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 int proc_pid_auxv(struct task_struct *task, char *buffer)
{
	struct mm_struct *mm = mm_for_maps(task);
	int res = PTR_ERR(mm);
	if (mm && !IS_ERR(mm)) {
		unsigned int nwords = 0;
		do {
			nwords += 2;
		} while (mm->saved_auxv[nwords - 2] != 0); /* AT_NULL */
		res = nwords * sizeof(mm->saved_auxv[0]);
		if (res > PAGE_SIZE)
			res = PAGE_SIZE;
		memcpy(buffer, mm->saved_auxv, res);
		mmput(mm);
	}
	return res;
}

static int proc_pid_auxv(struct task_struct *task, char *buffer)
{
	struct mm_struct *mm = mm_for_maps(task);
	int res = PTR_ERR(mm);
	if (mm && !IS_ERR(mm)) {
		unsigned int nwords = 0;
		do {
			nwords += 2;
		} while (mm->saved_auxv[nwords - 2] != 0); /* AT_NULL */
		res = nwords * sizeof(mm->saved_auxv[0]);
		if (res > PAGE_SIZE)
			res = PAGE_SIZE;
		memcpy(buffer, mm->saved_auxv, res);
		mmput(mm);
	}
	return res;
}
