auth2-pubkey.c

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/*
 * Copyright (c) 2000 Markus Friedl.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "includes.h"
RCSID("$OpenBSD: auth2-pubkey.c,v 1.9 2004/12/11 01:48:56 dtucker Exp $");

#include "ssh.h"
#include "ssh2.h"
#include "xmalloc.h"
#include "packet.h"
#include "buffer.h"
#include "log.h"
#include "servconf.h"
#include "compat.h"
#include "bufaux.h"
#include "auth.h"
#include "key.h"
#include "pathnames.h"
#include "uidswap.h"
#include "auth-options.h"
#include "canohost.h"
#include "monitor_wrap.h"
#include "misc.h"

/* import */
extern ServerOptions options;
extern u_char *session_id2;
extern u_int session_id2_len;

static int
userauth_pubkey(Authctxt *authctxt)
{
        Buffer b;
        Key *key = NULL;
        char *pkalg;
        u_char *pkblob, *sig;
        u_int alen, blen, slen;
        int have_sig, pktype;
        int authenticated = 0;

        if (!authctxt->valid) {
                debug2("userauth_pubkey: disabled because of invalid user");
                return 0;
        }
        have_sig = packet_get_char();
        if (datafellows & SSH_BUG_PKAUTH) {
                debug2("userauth_pubkey: SSH_BUG_PKAUTH");
                /* no explicit pkalg given */
                pkblob = packet_get_string(&blen);
                buffer_init(&b);
                buffer_append(&b, pkblob, blen);
                /* so we have to extract the pkalg from the pkblob */
                pkalg = buffer_get_string(&b, &alen);
                buffer_free(&b);
        } else {
                pkalg = packet_get_string(&alen);
                pkblob = packet_get_string(&blen);
        }
        pktype = key_type_from_name(pkalg);
        if (pktype == KEY_UNSPEC) {
                /* this is perfectly legal */
                logit("userauth_pubkey: unsupported public key algorithm: %s",
                    pkalg);
                goto done;
        }
        key = key_from_blob(pkblob, blen);
        if (key == NULL) {
                error("userauth_pubkey: cannot decode key: %s", pkalg);
                goto done;
        }
        if (key->type != pktype) {
                error("userauth_pubkey: type mismatch for decoded key "
                    "(received %d, expected %d)", key->type, pktype);
                goto done;
        }
        if (have_sig) {
                sig = packet_get_string(&slen);
                packet_check_eom();
                buffer_init(&b);
                if (datafellows & SSH_OLD_SESSIONID) {
                        buffer_append(&b, session_id2, session_id2_len);
                } else {
                        buffer_put_string(&b, session_id2, session_id2_len);
                }
                /* reconstruct packet */
                buffer_put_char(&b, SSH2_MSG_USERAUTH_REQUEST);
                buffer_put_cstring(&b, authctxt->user);
                buffer_put_cstring(&b,
                    datafellows & SSH_BUG_PKSERVICE ?
                    "ssh-userauth" :
                    authctxt->service);
                if (datafellows & SSH_BUG_PKAUTH) {
                        buffer_put_char(&b, have_sig);
                } else {
                        buffer_put_cstring(&b, "publickey");
                        buffer_put_char(&b, have_sig);
                        buffer_put_cstring(&b, pkalg);
                }
                buffer_put_string(&b, pkblob, blen);
#ifdef DEBUG_PK
                buffer_dump(&b);
#endif
                /* test for correct signature */
                authenticated = 0;
                if (PRIVSEP(user_key_allowed(authctxt->pw, key)) &&
                    PRIVSEP(key_verify(key, sig, slen, buffer_ptr(&b),
                    buffer_len(&b))) == 1)
                        authenticated = 1;
                buffer_free(&b);
                xfree(sig);
        } else {
                debug("test whether pkalg/pkblob are acceptable");
                packet_check_eom();

                /* XXX fake reply and always send PK_OK ? */
                /*
                 * XXX this allows testing whether a user is allowed
                 * to login: if you happen to have a valid pubkey this
                 * message is sent. the message is NEVER sent at all
                 * if a user is not allowed to login. is this an
                 * issue? -markus
                 */
                if (PRIVSEP(user_key_allowed(authctxt->pw, key))) {
                        packet_start(SSH2_MSG_USERAUTH_PK_OK);
                        packet_put_string(pkalg, alen);
                        packet_put_string(pkblob, blen);
                        packet_send();
                        packet_write_wait();
                        authctxt->postponed = 1;
                }
        }
        if (authenticated != 1)
                auth_clear_options();
done:
        debug2("userauth_pubkey: authenticated %d pkalg %s", authenticated, pkalg);
        if (key != NULL)
                key_free(key);
        xfree(pkalg);
        xfree(pkblob);
#ifdef HAVE_CYGWIN
        if (check_nt_auth(0, authctxt->pw) == 0)
                authenticated = 0;
#endif
        return authenticated;
}

/* return 1 if user allows given key */
static int
user_key_allowed2(struct passwd *pw, Key *key, char *file)
{
        char line[SSH_MAX_PUBKEY_BYTES];
        int found_key = 0;
        FILE *f;
        u_long linenum = 0;
        struct stat st;
        Key *found;
        char *fp;

        /* Temporarily use the user's uid. */
        temporarily_use_uid(pw);

        debug("trying public key file %s", file);

        /* Fail quietly if file does not exist */
        if (stat(file, &st) < 0) {
                /* Restore the privileged uid. */
                restore_uid();
                return 0;
        }
        /* Open the file containing the authorized keys. */
        f = fopen(file, "r");
        if (!f) {
                /* Restore the privileged uid. */
                restore_uid();
                return 0;
        }
        if (options.strict_modes &&
            secure_filename(f, file, pw, line, sizeof(line)) != 0) {
                fclose(f);
                logit("Authentication refused: %s", line);
                restore_uid();
                return 0;
        }

        found_key = 0;
        found = key_new(key->type);

        while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
                char *cp, *key_options = NULL;

                /* Skip leading whitespace, empty and comment lines. */
                for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
                        ;
                if (!*cp || *cp == '\n' || *cp == '#')
                        continue;

                if (key_read(found, &cp) != 1) {
                        /* no key?  check if there are options for this key */
                        int quoted = 0;
                        debug2("user_key_allowed: check options: '%s'", cp);
                        key_options = cp;
                        for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
                                if (*cp == '\\' && cp[1] == '"')
                                        cp++;   /* Skip both */
                                else if (*cp == '"')
                                        quoted = !quoted;
                        }
                        /* Skip remaining whitespace. */
                        for (; *cp == ' ' || *cp == '\t'; cp++)
                                ;
                        if (key_read(found, &cp) != 1) {
                                debug2("user_key_allowed: advance: '%s'", cp);
                                /* still no key?  advance to next line*/
                                continue;
                        }
                }
                if (key_equal(found, key) &&
                    auth_parse_options(pw, key_options, file, linenum) == 1) {
                        found_key = 1;
                        debug("matching key found: file %s, line %lu",
                            file, linenum);
                        fp = key_fingerprint(found, SSH_FP_MD5, SSH_FP_HEX);
                        verbose("Found matching %s key: %s",
                            key_type(found), fp);
                        xfree(fp);
                        break;
                }
        }
        restore_uid();
        fclose(f);
        key_free(found);
        if (!found_key)
                debug2("key not found");
        return found_key;
}

/* check whether given key is in .ssh/authorized_keys* */
int
user_key_allowed(struct passwd *pw, Key *key)
{
        int success;
        char *file;

        file = authorized_keys_file(pw);
        success = user_key_allowed2(pw, key, file);
        xfree(file);
        if (success)
                return success;

        /* try suffix "2" for backward compat, too */
        file = authorized_keys_file2(pw);
        success = user_key_allowed2(pw, key, file);
        xfree(file);
        return success;
}

Authmethod method_pubkey = {
        "publickey",
        userauth_pubkey,
        &options.pubkey_authentication
};

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