proc: restrict access to /proc/PID/io

/proc/PID/io may be used for gathering private information.  E.g.  for
openssh and vsftpd daemons wchars/rchars may be used to learn the
precise password length.  Restrict it to processes being able to ptrace
the target process.

ptrace_may_access() is needed to prevent keeping open file descriptor of
"io" file, executing setuid binary and gathering io information of the
setuid'ed process.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static void *proc_self_follow_link(struct dentry *dentry, struct nameidata *nd)
{
	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
	pid_t tgid = task_tgid_nr_ns(current, ns);
	char *name = ERR_PTR(-ENOENT);
	if (tgid) {
		name = __getname();
		if (!name)
			name = ERR_PTR(-ENOMEM);
		else
			sprintf(name, "%d", tgid);
	}
	nd_set_link(nd, name);
	return NULL;
}

static void *proc_self_follow_link(struct dentry *dentry, struct nameidata *nd)
{
	struct pid_namespace *ns = dentry->d_sb->s_fs_info;
	pid_t tgid = task_tgid_nr_ns(current, ns);
	char *name = ERR_PTR(-ENOENT);
	if (tgid) {
		name = __getname();
		if (!name)
			name = ERR_PTR(-ENOMEM);
		else
			sprintf(name, "%d", tgid);
	}
	nd_set_link(nd, name);
	return NULL;
}
