acl/lib_tls/tls/tls_verify.c

/*++
 * NAME
 *	tls_verify 3
 * SUMMARY
 *	peer name and peer certificate verification
 * SYNOPSIS
 *	#include <tls.h>
 *	#include <tls_private.h>
 *
 *	char *tls_peer_CN(peercert, TLScontext)
 *	X509   *peercert;
 *	TLS_SESS_STATE *TLScontext;
 *
 *	char *tls_issuer_CN(peercert, TLScontext)
 *	X509   *peercert;
 *	TLS_SESS_STATE *TLScontext;
 *
 *	const char *tls_dns_name(gn, TLScontext)
 *	const GENERAL_NAME *gn;
 *	TLS_SESS_STATE *TLScontext;
 *
 *	char *tls_fingerprint(peercert, dgst)
 *	X509   *peercert;
 *	const char *dgst;
 *
 *	int	tls_verify_certificate_callback(ok, ctx)
 *	int	ok;
 *	X509_STORE_CTX *ctx;
 * DESCRIPTION
 *	tls_peer_CN() returns the text CommonName for the peer
 *	certificate subject, or an empty string if no CommonName was
 *	found. The result is allocated with acl_mymalloc() and must be
 *	freed by the caller; it contains UTF-8 without non-printable
 *	ASCII characters.
 *
 *	tls_issuer_CN() returns the text CommonName for the peer
 *	certificate issuer, or an empty string if no CommonName was
 *	found. The result is allocated with acl_mymalloc() and must be
 *	freed by the caller; it contains UTF-8 without non-printable
 *	ASCII characters.
 *
 *	tls_dns_name() returns the string value of a GENERAL_NAME
 *	from a DNS subjectAltName extension. If non-printable characters
 *	are found, a null string is returned instead. Further sanity
 *	checks may be added if the need arises.
 *
 * 	tls_fingerprint() returns a fingerprint of the the given
 *	certificate using the requested message digest. Panics if the
 *	(previously verified) digest algorithm is not found. The return
 *	value is dynamically allocated with acl_mymalloc(), and the caller
 *	must eventually free it with acl_myfree().
 *
 *	tls_verify_callback() is called several times (directly or
 *	indirectly) from crypto/x509/x509_vfy.c. It is called as
 *	a final check, and if it returns "0", the handshake is
 *	immediately shut down and the connection fails.
 *
 *	Postfix/TLS has two modes, the "opportunistic" mode and
 *	the "enforce" mode:
 *
 *	In the "opportunistic" mode we never want the connection
 *	to fail just because there is something wrong with the
 *	peer's certificate. After all, we would have sent or received
 *	the mail even if TLS weren't available.  Therefore the
 *	return value is always "1".
 *
 *	The SMTP client or server may require TLS (e.g. to protect
 *	passwords), while peer certificates are optional.  In this
 *	case we must return "1" even when we are unhappy with the
 *	peer certificate.  Only when peer certificates are required,
 *      certificate verification failure will result in immediate
 *	termination (return 0).
 *
 *	The only error condition not handled inside the OpenSSL
 *	library is the case of a too-long certificate chain. We
 *	test for this condition only if "ok = 1", that is, if
 *	verification didn't fail because of some earlier problem.
 *
 *	Arguments:
 * .IP ok
 *	Result of prior verification: non-zero means success.  In
 *	order to reduce the noise level, some tests or error reports
 *	are disabled when verification failed because of some
 *	earlier problem.
 * .IP ctx
 *	SSL application context. This links to the Postfix TLScontext
 *	with enforcement and logging options.
 * .IP gn
 *	An OpenSSL GENERAL_NAME structure holding a DNS subjectAltName
 *	to be decoded and checked for validity.
 * .IP peercert
 *	Server or client X.509 certificate.
 * .IP dgst
 *	Name of a message digest algorithm suitable for computing secure
 *	(1st pre-image resistant) message digests of certificates. For now,
 *	md5, sha1, or member of SHA-2 family if supported by OpenSSL.
 * .IP TLScontext
 *	Server or client context for warning messages.
 * DIAGNOSTICS
 *	tls_peer_CN(), tls_issuer_CN() and tls_dns_name() log a warning
 *	when 1) the requested information is not available in the specified
 *	certificate, 2) the result exceeds a fixed limit, 3) the result
 *	contains NUL characters or the result contains non-printable or
 *	non-ASCII characters.
 * LICENSE
 * .ad
 * .fi
 *	This software is free. You can do with it whatever you want.
 *	The original author kindly requests that you acknowledge
 *	the use of his software.
 * AUTHOR(S)
 *	Originally written by:
 *	Lutz Jaenicke
 *	BTU Cottbus
 *	Allgemeine Elektrotechnik
 *	Universitaetsplatz 3-4
 *	D-03044 Cottbus, Germany
 *
 *	Updated by:
 *	Wietse Venema
 *	IBM T.J. Watson Research
 *	P.O. Box 704
 *	Yorktown Heights, NY 10598, USA
 *
 *	Victor Duchovni
 *	Morgan Stanley
 *--*/

#include "StdAfx.h"
#include <ctype.h>
#include <limits.h>

#include "../util/stringops.h"

#ifdef USE_TLS
#include <string.h>

/* TLS library. */

#include "tls.h"
#include "tls_private.h"

/* Application-specific. */

static const char hexcodes[] = "0123456789ABCDEF";

/* tls_verify_certificate_callback - verify peer certificate info */

int     tls_verify_certificate_callback(int ok, X509_STORE_CTX *ctx)
{
    char    buf[CCERT_BUFSIZ];
    X509   *cert;
    int     err;
    int     depth;
    SSL    *con;
    TLS_SESS_STATE *TLScontext;

    depth = X509_STORE_CTX_get_error_depth(ctx);
    cert = X509_STORE_CTX_get_current_cert(ctx);
    con = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
    TLScontext = SSL_get_ex_data(con, TLScontext_index);

    /*
     * The callback function is called repeatedly, first with the root
     * certificate, and then with each intermediate certificate ending with
     * the peer certificate.
     * 
     * With each call, the validity of the current certificate (usage bits,
     * attributes, expiration, ... checked by the OpenSSL library) is
     * available in the "ok" argument. Error details are available via
     * X509_STORE_CTX API.
     * 
     * We never terminate the SSL handshake in the verification callback, rather
     * we allow the TLS handshake to continue, but mark the session as
     * unverified. The application is responsible for closing any sessions
     * with unverified credentials.
     * 
     * Certificate chain depth limit violations are mis-reported by the OpenSSL
     * library, from SSL_CTX_set_verify(3):
     * 
     * The certificate verification depth set with SSL[_CTX]_verify_depth()
     * stops the verification at a certain depth. The error message produced
     * will be that of an incomplete certificate chain and not
     * X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.
     * 
     * We set a limit that is one higher than the user requested limit. If this
     * higher limit is reached, we raise an error even a trusted root CA is
     * present at this depth. This disambiguates trust chain truncation from
     * an incomplete trust chain.
     */
    if (depth >= SSL_get_verify_depth(con)) {
	ok = 0;
	X509_STORE_CTX_set_error(ctx, X509_V_ERR_CERT_CHAIN_TOO_LONG);
    }
    if (TLScontext->log_level >= 2) {
	X509_NAME_oneline(X509_get_subject_name(cert), buf, sizeof(buf));
	acl_msg_info("%s: certificate verification depth=%d verify=%d subject=%s",
		 TLScontext->namaddr, depth, ok, printable(buf, '?'));
    }

    /*
     * If no errors, or we are not logging verification errors, we are done.
     */
    if (ok || (TLScontext->peer_status & TLS_CERT_FLAG_LOGGED) != 0)
	return (1);

    /*
     * One counter-example is enough.
     */
    TLScontext->peer_status |= TLS_CERT_FLAG_LOGGED;

#define PURPOSE ((depth>0) ? "CA": TLScontext->am_server ? "client": "server")

    /*
     * Specific causes for verification failure.
     */
    switch (err = X509_STORE_CTX_get_error(ctx)) {
    case X509_V_ERR_DEPTH_ZERO_SELF_SIGNED_CERT:
	acl_msg_info("certificate verification failed for %s: "
		 "self-signed certificate", TLScontext->namaddr);
	break;
    case X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY:
    case X509_V_ERR_SELF_SIGNED_CERT_IN_CHAIN:

	/*
	 * There is no difference between issuing cert not provided and
	 * provided, but not found in CAfile/CApath. Either way, we don't
	 * trust it.
	 */
	X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert),
			  buf, sizeof(buf));
	acl_msg_info("certificate verification failed for %s: untrusted issuer %s",
		 TLScontext->namaddr, printable(buf, '?'));
	break;
    case X509_V_ERR_CERT_NOT_YET_VALID:
    case X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD:
	acl_msg_info("%s certificate verification failed for %s: certificate not"
		 " yet valid", PURPOSE, TLScontext->namaddr);
	break;
    case X509_V_ERR_CERT_HAS_EXPIRED:
    case X509_V_ERR_ERROR_IN_CERT_NOT_AFTER_FIELD:
	acl_msg_info("%s certificate verification failed for %s: certificate has"
		 " expired", PURPOSE, TLScontext->namaddr);
	break;
    case X509_V_ERR_INVALID_PURPOSE:
	acl_msg_info("certificate verification failed for %s: not designated for "
		 "use as a %s certificate", TLScontext->namaddr, PURPOSE);
	break;
    case X509_V_ERR_CERT_CHAIN_TOO_LONG:
	acl_msg_info("certificate verification failed for %s: "
		 "certificate chain longer than limit(%d)",
		 TLScontext->namaddr, SSL_get_verify_depth(con) - 1);
	break;
    default:
	acl_msg_info("%s certificate verification failed for %s: num=%d:%s",
		 PURPOSE, TLScontext->namaddr, err,
		 X509_verify_cert_error_string(err));
	break;
    }

    return (1);
}

#ifndef DONT_GRIPE
#define DONT_GRIPE 0
#define DO_GRIPE 1
#endif

/* tls_text_name - extract certificate property value by name */

static char *tls_text_name(X509_NAME *name, int nid, const char *label,
        const TLS_SESS_STATE *TLScontext, int gripe)
{
    const char *myname = "tls_text_name";
    int     pos;
    X509_NAME_ENTRY *entry;
    ASN1_STRING *entry_str;
    int     asn1_type;
    int     utf8_length;
    unsigned char *utf8_value;
    int     ch;
    unsigned char *cp;

    if (name == 0 || (pos = X509_NAME_get_index_by_NID(name, nid, -1)) < 0) {
	if (gripe != DONT_GRIPE) {
	    acl_msg_warn("%s: %s: peer certificate has no %s",
		     myname, TLScontext->namaddr, label);
	    tls_print_errors();
	}
	return (0);
    }
#if 0

    /*
     * If the match is required unambiguous, insist that that no other values
     * be present.
     */
    if (X509_NAME_get_index_by_NID(name, nid, pos) >= 0) {
	acl_msg_warn("%s: %s: multiple %ss in peer certificate",
		 myname, TLScontext->namaddr, label);
	return (0);
    }
#endif

    if ((entry = X509_NAME_get_entry(name, pos)) == 0) {
	/* This should not happen */
	acl_msg_warn("%s: %s: error reading peer certificate %s entry",
		 myname, TLScontext->namaddr, label);
	tls_print_errors();
	return (0);
    }
    if ((entry_str = X509_NAME_ENTRY_get_data(entry)) == 0) {
	/* This should not happen */
	acl_msg_warn("%s: %s: error reading peer certificate %s data",
		 myname, TLScontext->namaddr, label);
	tls_print_errors();
	return (0);
    }

    /*
     * XXX Convert everything into UTF-8. This is a super-set of ASCII, so we
     * don't have to bother with separate code paths for ASCII-like content.
     * If the payload is ASCII then we won't waste lots of CPU cycles
     * converting it into UTF-8. It's up to OpenSSL to do something
     * reasonable when converting ASCII formats that contain non-ASCII
     * content.
     * 
     * XXX Don't bother optimizing the string length error check. It is not
     * worth the complexity.
     */
    asn1_type = ASN1_STRING_type(entry_str);
    if ((utf8_length = ASN1_STRING_to_UTF8(&utf8_value, entry_str)) < 0) {
	acl_msg_warn("%s: %s: error decoding peer %s of ASN.1 type=%d",
		 myname, TLScontext->namaddr, label, asn1_type);
	tls_print_errors();
	return (0);
    }

    /*
     * No returns without cleaning up. A good optimizer will replace multiple
     * blocks of identical code by jumps to just one such block.
     */
#define TLS_TEXT_NAME_RETURN(x) do { \
	char *__tls_text_name_temp = (x); \
	OPENSSL_free(utf8_value); \
	return (__tls_text_name_temp); \
    } while (0)

    /*
     * Remove trailing null characters. They would give false alarms with the
     * length check and with the embedded null check.
     */
#define TRIM0(s, l) do { while ((l) > 0 && (s)[(l)-1] == 0) --(l); } while (0)

    TRIM0(utf8_value, utf8_length);

    /*
     * Enforce the length limit, because the caller will copy the result into
     * a fixed-length buffer.
     */
    if (utf8_length >= CCERT_BUFSIZ) {
	acl_msg_warn("%s: %s: peer %s too long: %d",
		 myname, TLScontext->namaddr, label, utf8_length);
	TLS_TEXT_NAME_RETURN(0);
    }

    /*
     * Reject embedded nulls in ASCII or UTF-8 names. OpenSSL is responsible
     * for producing properly-formatted UTF-8.
     */
    if (utf8_length != (int) strlen((char *) utf8_value)) {
	acl_msg_warn("%s: %s: NULL character in peer %s",
		 myname, TLScontext->namaddr, label);
	TLS_TEXT_NAME_RETURN(0);
    }

    /*
     * Reject non-printable ASCII characters in UTF-8 content.
     * 
     * Note: the code below does not find control characters in illegal UTF-8
     * sequences. It's OpenSSL's job to produce valid UTF-8, and reportedly,
     * it does validation.
     */
    for (cp = utf8_value; (ch = *cp) != 0; cp++) {
	if (ACL_ISASCII(ch) && !ACL_ISPRINT(ch)) {
	    acl_msg_warn("%s: %s: non-printable content in peer %s",
		     myname, TLScontext->namaddr, label);
	    TLS_TEXT_NAME_RETURN(0);
	}
    }
    TLS_TEXT_NAME_RETURN(acl_mystrdup((char *) utf8_value));
}

/* tls_dns_name - Extract valid DNS name from subjectAltName value */

const char *tls_dns_name(const GENERAL_NAME *gn, const TLS_SESS_STATE *TLScontext)
{
    const char *myname = "tls_dns_name";
    char   *cp;
    const char *dnsname;
    int     len;

    /*
     * Peername checks are security sensitive, carefully scrutinize the
     * input!
     */
    if (gn->type != GEN_DNS)
	acl_msg_panic("%s: Non DNS input argument", myname);

    /*
     * We expect the OpenSSL library to construct GEN_DNS extesion objects as
     * ASN1_IA5STRING values. Check we got the right union member.
     */
    if (ASN1_STRING_type(gn->d.ia5) != V_ASN1_IA5STRING) {
	acl_msg_warn("%s: %s: invalid ASN1 value type in subjectAltName",
		 myname, TLScontext->namaddr);
	return (0);
    }

    /*
     * Safe to treat as an ASCII string possibly holding a DNS name
     */
    dnsname = (char *) ASN1_STRING_data(gn->d.ia5);
    len = ASN1_STRING_length(gn->d.ia5);
    TRIM0(dnsname, len);

    /*
     * Per Dr. Steven Henson of the OpenSSL development team, ASN1_IA5STRING
     * values can have internal ASCII NUL values in this context because
     * their length is taken from the decoded ASN1 buffer, a trailing NUL is
     * always appended to make sure that the string is terminated, but the
     * ASN.1 length may differ from strlen().
     */
    if (len != (int) strlen(dnsname)) {
	acl_msg_warn("%s: %s: internal NUL in subjectAltName",
		 myname, TLScontext->namaddr);
	return 0;
    }

    /*
     * XXX: Should we be more strict and call valid_hostname()? So long as
     * the name is safe to handle, if it is not a valid hostname, it will not
     * compare equal to the expected peername, so being more strict than
     * "printable" is likely excessive...
     */
    if (*dnsname && !allprint(dnsname)) {
	cp = acl_mystrdup(dnsname);
	acl_msg_warn("%s: %s: non-printable characters in subjectAltName: %.100s",
		 myname, TLScontext->namaddr, printable(cp, '?'));
	acl_myfree(cp);
	return 0;
    }
    return (dnsname);
}

/* tls_peer_CN - extract peer common name from certificate */

char   *tls_peer_CN(X509 *peercert, const TLS_SESS_STATE *TLScontext)
{
    char   *cn;

    cn = tls_text_name(X509_get_subject_name(peercert), NID_commonName,
		       "subject CN", TLScontext, DO_GRIPE);
    return (cn ? cn : acl_mystrdup(""));
}

/* tls_issuer_CN - extract issuer common name from certificate */

char   *tls_issuer_CN(X509 *peer, const TLS_SESS_STATE *TLScontext)
{
    X509_NAME *name;
    char   *cn;

    name = X509_get_issuer_name(peer);

    /*
     * If no issuer CN field, use Organization instead. CA certs without a CN
     * are common, so we only complain if the organization is also missing.
     */
    if ((cn = tls_text_name(name, NID_commonName,
			    "issuer CN", TLScontext, DONT_GRIPE)) == 0)
	cn = tls_text_name(name, NID_organizationName,
			   "issuer Organization", TLScontext, DO_GRIPE);
    return (cn ? cn : acl_mystrdup(""));
}

/* tls_fingerprint - extract fingerprint from certificate */

char   *tls_fingerprint(X509 *peercert, const char *dgst)
{
    const char *myname = "tls_fingerprint";
    const EVP_MD *md_alg;
    unsigned char md_buf[EVP_MAX_MD_SIZE];
    unsigned int md_len;
    int     i;
    char   *result = 0;

    /* Previously available in "init" routine. */
    if ((md_alg = EVP_get_digestbyname(dgst)) == 0)
	acl_msg_panic("%s: digest algorithm \"%s\" not found", myname, dgst);

    /* Fails when serialization to ASN.1 runs out of memory */
    if (X509_digest(peercert, md_alg, md_buf, &md_len) == 0)
	acl_msg_fatal("%s: error computing certificate %s digest (out of memory?)",
		  myname, dgst);

    /* Check for OpenSSL contract violation */
    if (md_len > EVP_MAX_MD_SIZE || (int) md_len >= INT_MAX / 3)
	acl_msg_panic("%s: unexpectedly large %s digest size: %u",
		  myname, dgst, md_len);

    result = acl_mymalloc(md_len * 3);
    for (i = 0; i < (int) md_len; i++) {
	result[i * 3] = hexcodes[(md_buf[i] & 0xf0) >> 4U];
	result[(i * 3) + 1] = hexcodes[(md_buf[i] & 0x0f)];
	result[(i * 3) + 2] = (i + 1 != (int) md_len) ? ':' : '\0';
    }
    return (result);
}

#endif