introduce sandboxing of the pre-auth privsep child using systrace(4).

This introduces a new "UsePrivilegeSeparation=sandbox" option for
sshd_config that applies mandatory restrictions on the syscalls the
privsep child can perform. This prevents a compromised privsep child
from being used to attack other hosts (by opening sockets and proxying)
or probing local kernel attack surface.

The sandbox is implemented using systrace(4) in unsupervised "fast-path"
mode, where a list of permitted syscalls is supplied. Any syscall not
on the list results in SIGKILL being sent to the privsep child. Note
that this requires a kernel with the new SYSTR_POLICY_KILL option.

UsePrivilegeSeparation=sandbox will become the default in the future
so please start testing it now.

feedback dtucker@; ok markus@

8 files changed, 342 insertions, 13 deletions

diff --git a/usr.bin/ssh/sandbox-rlimit.c b/usr.bin/ssh/sandbox-rlimit.c
new file mode 100644
index 00000000000..b0c80e5f031
--- /dev/null
+++ b/usr.bin/ssh/sandbox-rlimit.c
@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2011 Damien Miller <djm@mindrot.org>
+ *
+ * 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 <sys/types.h>
+#include <sys/param.h>
+#include <sys/time.h>
+#include <sys/resource.h>
+
+#include <errno.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "log.h"
+#include "sandbox.h"
+#include "xmalloc.h"
+
+/* Minimal sandbox that sets zero nfiles, nprocs and filesize rlimits */
+
+struct ssh_sandbox {
+	pid_t child_pid;
+};
+
+struct ssh_sandbox *
+ssh_sandbox_init(void)
+{
+	struct ssh_sandbox *box;
+
+	/*
+	 * Strictly, we don't need to maintain any state here but we need
+	 * to return non-NULL to satisfy the API.
+	 */
+	debug3("%s: preparing rlimit sandbox", __func__);
+	box = xcalloc(1, sizeof(*box));
+	box->child_pid = 0;
+
+	return box;
+}
+
+void
+ssh_sandbox_child(struct ssh_sandbox *box)
+{
+	struct rlimit rl_zero;
+
+	rl_zero.rlim_cur = rl_zero.rlim_max = 0;
+
+	if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
+		fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
+			__func__, strerror(errno));
+	if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
+		fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
+			__func__, strerror(errno));
+	if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
+		fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
+			__func__, strerror(errno));
+}
+
+void
+ssh_sandbox_parent_finish(struct ssh_sandbox *box)
+{
+	free(box);
+	debug3("%s: finished", __func__);
+}
+
+void
+ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
+{
+	box->child_pid = child_pid;
+	/* Nothing to do here */
+}
+
diff --git a/usr.bin/ssh/sandbox-systrace.c b/usr.bin/ssh/sandbox-systrace.c
new file mode 100644
index 00000000000..ecea3ece85a
--- /dev/null
+++ b/usr.bin/ssh/sandbox-systrace.c
@@ -0,0 +1,181 @@
+/*
+ * Copyright (c) 2011 Damien Miller <djm@mindrot.org>
+ *
+ * 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 <sys/types.h>
+#include <sys/param.h>
+#include <sys/ioctl.h>
+#include <sys/syscall.h>
+#include <sys/socket.h>
+
+#include <dev/systrace.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "atomicio.h"
+#include "log.h"
+#include "sandbox.h"
+#include "xmalloc.h"
+
+static const int preauth_policy[] = {
+	SYS___sysctl,
+	SYS_close,
+	SYS_exit,
+	SYS_getpid,
+	SYS_gettimeofday,
+	SYS_madvise,
+	SYS_mmap,
+	SYS_mprotect,
+	SYS_poll,
+	SYS_munmap,
+	SYS_read,
+	SYS_select,
+	SYS_sigprocmask,
+	SYS_write,
+	-1
+};
+
+struct ssh_sandbox {
+	int child_sock;
+	int parent_sock;
+	int systrace_fd;
+	pid_t child_pid;
+	struct systrace_policy policy;
+};
+
+struct ssh_sandbox *
+ssh_sandbox_init(void)
+{
+	struct ssh_sandbox *box;
+	int s[2];
+
+	debug3("%s: preparing systrace sandbox", __func__);
+	box = xcalloc(1, sizeof(*box));
+	if (socketpair(AF_UNIX, SOCK_STREAM, 0, s) == -1)
+		fatal("%s: socketpair: %s", __func__, strerror(errno));
+	box->child_sock = s[0];
+	box->parent_sock = s[1];
+	box->systrace_fd = -1;
+	box->child_pid = 0;
+
+	return box;
+}
+
+void
+ssh_sandbox_child(struct ssh_sandbox *box)
+{
+	char whatever = 0;
+
+	close(box->parent_sock);
+	/* Signal parent that we are ready */
+	debug3("%s: ready", __func__);
+	if (atomicio(vwrite, box->child_sock, &whatever, 1) != 1)
+		fatal("%s: write: %s", __func__, strerror(errno));
+	/* Wait for parent to signal for us to go */
+	if (atomicio(read, box->child_sock, &whatever, 1) != 1)
+		fatal("%s: read: %s", __func__, strerror(errno));
+	debug3("%s: started", __func__);
+	close(box->child_sock);
+}
+
+static void
+ssh_sandbox_parent(struct ssh_sandbox *box, pid_t child_pid,
+    const int *allowed_syscalls)
+{
+	int dev_systrace, i, j, found;
+	char whatever = 0;
+
+	debug3("%s: wait for child %ld", __func__, (long)child_pid);
+	box->child_pid = child_pid;
+	close(box->child_sock);
+	/* Wait for child to signal that it is ready */
+	if (atomicio(read, box->parent_sock, &whatever, 1) != 1)
+		fatal("%s: read: %s", __func__, strerror(errno));
+	debug3("%s: child %ld ready", __func__, (long)child_pid);
+
+	/* Set up systracing of child */
+	if ((dev_systrace = open("/dev/systrace", O_RDONLY)) == -1)
+		fatal("%s: open(\"/dev/systrace\"): %s", __func__,
+		    strerror(errno));
+	if (ioctl(dev_systrace, STRIOCCLONE, &box->systrace_fd) == -1)
+		fatal("%s: ioctl(STRIOCCLONE, %d): %s", __func__,
+		    dev_systrace, strerror(errno));
+	close(dev_systrace);
+	debug3("%s: systrace attach, fd=%d", __func__, box->systrace_fd);
+	if (ioctl(box->systrace_fd, STRIOCATTACH, &child_pid) == -1)
+		fatal("%s: ioctl(%d, STRIOCATTACH, %d): %s", __func__,
+		    box->systrace_fd, child_pid, strerror(errno));
+
+	/* Allocate and assign policy */
+	bzero(&box->policy, sizeof(box->policy));
+	box->policy.strp_op = SYSTR_POLICY_NEW;
+	box->policy.strp_maxents = SYS_MAXSYSCALL;
+	if (ioctl(box->systrace_fd, STRIOCPOLICY, &box->policy) == -1)
+		fatal("%s: ioctl(%d, STRIOCPOLICY (new)): %s", __func__,
+		    box->systrace_fd, strerror(errno));
+
+	box->policy.strp_op = SYSTR_POLICY_ASSIGN;
+	box->policy.strp_pid = box->child_pid;
+	if (ioctl(box->systrace_fd, STRIOCPOLICY, &box->policy) == -1)
+		fatal("%s: ioctl(%d, STRIOCPOLICY (assign)): %s",
+		    __func__, box->systrace_fd, strerror(errno));
+
+	/* Set per-syscall policy */
+	for (i = 0; i < SYS_MAXSYSCALL; i++) {
+		for (j = found = 0; allowed_syscalls[j] != -1 && !found; j++) {
+			if (allowed_syscalls[j] == i)
+				found = 1;
+		}
+		box->policy.strp_op = SYSTR_POLICY_MODIFY;
+		box->policy.strp_code = i;
+		box->policy.strp_policy = found ?
+		    SYSTR_POLICY_PERMIT : SYSTR_POLICY_KILL;
+		if (found)
+			debug3("%s: policy: enable syscall %d", __func__, i);
+		if (ioctl(box->systrace_fd, STRIOCPOLICY,
+		    &box->policy) == -1)
+			fatal("%s: ioctl(%d, STRIOCPOLICY (modify)): %s",
+			    __func__, box->systrace_fd, strerror(errno));
+	}
+
+	/* Signal the child to start running */
+	debug3("%s: start child %ld", __func__, (long)child_pid);
+	if (atomicio(vwrite, box->parent_sock, &whatever, 1) != 1)
+		fatal("%s: write: %s", __func__, strerror(errno));
+	close(box->parent_sock);
+}
+
+void
+ssh_sandbox_parent_finish(struct ssh_sandbox *box)
+{
+	/* Closing this before the child exits will terminate it */
+	close(box->systrace_fd);
+
+	free(box);
+	debug3("%s: finished", __func__);
+}
+
+void
+ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
+{
+	ssh_sandbox_parent(box, child_pid, preauth_policy);
+}
diff --git a/usr.bin/ssh/sandbox.h b/usr.bin/ssh/sandbox.h
new file mode 100755
index 00000000000..7593dedea32
--- /dev/null
+++ b/usr.bin/ssh/sandbox.h
@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2011 Damien Miller <djm@mindrot.org>
+ *
+ * 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.
+ */
+
+struct ssh_sandbox;
+
+struct ssh_sandbox *ssh_sandbox_init(void);
+void ssh_sandbox_child(struct ssh_sandbox *);
+void ssh_sandbox_parent_finish(struct ssh_sandbox *);
+void ssh_sandbox_parent_preauth(struct ssh_sandbox *, pid_t);
diff --git a/usr.bin/ssh/servconf.c b/usr.bin/ssh/servconf.c
index eaa127f86aa..597e2b6e4f7 100644
--- a/usr.bin/ssh/servconf.c
+++ b/usr.bin/ssh/servconf.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.c,v 1.221 2011/06/22 21:47:28 djm Exp $ */
+/* $OpenBSD: servconf.c,v 1.222 2011/06/22 21:57:01 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -266,7 +266,7 @@ fill_default_server_options(ServerOptions *options)
 
 	/* Turn privilege separation on by default */
 	if (use_privsep == -1)
-		use_privsep = 1;
+		use_privsep = PRIVSEP_ON;
 }
 
 /* Keyword tokens. */
@@ -662,6 +662,12 @@ static const struct multistate multistate_gatewayports[] = {
 	{ "no",				0 },
 	{ NULL, -1 }
 };
+static const struct multistate multistate_privsep[] = {
+	{ "sandbox",			PRIVSEP_SANDBOX },
+	{ "yes",			PRIVSEP_ON },
+	{ "no",				PRIVSEP_OFF },
+	{ NULL, -1 }
+};
 
 int
 process_server_config_line(ServerOptions *options, char *line,
@@ -1021,7 +1027,8 @@ process_server_config_line(ServerOptions *options, char *line,
 
 	case sUsePrivilegeSeparation:
 		intptr = &use_privsep;
-		goto parse_flag;
+		multistate_ptr = multistate_privsep;
+		goto parse_multistate;
 
 	case sAllowUsers:
 		while ((arg = strdelim(&cp)) && *arg != '\0') {
@@ -1529,6 +1536,8 @@ fmt_intarg(ServerOpCodes code, int val)
 		return fmt_multistate_int(val, multistate_gatewayports);
 	case sCompression:
 		return fmt_multistate_int(val, multistate_compression);
+	case sUsePrivilegeSeparation:
+		return fmt_multistate_int(val, multistate_privsep);
 	case sProtocol:
 		switch (val) {
 		case SSH_PROTO_1:
diff --git a/usr.bin/ssh/servconf.h b/usr.bin/ssh/servconf.h
index 84cc33c0c6a..d7ca51b44fe 100644
--- a/usr.bin/ssh/servconf.h
+++ b/usr.bin/ssh/servconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.98 2011/05/23 03:30:07 djm Exp $ */
+/* $OpenBSD: servconf.h,v 1.99 2011/06/22 21:57:01 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -36,6 +36,11 @@
 #define	PERMIT_NO_PASSWD	2
 #define	PERMIT_YES		3
 
+/* use_privsep */
+#define PRIVSEP_OFF		0
+#define PRIVSEP_ON		1
+#define PRIVSEP_SANDBOX		2
+
 #define DEFAULT_AUTH_FAIL_MAX	6	/* Default for MaxAuthTries */
 #define DEFAULT_SESSIONS_MAX	10	/* Default for MaxSessions */
 
diff --git a/usr.bin/ssh/sshd.c b/usr.bin/ssh/sshd.c
index 27423765181..4cc6a6f2296 100644
--- a/usr.bin/ssh/sshd.c
+++ b/usr.bin/ssh/sshd.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshd.c,v 1.383 2011/06/17 21:44:31 djm Exp $ */
+/* $OpenBSD: sshd.c,v 1.384 2011/06/22 21:57:01 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -102,6 +102,7 @@
 #endif
 #include "monitor_wrap.h"
 #include "roaming.h"
+#include "sandbox.h"
 #include "version.h"
 
 #ifdef LIBWRAP
@@ -611,18 +612,23 @@ privsep_preauth(Authctxt *authctxt)
 {
 	int status;
 	pid_t pid;
+	struct ssh_sandbox *box = NULL;
 
 	/* Set up unprivileged child process to deal with network data */
 	pmonitor = monitor_init();
 	/* Store a pointer to the kex for later rekeying */
 	pmonitor->m_pkex = &xxx_kex;
 
+	if (use_privsep == PRIVSEP_SANDBOX)
+		box = ssh_sandbox_init();
 	pid = fork();
 	if (pid == -1) {
 		fatal("fork of unprivileged child failed");
 	} else if (pid != 0) {
 		debug2("Network child is on pid %ld", (long)pid);
 
+		if (box != NULL)
+			ssh_sandbox_parent_preauth(box, pid);
 		pmonitor->m_pid = pid;
 		monitor_child_preauth(authctxt, pmonitor);
 
@@ -630,10 +636,21 @@ privsep_preauth(Authctxt *authctxt)
 		monitor_sync(pmonitor);
 
 		/* Wait for the child's exit status */
-		while (waitpid(pid, &status, 0) < 0)
+		while (waitpid(pid, &status, 0) < 0) {
 			if (errno != EINTR)
-				break;
-		return (1);
+				fatal("%s: waitpid: %s", __func__,
+				    strerror(errno));
+		}
+		if (WIFEXITED(status)) {
+			if (WEXITSTATUS(status) != 0)
+				fatal("%s: preauth child exited with status %d",
+				    __func__, WEXITSTATUS(status));
+		} else if (WIFSIGNALED(status))
+			fatal("%s: preauth child terminated by signal %d",
+			    __func__, WTERMSIG(status));
+		if (box != NULL)
+			ssh_sandbox_parent_finish(box);
+		return 1;
 	} else {
 		/* child */
 		close(pmonitor->m_sendfd);
@@ -646,8 +663,11 @@ privsep_preauth(Authctxt *authctxt)
 		if (getuid() == 0 || geteuid() == 0)
 			privsep_preauth_child();
 		setproctitle("%s", "[net]");
+		if (box != NULL)
+			ssh_sandbox_child(box);
+
+		return 0;
 	}
-	return (0);
 }
 
 static void
diff --git a/usr.bin/ssh/sshd/Makefile b/usr.bin/ssh/sshd/Makefile
index a479ec7ca21..15417811a83 100644
--- a/usr.bin/ssh/sshd/Makefile
+++ b/usr.bin/ssh/sshd/Makefile
@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.72 2010/10/13 08:14:22 jsg Exp $
+#	$OpenBSD: Makefile,v 1.73 2011/06/22 21:57:01 djm Exp $
 
 .PATH:		${.CURDIR}/..
 
@@ -16,7 +16,7 @@ SRCS=	sshd.c auth-rhosts.c auth-passwd.c auth-rsa.c auth-rh-rsa.c \
 	auth2-none.c auth2-passwd.c auth2-pubkey.c \
 	monitor_mm.c monitor.c monitor_wrap.c \
 	kexdhs.c kexgexs.c kexecdhs.c sftp-server.c sftp-common.c \
-	roaming_common.c roaming_serv.c
+	roaming_common.c roaming_serv.c sandbox-systrace.c
 
 .include <bsd.own.mk> # for KERBEROS and AFS
 
diff --git a/usr.bin/ssh/sshd_config.5 b/usr.bin/ssh/sshd_config.5
index 69de8d4d2b9..a8f46c4d256 100644
--- a/usr.bin/ssh/sshd_config.5
+++ b/usr.bin/ssh/sshd_config.5
@@ -33,8 +33,8 @@
 .\" (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 1.133 2011/05/23 07:10:21 jmc Exp $
-.Dd $Mdocdate: May 23 2011 $
+.\" $OpenBSD: sshd_config.5,v 1.134 2011/06/22 21:57:01 djm Exp $
+.Dd $Mdocdate: June 22 2011 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -1048,6 +1048,12 @@ The goal of privilege separation is to prevent privilege
 escalation by containing any corruption within the unprivileged processes.
 The default is
 .Dq yes .
+If
+.Cm UsePrivilegeSeparation
+is set to
+.Dq sandbox
+then the pre-authentication unprivileged process is subject to additional
+restrictions.
 .It Cm X11DisplayOffset
 Specifies the first display number available for
 .Xr sshd 8 Ns 's