see below.

> +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;
[...]
> +error:
> +	proc_map_release(inode, file);
> +	return ret;

I don't think this is correct. Have a look at the other callers of
do_maps_open() - none of them do any cleanup steps on error, they
just return. I think the "goto error" here should be a return
instead.

Have a look at the error cases that can cause do_maps_open() to
fail: do_maps_open() just calls proc_maps_open(). If the
__seq_open_private() call fails because of memory pressure,
file->private_data is still NULL, and your newly added NULL check
in proc_map_release() causes proc_map_release() to be a no-op
there. But if proc_maps_open() fails later on, things get nasty:
If, for example, proc_mem_open() fails because of a ptrace
permission denial, __seq_open_file -> seq_open has already set
file->private_data to a struct seq_file *, and then
proc_maps_open(), prior to passing on the error code, calls
seq_release_private -> seq_release, which frees that
struct seq_file * without NULLing the private_data pointer.
As far as I can tell, proc_map_release() would then run into
a use-after-free scenario.

> +	priv->task = get_proc_task(inode);
> +	if (!priv->task) {
> +		ret = -ESRCH;
> +		goto error;
> +	}

You're not actually using ->task anywhere in the current version,
right? Can this be deleted?

> +const struct file_operations proc_totmaps_operations = {
[...]
> +	.release	= proc_map_release,

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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