- djm@cvs.openbsd.org 2008/02/08 23:24:07

[servconf.c servconf.h session.c sftp-server.c sftp.h sshd_config]
     [sshd_config.5]
     add sshd_config ChrootDirectory option to chroot(2) users to a directory
     and tweak internal sftp server to work with it (no special files in
     chroot required). ok markus@

Makefile.in

@@ -1,4 +1,4 @@
-# $Id: Makefile.in,v 1.285 2007/06/11 04:01:42 djm Exp $
+# $Id: Makefile.in,v 1.286 2008/02/10 11:40:12 djm Exp $
 
 # uncomment if you run a non bourne compatable shell. Ie. csh
 #SHELL = @SH@
@@ -156,8 +156,8 @@ ssh-keysign$(EXEEXT): $(LIBCOMPAT) libssh.a ssh-keysign.o
 ssh-keyscan$(EXEEXT): $(LIBCOMPAT) libssh.a ssh-keyscan.o
 	$(LD) -o $@ ssh-keyscan.o $(LDFLAGS) -lssh -lopenbsd-compat -lssh $(LIBS)
 
-sftp-server$(EXEEXT): $(LIBCOMPAT) libssh.a sftp.o sftp-common.o sftp-server.o
+sftp-server$(EXEEXT): $(LIBCOMPAT) libssh.a sftp.o sftp-common.o sftp-server.o sftp-server-main.o
-	$(LD) -o $@ sftp-server.o sftp-common.o $(LDFLAGS) -lssh -lopenbsd-compat $(LIBS)
+	$(LD) -o $@ sftp-server.o sftp-common.o sftp-server-main.o $(LDFLAGS) -lssh -lopenbsd-compat $(LIBS)
 
 sftp$(EXEEXT): $(LIBCOMPAT) libssh.a sftp.o sftp-client.o sftp-common.o sftp-glob.o progressmeter.o
 	$(LD) -o $@ progressmeter.o sftp.o sftp-client.o sftp-common.o sftp-glob.o $(LDFLAGS) -lssh -lopenbsd-compat $(LIBS) $(LIBEDIT)

servconf.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.c,v 1.175 2008/01/01 09:27:33 dtucker Exp $ */
+/* $OpenBSD: servconf.c,v 1.176 2008/02/08 23:24:08 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -122,6 +122,7 @@ initialize_server_options(ServerOptions *options)
 	options->permit_tun = -1;
 	options->num_permitted_opens = -1;
 	options->adm_forced_command = NULL;
+	options->chroot_directory = NULL;
 }
 
 void
@@ -291,7 +292,7 @@ typedef enum {
 	sHostbasedUsesNameFromPacketOnly, sClientAliveInterval,
 	sClientAliveCountMax, sAuthorizedKeysFile, sAuthorizedKeysFile2,
 	sGssAuthentication, sGssCleanupCreds, sAcceptEnv, sPermitTunnel,
-	sMatch, sPermitOpen, sForceCommand,
+	sMatch, sPermitOpen, sForceCommand, sChrootDirectory,
 	sUsePrivilegeSeparation,
 	sDeprecated, sUnsupported
 } ServerOpCodes;
@@ -403,6 +404,7 @@ static struct {
  	{ "match", sMatch, SSHCFG_ALL },
 	{ "permitopen", sPermitOpen, SSHCFG_ALL },
 	{ "forcecommand", sForceCommand, SSHCFG_ALL },
+	{ "chrootdirectory", sChrootDirectory, SSHCFG_ALL },
 	{ NULL, sBadOption, 0 }
 };
 
@@ -1147,6 +1149,7 @@ parse_flag:
 	case sBanner:
 		charptr = &options->banner;
 		goto parse_filename;
+
 	/*
 	 * These options can contain %X options expanded at
 	 * connect time, so that you can specify paths like:
@@ -1255,6 +1258,10 @@ parse_flag:
 			options->adm_forced_command = xstrdup(cp + len);
 		return 0;
 
+	case sChrootDirectory:
+		charptr = &options->chroot_directory;
+		goto parse_filename;
+
 	case sDeprecated:
 		logit("%s line %d: Deprecated option %s",
 		    filename, linenum, arg);
@@ -1363,6 +1370,7 @@ copy_set_server_options(ServerOptions *dst, ServerOptions *src, int preauth)
 	if (preauth)
 		return;
 	M_CP_STROPT(adm_forced_command);
+	M_CP_STROPT(chroot_directory);
 }
 
 #undef M_CP_INTOPT

servconf.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.80 2007/02/19 10:45:58 dtucker Exp $ */
+/* $OpenBSD: servconf.h,v 1.81 2008/02/08 23:24:08 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -141,6 +141,8 @@ typedef struct {
 	int	permit_tun;
 
 	int	num_permitted_opens;
+
+	char   *chroot_directory;
 }       ServerOptions;
 
 void	 initialize_server_options(ServerOptions *);

session.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: session.c,v 1.225 2008/02/04 21:53:00 markus Exp $ */
+/* $OpenBSD: session.c,v 1.226 2008/02/08 23:24:07 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -84,6 +84,7 @@
 #include "sshlogin.h"
 #include "serverloop.h"
 #include "canohost.h"
+#include "misc.h"
 #include "session.h"
 #include "kex.h"
 #include "monitor_wrap.h"
@@ -93,6 +94,9 @@
 #include <kafs.h>
 #endif
 
+/* Magic name for internal sftp-server */
+#define INTERNAL_SFTP_NAME	"internal-sftp"
+
 /* func */
 
 Session *session_new(void);
@@ -130,7 +134,7 @@ extern Buffer loginmsg;
 const char *original_command = NULL;
 
 /* data */
-#define MAX_SESSIONS 10
+#define MAX_SESSIONS 20
 Session	sessions[MAX_SESSIONS];
 
 #define SUBSYSTEM_NONE		0
@@ -688,13 +692,17 @@ do_exec(Session *s, const char *command)
 	if (options.adm_forced_command) {
 		original_command = command;
 		command = options.adm_forced_command;
-		if (s->is_subsystem)
+		if (strcmp(INTERNAL_SFTP_NAME, command) == 0)
+			s->is_subsystem = SUBSYSTEM_INT_SFTP;
+		else if (s->is_subsystem)
 			s->is_subsystem = SUBSYSTEM_EXT;
 		debug("Forced command (config) '%.900s'", command);
 	} else if (forced_command) {
 		original_command = command;
 		command = forced_command;
-		if (s->is_subsystem)
+		if (strcmp(INTERNAL_SFTP_NAME, command) == 0)
+			s->is_subsystem = SUBSYSTEM_INT_SFTP;
+		else if (s->is_subsystem)
 			s->is_subsystem = SUBSYSTEM_EXT;
 		debug("Forced command (key option) '%.900s'", command);
 	}
@@ -710,7 +718,6 @@ do_exec(Session *s, const char *command)
 		PRIVSEP(audit_run_command(shell));
 	}
 #endif
-
 	if (s->ttyfd != -1)
 		do_exec_pty(s, command);
 	else
@@ -1293,6 +1300,61 @@ do_nologin(struct passwd *pw)
 	}
 }
 
+/*
+ * Chroot into a directory after checking it for safety: all path components
+ * must be root-owned directories with strict permissions.
+ */
+static void
+safely_chroot(const char *path, uid_t uid)
+{
+	const char *cp;
+	char component[MAXPATHLEN];
+	struct stat st;
+
+	if (*path != '/')
+		fatal("chroot path does not begin at root");
+	if (strlen(path) >= sizeof(component))
+		fatal("chroot path too long");
+
+	/*
+	 * Descend the path, checking that each component is a
+	 * root-owned directory with strict permissions.
+	 */
+	for (cp = path; cp != NULL;) {
+		if ((cp = strchr(cp, '/')) == NULL)
+			strlcpy(component, path, sizeof(component));
+		else {
+			cp++;
+			memcpy(component, path, cp - path);
+			component[cp - path] = '\0';
+		}
+	
+		debug3("%s: checking '%s'", __func__, component);
+
+		if (stat(component, &st) != 0)
+			fatal("%s: stat(\"%s\"): %s", __func__,
+			    component, strerror(errno));
+		if (st.st_uid != 0 || (st.st_mode & 022) != 0)
+			fatal("bad ownership or modes for chroot "
+			    "directory %s\"%s\"", 
+			    cp == NULL ? "" : "component ", component);
+		if (!S_ISDIR(st.st_mode))
+			fatal("chroot path %s\"%s\" is not a directory",
+			    cp == NULL ? "" : "component ", component);
+
+	}
+
+	if (chdir(path) == -1)
+		fatal("Unable to chdir to chroot path \"%s\": "
+		    "%s", path, strerror(errno));
+	if (chroot(path) == -1)
+		fatal("chroot(\"%s\"): %s", path, strerror(errno));
+	if (chdir("/") == -1)
+		fatal("%s: chdir(/) after chroot: %s",
+		    __func__, strerror(errno));
+	verbose("Changed root directory to \"%s\"", path);
+}
+
 /* Set login name, uid, gid, and groups. */
 void
 do_setusercontext(struct passwd *pw)
@@ -1324,7 +1386,7 @@ do_setusercontext(struct passwd *pw)
 		}
 # endif /* USE_PAM */
 		if (setusercontext(lc, pw, pw->pw_uid,
-		    (LOGIN_SETALL & ~LOGIN_SETPATH)) < 0) {
+		    (LOGIN_SETALL & ~(LOGIN_SETPATH|LOGIN_SETUSER))) < 0) {
 			perror("unable to set user context");
 			exit(1);
 		}
@@ -1376,6 +1438,24 @@ do_setusercontext(struct passwd *pw)
 			exit(1);
 		}
 # endif /* USE_LIBIAF */
+#endif
+
+		if (options.chroot_directory != NULL &&
+		    strcasecmp(options.chroot_directory, "none") != 0) {
+			char *chroot_path;
+
+			chroot_path = percent_expand(options.chroot_directory,
+			    "h", pw->pw_dir, "u", pw->pw_name, (char *)NULL);
+			safely_chroot(chroot_path, pw->pw_uid);
+			free(chroot_path);
+		}
+
+#ifdef HAVE_LOGIN_CAP
+		if (setusercontext(lc, pw, pw->pw_uid, LOGIN_SETUSER) < 0) {
+			perror("unable to set user context (setuser)");
+			exit(1);
+		}
+#else
 		/* Permanently switch to the desired uid. */
 		permanently_set_uid(pw);
 #endif
@@ -1625,7 +1705,7 @@ do_child(Session *s, const char *command)
 		argv[i] = NULL;
 		optind = optreset = 1;
 		__progname = argv[0];
-		exit(sftp_server_main(i, argv));
+		exit(sftp_server_main(i, argv, s->pw));
 	}
 
 	if (options.use_login) {
@@ -1900,7 +1980,7 @@ session_subsystem_req(Session *s)
 		if (strcmp(subsys, options.subsystem_name[i]) == 0) {
 			prog = options.subsystem_command[i];
 			cmd = options.subsystem_args[i];
-			if (!strcmp("internal-sftp", prog)) {
+			if (!strcmp(INTERNAL_SFTP_NAME, prog)) {
 				s->is_subsystem = SUBSYSTEM_INT_SFTP;
 			} else if (stat(prog, &st) < 0) {
 				error("subsystem: cannot stat %s: %s", prog,

sftp-server-main.c

@@ -0,0 +1,50 @@
+/* $OpenBSD: */
+/*
+ * Copyright (c) 2008 Markus Friedl.  All rights reserved.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "includes.h"
+
+#include <sys/types.h>
+#include <pwd.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "log.h"
+#include "sftp.h"
+#include "misc.h"
+
+void
+cleanup_exit(int i)
+{
+	sftp_server_cleanup_exit(i);
+}
+
+int
+main(int argc, char **argv)
+{
+	struct passwd *user_pw;
+
+	/* Ensure that fds 0, 1 and 2 are open or directed to /dev/null */
+	sanitise_stdfd();
+
+	if ((user_pw = getpwuid(getuid())) == NULL) {
+		fprintf(stderr, "No user found for uid %lu", (u_long)getuid());
+		return 1;
+	}
+
+	return (sftp_server_main(argc, argv, user_pw));
+}

sftp-server.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: sftp-server.c,v 1.76 2008/02/04 21:53:00 markus Exp $ */
+/* $OpenBSD: sftp-server.c,v 1.77 2008/02/08 23:24:07 djm Exp $ */
 /*
  * Copyright (c) 2000-2004 Markus Friedl.  All rights reserved.
  *
@@ -1219,7 +1219,7 @@ sftp_server_usage(void)
 }
 
 int
-sftp_server_main(int argc, char **argv)
+sftp_server_main(int argc, char **argv, struct passwd *user_pw)
 {
 	fd_set *rset, *wset;
 	int in, out, max, ch, skipargs = 0, log_stderr = 0;
@@ -1277,11 +1277,7 @@ sftp_server_main(int argc, char **argv)
 	} else
 		client_addr = xstrdup("UNKNOWN");
 
-	if ((pw = getpwuid(getuid())) == NULL) {
+	pw = pwcopy(user_pw);
-		error("No user found for uid %lu", (u_long)getuid());
-		sftp_server_cleanup_exit(255);
-	}
-	pw = pwcopy(pw);
 
 	logit("session opened for local user %s from [%s]",
 	    pw->pw_name, client_addr);

sftp.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: sftp.h,v 1.6 2008/02/04 21:53:00 markus Exp $ */
+/* $OpenBSD: sftp.h,v 1.7 2008/02/08 23:24:07 djm Exp $ */
 
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
@@ -91,5 +91,7 @@
 #define SSH2_FX_OP_UNSUPPORTED		8
 #define SSH2_FX_MAX			8
 
-int	sftp_server_main(int, char **);
+struct passwd;
+
+int	sftp_server_main(int, char **, struct passwd *);
 void	sftp_server_cleanup_exit(int) __dead;

sshd_config

@@ -1,4 +1,4 @@
-#	$OpenBSD: sshd_config,v 1.76 2007/08/23 03:22:16 djm Exp $
+#	$OpenBSD: sshd_config,v 1.77 2008/02/08 23:24:07 djm Exp $
 
 # This is the sshd server system-wide configuration file.  See
 # sshd_config(5) for more information.
@@ -102,6 +102,7 @@ Protocol 2
 #PidFile /var/run/sshd.pid
 #MaxStartups 10
 #PermitTunnel no
+#ChrootDirectory none
 
 # no default banner path
 #Banner none

sshd_config.5

@@ -34,8 +34,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: sshd_config.5,v 1.79 2008/01/01 09:27:33 dtucker Exp $
+.\" $OpenBSD: sshd_config.5,v 1.80 2008/02/08 23:24:07 djm Exp $
-.Dd $Mdocdate: January 1 2008 $
+.Dd $Mdocdate: February 8 2008 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -173,6 +173,45 @@ All authentication styles from
 are supported.
 The default is
 .Dq yes .
+.It Cm ChrootDirectory
+Specifies a path to
+.Xr chroot 2
+to after authentication.
+This path, and all its components, must be root-owned directories that are
+not writable by any other user or group.
+.Pp
+The path may contain the following tokens that are expanded at runtime once
+the connecting user has been authenticated: %% is replaced by a literal '%',
+%h is replaced by the home directory of the user being authenticated, and
+%u is replaced by the username of that user.
+.Pp
+The
+.Cm ChrootDirectory
+must contain the necessary files and directories to support the
+users' session.
+For an interactive session this requires at least a shell, typically
+.Xr sh 1 ,
+and basic
+.Pa /dev
+nodes such as
+.Xr null 4 ,
+.Xr zero 4 ,
+.Xr stdin 4 ,
+.Xr stdout 4 ,
+.Xr stderr 4 ,
+.Xr arandom 4
+and
+.Xr tty 4
+devices.
+For file transfer sessions using
+.Dq sftp ,
+no additional configuration of the environment is necessary if the
+in-process sftp server is used (see
+.Cm Subsystem
+for details.
+.Pp
+The default is not to
+.Xr chroot 2 .
 .It Cm Ciphers
 Specifies the ciphers allowed for protocol version 2.
 Multiple ciphers must be comma-separated.
@@ -740,11 +779,22 @@ The default is
 Configures an external subsystem (e.g. file transfer daemon).
 Arguments should be a subsystem name and a command (with optional arguments)
 to execute upon subsystem request.
+.Pp
 The command
 .Xr sftp-server 8
 implements the
 .Dq sftp
 file transfer subsystem.
+.Pp
+Alternately the name
+.Dq internal-sftp
+implements an in-process
+.Dq sftp
+server.
+This may simplify configurations using
+.Cm ChrootDirectory
+to force a different filesystem root on clients.
+.Pp
 By default no subsystems are defined.
 Note that this option applies to protocol version 2 only.
 .It Cm SyslogFacility