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Message-ID: <1432006160.2870.138.camel@perches.com>
Subject: Re: [PATCH v5] procfs: Always expose /proc/<pid>/map_files/ and
 make it readable
From: Joe Perches <joe@perches.com>
To: Calvin Owens <calvinowens@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Miklos Szeredi <miklos@szeredi.hu>,
        Zefan Li <lizefan@huawei.com>, Oleg Nesterov <oleg@redhat.com>,
        David Howells <dhowells@redhat.com>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, Andy Lutomirski <luto@amacapital.net>,
        Kees Cook <keescook@chromium.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Date: Mon, 18 May 2015 20:29:20 -0700
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References: <20150214204009.GA1763278@mail.thefacebook.com>
	 <1432005006-3428-1-git-send-email-calvinowens@fb.com>
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On Mon, 2015-05-18 at 20:10 -0700, Calvin Owens wrote:
> Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> is only exposed if CONFIG_CHECKPOINT_RESTORE is set. This interface is
> very useful for enumerating the files mapped into a process when the
> more verbose information in /proc/<pid>/maps is not needed. It also
> allows access to file descriptors for files that have been deleted and
> closed but are still mmapped into a process, which can be very useful
> for introspection and debugging.

style trivia:

> diff --git a/fs/proc/base.c b/fs/proc/base.c
[]
> +/*
> + * Enforce stronger PTRACE_MODE_ATTACH permissions on the symlinks under
> + * /proc/<pid>/map_files, since these links may refer to deleted or O_TMPFILE
> + * files that users might assume are inaccessible regardless of their
> + * ownership/permissions.
> + */
> +static void *proc_map_files_follow_link(struct dentry *dentry, struct nameidata *nd)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct task_struct *task;
> +	int allowed = 0;
> +
> +	task = get_proc_task(inode);
> +	if (task) {
> +		allowed = ptrace_may_access(task, PTRACE_MODE_ATTACH);
> +		put_task_struct(task);
> +	} else {
> +		return ERR_PTR(-ESRCH);
> +	}
> +
> +	if (!allowed)
> +		return ERR_PTR(-EACCES);
> +
> +	return proc_pid_follow_link(dentry, nd);
> +}

It'd perhaps be clearer to read this with an
immediate return after a failure in get_proc_task.

Maybe something like (move initializations as desired):

static void *proc_map_files_follow_link(struct dentry *dentry, struct nameidata *nd)
{
	int allowed;
	struct iode *inode = d_inode(dentry);
	struct task_struct task = get_proc_task(inode);

	if (!task)
		return ERR_PTR(-ESRCH);

	allowed = ptrace_may_access(task, PTRACE_MODE_ATTACH);

	put_task_struct(task);

	if (!allowed)
		return ERR_PTR(-EACCES);

	return proc_pic_follow_link(dentry, nd);
}