From 688a44577ac447362b8a7570764edc34b48b30a9 Mon Sep 17 00:00:00 2001
From: David Härdeman <david@hardeman.nu>
Date: Wed, 17 Jun 2020 11:36:49 +0200
Subject: Implement capability handling and user switching, disallow running as
 root

---
 main.c      | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 main.h      |  2 ++
 meson.build |  4 ++-
 3 files changed, 95 insertions(+), 8 deletions(-)

diff --git a/main.c b/main.c
index 1147709..598b9b2 100644
--- a/main.c
+++ b/main.c
@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
@@ -13,6 +14,9 @@
 #include <sys/eventfd.h>
 #include <signal.h>
 #include <systemd/sd-daemon.h>
+#include <cap-ng.h>
+#include <pwd.h>
+#include <grp.h>
 
 #include "main.h"
 #include "uring.h"
@@ -226,6 +230,8 @@ cfg_init(int argc, char **argv)
 	if (!cfg)
 		perrordie("malloc");
 
+	cfg->uid = geteuid();
+	cfg->gid = getegid();
 	uring_task_init(&cfg->task, "cfg", NULL, cfg_free);
 	list_init(&cfg->servers);
 
@@ -235,10 +241,11 @@ cfg_init(int argc, char **argv)
 			{ "homedir",	required_argument,	0, 'h' },
 			{ "debug",	required_argument,	0, 'd' },
 			{ "verbose",	no_argument,		0, 'v' },
+			{ "user",	required_argument,	0, 'v' },
 			{ 0,		0,			0,  0  }
 		};
 
-		c = getopt_long(argc, argv, ":h:d:v",
+		c = getopt_long(argc, argv, ":h:d:vu:",
 				long_options, &option_index);
 		if (c == -1)
 			break;
@@ -250,6 +257,28 @@ cfg_init(int argc, char **argv)
 		case 'v':
 			debug_mask |= DBG_VERBOSE;
 			break;
+		case 'u': {
+			struct passwd *pwd;
+
+			errno = 0;
+			pwd = getpwnam(optarg);
+			if (!pwd) {
+				if (errno == 0)
+					errno = ESRCH;
+				if (errno == ESRCH)
+					die("failed to find user %s", optarg);
+				else
+					die("failed to find user %s (%m)", optarg);
+			}
+
+			debug(DBG_CFG, "asked to execute with uid %ji gid %ji\n",
+			      (intmax_t)pwd->pw_uid,
+			      (intmax_t)pwd->pw_gid);
+			cfg->uid = pwd->pw_uid;
+			cfg->gid = pwd->pw_gid;
+			break;
+		}
+
 		case 'd':
 			if (!strcasecmp(optarg, "all")) {
 				debug_mask = ~0;
@@ -284,11 +313,50 @@ cfg_init(int argc, char **argv)
 	if (!cfg->homedir)
 		cfg->homedir = DEFAULT_HOMEDIR_PATH;
 
-	verbose("Homedir is %s\n", cfg->homedir);
+	return cfg;
+}
+
+static void
+cfg_apply(struct cfg *cfg)
+{
+	if (cfg->uid == 0 || cfg->gid == 0)
+		/* This catches both -u root and running as root without -u */
+		die("Execution as root is not supported (use -u <someuser>)");
+
+	capng_clear(CAPNG_SELECT_BOTH);
+	if (capng_updatev(CAPNG_ADD,
+			  CAPNG_EFFECTIVE | CAPNG_PERMITTED,
+			  CAP_NET_RAW, CAP_NET_BIND_SERVICE, -1))
+		die("capng_updatev failed");
+
+	if (geteuid() != cfg->uid) {
+		if (capng_change_id(cfg->uid,
+				    cfg->gid,
+				    CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING))
+			die("capng_change_id failed");
+	} else {
+		if (capng_apply(CAPNG_SELECT_BOTH)) {
+			capng_clear(CAPNG_SELECT_BOTH);
+			if (capng_apply(CAPNG_SELECT_BOTH))
+				die("capng_apply failed");
+		}
+		setgroups(0, NULL);
+	}
+
+	/*
+	 * Do this after caps have been dropped to make sure we're not
+	 * accessing a directory we should have permissions to.
+	 */
 	if (chdir(cfg->homedir))
-		perrordie("chdir");
+		perrordie("chdir(%s)", cfg->homedir);
 
-	return cfg;
+	if (debug_enabled(DBG_VERBOSE)) {
+		char *wd;
+
+		wd = get_current_dir_name();
+		verbose("Homedir: %s\n", wd ? wd : "<unknown>");
+		free(wd);
+	}
 }
 
 struct signalfd_ev {
@@ -467,13 +535,30 @@ main(int argc, char **argv)
 	unsigned server_count;
 
 	cfg = cfg_init(argc, argv);
-
 	cfghack = cfg;
 
+	cfg_apply(cfg);
+
 	cfg_read(cfg);
 
 	uring_init(cfg);
 
+	igmp_init(cfg);
+
+	/* Drop CAP_NET_RAW (if we have it), only used for igmp */
+	capng_clear(CAPNG_SELECT_BOTH);
+	if (capng_update(CAPNG_ADD,
+			 CAPNG_EFFECTIVE | CAPNG_PERMITTED,
+			 CAP_NET_BIND_SERVICE))
+		die("capng_update failed");
+
+	if (capng_apply(CAPNG_SELECT_BOTH)) {
+		/* Try clearing all caps, shouldn't fail */
+		capng_clear(CAPNG_SELECT_BOTH);
+		if (capng_apply(CAPNG_SELECT_BOTH))
+			die("capng_apply failed");
+	}
+
 	signalfd_init(cfg);
 
 	cfgdir_init(cfg);
@@ -482,8 +567,6 @@ main(int argc, char **argv)
 
 	announce_start(cfg->aev);
 
-	igmp_init(cfg);
-
 	uring_task_put(cfg, &cfg->task);
 
 	server_count = 0;
diff --git a/main.h b/main.h
index 0074bd9..282d827 100644
--- a/main.h
+++ b/main.h
@@ -88,6 +88,8 @@ struct uring_task {
 };
 
 struct cfg {
+	uid_t uid;
+	gid_t gid;
 	const char *homedir;
 	char *cfg_path;
 	bool do_igmp;
diff --git a/meson.build b/meson.build
index 69fd4e5..2425c45 100644
--- a/meson.build
+++ b/meson.build
@@ -2,6 +2,7 @@ project('mcproxy', 'c', default_options : ['c_std=gnu18'])
 
 liburing = dependency('liburing')
 libsystemd = dependency('libsystemd')
+libcapng = dependency('libcap-ng')
 
 mcproxy_sources = [
 		'main.c',
@@ -22,5 +23,6 @@ executable('stest', 'stest.c')
 executable('mcproxy',
 	   mcproxy_sources,
 	   dependencies : [liburing,
-			   libsystemd])
+			   libsystemd,
+			   libcapng])
 
-- 
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