Commit message: audit: fix a double fetch in audit_log_single_execve_arg()

There is a double fetch problem in audit_log_single_execve_arg()
where we first check the execve(2) argumnets for any "bad" characters
which would require hex encoding and then re-fetch the arguments for
logging in the audit record[1].  Of course this leaves a window of
opportunity for an unsavory application to munge with the data.

This patch reworks things by only fetching the argument data once[2]
into a buffer where it is scanned and logged into the audit
records(s).  In addition to fixing the double fetch, this patch
improves on the original code in a few other ways: better handling
of large arguments which require encoding, stricter record length
checking, and some performance improvements (completely unverified,
but we got rid of some strlen() calls, that's got to be a good
thing).

As part of the development of this patch, I've also created a basic
regression test for the audit-testsuite, the test can be tracked on
GitHub at the following link:

 * https://github.com/linux-audit/audit-testsuite/issues/25

[1] If you pay careful attention, there is actually a triple fetch
problem due to a strnlen_user() call at the top of the function.

[2] This is a tiny white lie, we do make a call to strnlen_user()
prior to fetching the argument data.  I don't like it, but due to the
way the audit record is structured we really have no choice unless we
copy the entire argument at once (which would require a rather
wasteful allocation).  The good news is that with this patch the
kernel no longer relies on this strnlen_user() value for anything
beyond recording it in the log, we also update it with a trustworthy
value whenever possible.

Reported-by: Pengfei Wang <wpengfeinudt@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>
CVE: CVE-2016-6136
CWE: CWE-362
Vulnerability description: Race condition in the audit_log_single_execve_arg function in kernel/auditsc.c in the Linux kernel through 4.7 allows local users to bypass intended character-set restrictions or disrupt system-call auditing by changing a certain string, aka a "double fetch" vulnerability.

Project: linux
File: auditsc.c

static int audit_log_single_execve_arg(struct audit_context *context,
					struct audit_buffer **ab,
					int arg_num,
					size_t *len_sent,
					const char __user *p,
					char *buf)
 {
	char arg_num_len_buf[12];
	const char __user *tmp_p = p;
	/* how many digits are in arg_num? 5 is the length of ' a=""' */
	size_t arg_num_len = snprintf(arg_num_len_buf, 12, "%d", arg_num) + 5;
	size_t len, len_left, to_send;
	size_t max_execve_audit_len = MAX_EXECVE_AUDIT_LEN;
	unsigned int i, has_cntl = 0, too_long = 0;
	int ret;
	/* strnlen_user includes the null we don't want to send */
	len_left = len = strnlen_user(p, MAX_ARG_STRLEN) - 1;
	/*
	 * We just created this mm, if we can't find the strings
	 * we just copied into it something is _very_ wrong. Similar
	 * for strings that are too long, we should not have created
	 * any.
	 */
	if (WARN_ON_ONCE(len < 0 || len > MAX_ARG_STRLEN - 1)) {
		send_sig(SIGKILL, current, 0);
		return -1;
 	}
 
	/* walk the whole argument looking for non-ascii chars */
 	do {
		if (len_left > MAX_EXECVE_AUDIT_LEN)
			to_send = MAX_EXECVE_AUDIT_LEN;
		else
			to_send = len_left;
		ret = copy_from_user(buf, tmp_p, to_send);
		/*
		 * There is no reason for this copy to be short. We just
		 * copied them here, and the mm hasn't been exposed to user-
		 * space yet.
		 */
		if (ret) {
			WARN_ON(1);
			send_sig(SIGKILL, current, 0);
			return -1;
		}
		buf[to_send] = '\0';
		has_cntl = audit_string_contains_control(buf, to_send);
		if (has_cntl) {
			/*
			 * hex messages get logged as 2 bytes, so we can only
			 * send half as much in each message
			 */
			max_execve_audit_len = MAX_EXECVE_AUDIT_LEN / 2;
			break;
		}
		len_left -= to_send;
		tmp_p += to_send;
	} while (len_left > 0);
	len_left = len;
	if (len > max_execve_audit_len)
		too_long = 1;
	/* rewalk the argument actually logging the message */
	for (i = 0; len_left > 0; i++) {
		int room_left;
		if (len_left > max_execve_audit_len)
			to_send = max_execve_audit_len;
		else
			to_send = len_left;
		/* do we have space left to send this argument in this ab? */
		room_left = MAX_EXECVE_AUDIT_LEN - arg_num_len - *len_sent;
		if (has_cntl)
			room_left -= (to_send * 2);
		else
			room_left -= to_send;
		if (room_left < 0) {
			*len_sent = 0;
			audit_log_end(*ab);
			*ab = audit_log_start(context, GFP_KERNEL, AUDIT_EXECVE);
			if (!*ab)
				return 0;
		}
 
		/*
		 * first record needs to say how long the original string was
		 * so we can be sure nothing was lost.
		 */
		if ((i == 0) && (too_long))
			audit_log_format(*ab, " a%d_len=%zu", arg_num,
					 has_cntl ? 2*len : len);
		/*
		 * normally arguments are small enough to fit and we already
		 * filled buf above when we checked for control characters
		 * so don't bother with another copy_from_user
		 */
		if (len >= max_execve_audit_len)
			ret = copy_from_user(buf, p, to_send);
		else
			ret = 0;
		if (ret) {
			WARN_ON(1);
			send_sig(SIGKILL, current, 0);
			return -1;
 		}
		buf[to_send] = '\0';
		/* actually log it */
		audit_log_format(*ab, " a%d", arg_num);
		if (too_long)
			audit_log_format(*ab, "[%d]", i);
		audit_log_format(*ab, "=");
		if (has_cntl)
			audit_log_n_hex(*ab, buf, to_send);
		else
			audit_log_string(*ab, buf);
		p += to_send;
		len_left -= to_send;
		*len_sent += arg_num_len;
		if (has_cntl)
			*len_sent += to_send * 2;
		else
			*len_sent += to_send;
	}
	/* include the null we didn't log */
	return len + 1;
}