tcpdump(8) monitor process privdrop

The privsep monitor process handles all privileged operations on behalf
of the unprivileged "packet parser" process. Once it enters its runtime
state, it only needs to:

* Perform DNS and other "numbers to names" lookups, sending results
back over a pipe/socketpair.
* Display the final packet statistics on ^C.

We can finally now drop root privileges in this process as well, as bpf
BIOCGSTATS is still permitted by non-root on open descriptors after it
has been permanently locked with BIOCLOCK. This provides some additional
protection, to go along with the already tight unveil(2) and pledge(2)
restrictions.

With this change tcpdump(8) completely drops root privileges at runtime.

ok mestre@, deraadt@

1 files changed, 40 insertions, 30 deletions

diff --git a/usr.sbin/tcpdump/privsep.c b/usr.sbin/tcpdump/privsep.c
index 790a67d6947..bb2279968df 100644
--- a/usr.sbin/tcpdump/privsep.c
+++ b/usr.sbin/tcpdump/privsep.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: privsep.c,v 1.51 2018/11/09 18:39:34 brynet Exp $	*/
+/*	$OpenBSD: privsep.c,v 1.52 2018/11/17 16:52:02 brynet Exp $	*/
 
 /*
  * Copyright (c) 2003 Can Erkin Acar
@@ -102,6 +102,8 @@ static volatile	sig_atomic_t cur_state = STATE_INIT;
 
 extern void	set_slave_signals(void);
 
+static void	drop_privs(int);
+
 static void	impl_open_bpf(int, int *);
 static void	impl_open_dump(int, const char *);
 static void	impl_open_pfosfp(int);
@@ -119,11 +121,42 @@ static void	impl_pcap_stats(int, int *);
 static void	test_state(int, int);
 static void	logmsg(int, const char *, ...);
 
+static void
+drop_privs(int nochroot)
+{
+	struct passwd *pw;
+
+	/*
+	 * If run as regular user, then tcpdump will rely on
+	 * pledge(2). If we are root, we want to chroot also..
+	 */
+	if (getuid() != 0)
+		return;
+
+	pw = getpwnam("_tcpdump");
+	if (pw == NULL)
+		errx(1, "unknown user _tcpdump");
+
+	if (!nochroot) {
+		if (chroot(pw->pw_dir) == -1)
+			err(1, "unable to chroot");
+		if (chdir("/") == -1)
+			err(1, "unable to chdir");
+	}
+
+	/* drop to _tcpdump */
+	if (setgroups(1, &pw->pw_gid) == -1)
+		err(1, "setgroups() failed");
+	if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) == -1)
+		err(1, "setresgid() failed");
+	if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) == -1)
+		err(1, "setresuid() failed");
+}
+
 int
 priv_init(int argc, char **argv)
 {
 	int i, nargc, socks[2];
-	struct passwd *pw;
 	sigset_t allsigs, oset;
 	char **privargv;
 
@@ -149,29 +182,7 @@ priv_init(int argc, char **argv)
 		set_slave_signals();
 		sigprocmask(SIG_SETMASK, &oset, NULL);
 
-		/*
-		 * If run as regular user, packet parser will rely on
-		 * pledge(2). If we are root, we want to chroot also..
-		 */
-		if (getuid() != 0)
-			return (0);
-
-		pw = getpwnam("_tcpdump");
-		if (pw == NULL)
-			errx(1, "unknown user _tcpdump");
-
-		if (chroot(pw->pw_dir) == -1)
-			err(1, "unable to chroot");
-		if (chdir("/") == -1)
-			err(1, "unable to chdir");
-
-		/* drop to _tcpdump */
-		if (setgroups(1, &pw->pw_gid) == -1)
-			err(1, "setgroups() failed");
-		if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) == -1)
-			err(1, "setresgid() failed");
-		if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) == -1)
-			err(1, "setresuid() failed");
+		drop_privs(0);
 
 		return (0);
 	}
@@ -256,10 +267,8 @@ priv_exec(int argc, char *argv[])
 	if (WFileName != NULL) {
 		if (strcmp(WFileName, "-") != 0)
 			allowed_ext[STATE_FILTER] |= ALLOW(PRIV_OPEN_OUTPUT);
-		else
-			allowed_ext[STATE_FILTER] |= ALLOW(PRIV_INIT_DONE);
-	} else
-		allowed_ext[STATE_FILTER] |= ALLOW(PRIV_INIT_DONE);
+	}
+	allowed_ext[STATE_FILTER] |= ALLOW(PRIV_INIT_DONE);
 	if (!nflag) {
 		allowed_ext[STATE_RUN] |= ALLOW(PRIV_GETHOSTBYADDR);
 		allowed_ext[STATE_FILTER] |= ALLOW(PRIV_ETHER_NTOHOST);
@@ -294,7 +303,7 @@ priv_exec(int argc, char *argv[])
 			impl_open_pfosfp(sock);
 			break;
 		case PRIV_OPEN_OUTPUT:
-			test_state(cmd, STATE_RUN);
+			test_state(cmd, STATE_FILTER);
 			impl_open_output(sock, WFileName);
 			break;
 		case PRIV_SETFILTER:
@@ -305,6 +314,7 @@ priv_exec(int argc, char *argv[])
 			test_state(cmd, STATE_RUN);
 			impl_init_done(sock, &bpfd);
 
+			drop_privs(1);
 			if (unveil("/etc/ethers", "r") == -1)
 				err(1, "unveil");
 			if (unveil("/etc/rpc", "r") == -1)