Commit message: Don't crash if crypt() fails.

It can fail, so make sure it doesn't before comparing its result with
the password.

This addresses Include Security issue F12: [libpcap] Remote Packet
Capture Daemon Null Pointer Dereference Denial of Service.
CVE: CVE-2019-15163
CWE: CWE-476
Vulnerability description: rpcapd/daemon.c in libpcap before 1.9.1 allows attackers to cause a denial of service (NULL pointer dereference and daemon crash) if a crypt() call fails.

Project: libpcap

daemon_AuthUserPwd(char *username, char *password, char *errbuf)
{
#ifdef _WIN32
	/*
	 * Warning: the user which launches the process must have the
	 * SE_TCB_NAME right.
	 * This corresponds to have the "Act as part of the Operating System"
	 * turned on (administrative tools, local security settings, local
	 * policies, user right assignment)
	 * However, it seems to me that if you run it as a service, this
	 * right should be provided by default.
	 *
	 * XXX - hopefully, this returns errors such as ERROR_LOGON_FAILURE,
	 * which merely indicates that the user name or password is
	 * incorrect, not whether it's the user name or the password
	 * that's incorrect, so a client that's trying to brute-force
	 * accounts doesn't know whether it's the user name or the
	 * password that's incorrect, so it doesn't know whether to
	 * stop trying to log in with a given user name and move on
	 * to another user name.
	 */
	HANDLE Token;
	if (LogonUser(username, ".", password, LOGON32_LOGON_NETWORK, LOGON32_PROVIDER_DEFAULT, &Token) == 0)
	{
		pcap_fmt_errmsg_for_win32_err(errbuf, PCAP_ERRBUF_SIZE,
		    GetLastError(), "LogonUser() failed");
		return -1;
	}

	if (ImpersonateLoggedOnUser(Token) == 0)
	{
		pcap_fmt_errmsg_for_win32_err(errbuf, PCAP_ERRBUF_SIZE,
		    GetLastError(), "ImpersonateLoggedOnUser() failed");
		CloseHandle(Token);
		return -1;
	}

	CloseHandle(Token);
	return 0;

#else
	/*
	 * See
	 *
	 *	http://www.unixpapa.com/incnote/passwd.html
	 *
	 * We use the Solaris/Linux shadow password authentication if
	 * we have getspnam(), otherwise we just do traditional
	 * authentication, which, on some platforms, might work, even
	 * with shadow passwords, if we're running as root.  Traditional
	 * authenticaion won't work if we're not running as root, as
	 * I think these days all UN*Xes either won't return the password
	 * at all with getpwnam() or will only do so if you're root.
	 *
	 * XXX - perhaps what we *should* be using is PAM, if we have
	 * it.  That might hide all the details of username/password
	 * authentication, whether it's done with a visible-to-root-
	 * only password database or some other authentication mechanism,
	 * behind its API.
	 */
	struct passwd *user;
	char *user_password;
 #ifdef HAVE_GETSPNAM
 	struct spwd *usersp;
 #endif
	char *crypt_password;
 
 	if ((user = getpwnam(username)) == NULL)
	{
		pcap_snprintf(errbuf, PCAP_ERRBUF_SIZE, "Authentication failed: user name or password incorrect");
		return -1;
	}

#ifdef HAVE_GETSPNAM
	if ((usersp = getspnam(username)) == NULL)
	{
		pcap_snprintf(errbuf, PCAP_ERRBUF_SIZE, "Authentication failed: user name or password incorrect");
		return -1;
	}
	user_password = usersp->sp_pwdp;
#else
	/*
	 * XXX - what about other platforms?
	 * The unixpapa.com page claims this Just Works on *BSD if you're
	 * running as root - it's from 2000, so it doesn't indicate whether
	 * macOS (which didn't come out until 2001, under the name Mac OS
	 * X) behaves like the *BSDs or not, and might also work on AIX.
	 * HP-UX does something else.
	 *
	 * Again, hopefully PAM hides all that.
	 */
 	user_password = user->pw_passwd;
 #endif
 
	crypt_password = crypt(password, user_password);
	if (crypt_password == NULL)
	{
		pcap_snprintf(errbuf, PCAP_ERRBUF_SIZE, "Authentication failed");
		return -1;
	}
	if (strcmp(user_password, crypt_password) != 0)
 	{
 		pcap_snprintf(errbuf, PCAP_ERRBUF_SIZE, "Authentication failed: user name or password incorrect");
 		return -1;
	}

	if (setuid(user->pw_uid))
	{
		pcap_fmt_errmsg_for_errno(errbuf, PCAP_ERRBUF_SIZE,
		    errno, "setuid");
		return -1;
	}

/*	if (setgid(user->pw_gid))
	{
		pcap_fmt_errmsg_for_errno(errbuf, PCAP_ERRBUF_SIZE,
		    errno, "setgid");
		return -1;
	}
*/
	return 0;

#endif

}