s3-client: Add a KRB5 wrapper for smbspool

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11690

We need a wrapper for smbspool to be able to authenticate with Kerberos.
This needs to replace the cups smb backend. The permission need to be
0700 and the owner root.

Note that Kerberos support is broken in CUPS 2.1.2 maybe earlier
versions. It works with 1.6.3.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Michael Adam <obnox@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>

Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Mon Jan 25 19:58:46 CET 2016 on sn-devel-144

(cherry picked from commit 62c68bd8c8f7a7e057e38a461707f1c195e62df0)

3 files changed, 234 insertions, 0 deletions

diff --git a/source3/client/README.smbspool b/source3/client/README.smbspool
new file mode 100644
index 00000000000..f73167a695a
--- /dev/null
+++ b/source3/client/README.smbspool
@@ -0,0 +1,17 @@
+smbspool
+=========
+
+smbspool is a very small print spooling program that sends a print file to an
+SMB printer. The command-line arguments are position-dependent for
+compatibility with the CUPS.
+
+For printing support with Kerberos, CUPS 1.5+ needs a wrapper for the backend
+which sets the correct location of the Kerberos credential cache.
+
+smbspool_krb5_wrapper
+======================
+
+This tool can be used to print using Kerberos credentials. To get this working
+smbspool_krb5_wrapper needs to be the smb backend of CUPS. It needs to be owned
+by root and the permissions for the binary need to be 0700. Once
+smbspool_krb5_wrapper switched to the user trying to print it executes smbspool.
diff --git a/source3/client/smbspool_krb5_wrapper.c b/source3/client/smbspool_krb5_wrapper.c
new file mode 100644
index 00000000000..e19fd92dc5b
--- /dev/null
+++ b/source3/client/smbspool_krb5_wrapper.c
@@ -0,0 +1,210 @@
+/*
+ * Unix SMB/CIFS implementation.
+ *
+ * CUPS printing backend helper to execute smbspool
+ *
+ * Copyright (C) 2010-2011 Andreas Schneider <asn@samba.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "includes.h"
+#include "system/filesys.h"
+#include "system/passwd.h"
+
+#include <errno.h>
+#include <string.h>
+
+#include <cups/backend.h>
+
+#include "dynconfig/dynconfig.h"
+
+enum cups_smb_dbglvl_e {
+	CUPS_SMB_LOG_DEBUG = 0,
+	CUPS_SMB_LOG_ERROR,
+};
+static void cups_smb_debug(enum cups_smb_dbglvl_e lvl, const char *format, ...);
+
+#define CUPS_SMB_DEBUG(...) cups_smb_debug(CUPS_SMB_LOG_DEBUG, __VA_ARGS__)
+#define CUPS_SMB_ERROR(...) cups_smb_debug(CUPS_SMB_LOG_DEBUG, __VA_ARGS__)
+
+static void cups_smb_debug(enum cups_smb_dbglvl_e lvl, const char *format, ...)
+{
+	const char *prefix = "DEBUG";
+	char buffer[1024];
+	va_list va;
+
+	va_start(va, format);
+	vsnprintf(buffer, sizeof(buffer), format, va);
+	va_end(va);
+
+	switch (lvl) {
+	case CUPS_SMB_LOG_DEBUG:
+		prefix = "DEBUG";
+		break;
+	case CUPS_SMB_LOG_ERROR:
+		prefix = "ERROR";
+		break;
+	}
+
+	fprintf(stderr,
+		"%s: SMBSPOOL_KRB5 - %s\n",
+		prefix,
+		buffer);
+}
+
+/*
+ * This is a helper binary to execute smbspool.
+ *
+ * It needs to be installed or symlinked as:
+ *      /usr/lib/cups/backend/smb
+ *
+ * The permissions of the binary need to be set to 0700 so that it is executed
+ * as root. The binary switches to the user which is passed via the environment
+ * variable AUTH_UID, so we can access the kerberos ticket.
+ */
+int main(int argc, char *argv[])
+{
+	char smbspool_cmd[PATH_MAX] = {0};
+	struct passwd *pwd;
+	char gen_cc[PATH_MAX] = {0};
+	struct stat sb;
+	char *env;
+	uid_t uid = (uid_t)-1;
+	gid_t gid = (gid_t)-1;
+	unsigned long tmp;
+	int cmp;
+	int rc;
+
+	uid = getuid();
+
+	CUPS_SMB_DEBUG("Started with uid=%d\n", uid);
+	if (uid != 0) {
+		goto smbspool;
+	}
+
+	/* Check if AuthInfoRequired is set to negotiate */
+	env = getenv("AUTH_INFO_REQUIRED");
+	if (env == NULL) {
+		CUPS_SMB_ERROR("AUTH_INFO_REQUIRED is not set");
+		fprintf(stderr, "ATTR: auth-info-required=negotiate\n");
+		return CUPS_BACKEND_AUTH_REQUIRED;
+	}
+
+	CUPS_SMB_DEBUG("AUTH_INFO_REQUIRED=%s", env);
+	cmp = strcmp(env, "negotiate");
+	if (cmp != 0) {
+		CUPS_SMB_ERROR("AUTH_INFO_REQUIRED is not set to negotiate");
+		fprintf(stderr, "ATTR: auth-info-required=negotiate\n");
+		return CUPS_BACKEND_AUTH_REQUIRED;
+	}
+
+	/*
+	 * AUTH_UID gets only set if we have an incoming connection over the
+	 * CUPS unix domain socket.
+	 */
+	env = getenv("AUTH_UID");
+	if (env == NULL) {
+		CUPS_SMB_ERROR("AUTH_UID is not set");
+		fprintf(stderr, "ATTR: auth-info-required=negotiate\n");
+		return CUPS_BACKEND_AUTH_REQUIRED;
+	}
+
+	if (strlen(env) > 10) {
+		CUPS_SMB_ERROR("Invalid AUTH_UID");
+		return CUPS_BACKEND_FAILED;
+	}
+
+	errno = 0;
+	tmp = strtoul(env, NULL, 10);
+	if (errno != 0 || tmp >= UINT32_MAX) {
+		CUPS_SMB_ERROR("Failed to convert AUTH_UID=%s", env);
+		return CUPS_BACKEND_FAILED;
+	}
+	uid = (uid_t)tmp;
+
+	pwd = getpwuid(uid);
+	if (pwd == NULL) {
+		CUPS_SMB_ERROR("Failed to find system user: %u - %s",
+			       uid, strerror(errno));
+		return CUPS_BACKEND_FAILED;
+	}
+	gid = pwd->pw_gid;
+
+	rc = setgroups(0, NULL);
+	if (rc != 0) {
+		CUPS_SMB_ERROR("Failed to clear groups - %s",
+			       strerror(errno));
+		return CUPS_BACKEND_FAILED;
+	}
+
+	CUPS_SMB_DEBUG("Switching to gid=%d", gid);
+	rc = setgid(gid);
+	if (rc != 0) {
+		CUPS_SMB_ERROR("Failed to switch to gid=%u",
+			       gid,
+			       strerror(errno));
+		return CUPS_BACKEND_FAILED;
+	}
+
+	CUPS_SMB_DEBUG("Switching to uid=%u", uid);
+	rc = setuid(uid);
+	if (rc != 0) {
+		CUPS_SMB_ERROR("Failed to switch to uid=%u",
+			       uid,
+			       strerror(errno));
+		return CUPS_BACKEND_FAILED;
+	}
+
+	snprintf(gen_cc, sizeof(gen_cc), "/tmp/krb5cc_%d", uid);
+
+	rc = lstat(gen_cc, &sb);
+	if (rc == 0) {
+		snprintf(gen_cc, sizeof(gen_cc), "FILE:/tmp/krb5cc_%d", uid);
+	} else {
+		snprintf(gen_cc, sizeof(gen_cc), "/run/user/%d/krb5cc", uid);
+
+		rc = lstat(gen_cc, &sb);
+		if (rc == 0 && S_ISDIR(sb.st_mode)) {
+			snprintf(gen_cc,
+				 sizeof(gen_cc),
+				 "DIR:/run/user/%d/krb5cc",
+				 uid);
+		} else {
+#if defined(__linux__)
+			snprintf(gen_cc,
+				 sizeof(gen_cc),
+				 "KEYRING:persistent:%d",
+				 uid);
+#endif
+		}
+	}
+
+	/*
+	 * Make sure we do not have LD_PRELOAD or other security relevant
+	 * environment variables set.
+	 */
+	clearenv();
+
+	CUPS_SMB_DEBUG("Setting KRB5CCNAME to '%s'", gen_cc);
+	setenv("KRB5CCNAME", gen_cc, 1);
+
+smbspool:
+	snprintf(smbspool_cmd,
+		 sizeof(smbspool_cmd),
+		 "%s/smbspool",
+		 get_dyn_BINDIR());
+
+	return execv(smbspool_cmd, argv);
+}
diff --git a/source3/wscript_build b/source3/wscript_build
index c0296c00896..657d79875c2 100755
--- a/source3/wscript_build
+++ b/source3/wscript_build
@@ -1186,6 +1186,13 @@ bld.SAMBA3_BINARY('smbspool',
                  libsmb
                  samba3core''')
 
+bld.SAMBA3_BINARY('smbspool_krb5_wrapper',
+                 source='client/smbspool_krb5_wrapper.c',
+                 deps='''
+                 DYNCONFIG
+                 cups
+                 ''')
+
 bld.SAMBA3_BINARY('testparm',
                  source='utils/testparm.c',
                  deps='''