Implement libcap support

Use libcap for saving CAP_NET_ADMIN (if --transparent is given) over a
setuid(). We don’t need CAP_NET_BIND_SERVICE as the listening sockets
are established before dropping root.

Makefile

@@ -3,6 +3,7 @@
 VERSION=$(shell ./genver.sh -r)
 USELIBCONFIG=1	# Use libconfig? (necessary to use configuration files)
 USELIBWRAP=	# Use libwrap?
+USELIBCAP=	# Use libcap?
 COV_TEST= 	# Perform test coverage?
 PREFIX=/usr/local
 
@@ -31,6 +32,11 @@ ifneq ($(strip $(USELIBCONFIG)),)
 	CPPFLAGS+=-DLIBCONFIG
 endif
 
+ifneq ($(strip $(USELIBCAP)),)
+	LIBS:=$(LIBS) -lcap
+	CPPFLAGS+=-DLIBCAP
+endif
+
 all: sslh $(MAN) echosrv
 
 .c.o: *.h

README

@@ -157,16 +157,21 @@ in inetd mode.
 sslh options: -i for inetd mode, --http to forward http
 connexions to port 80, and SSH connexions to port 22.
 
-==== capapbilities support ====
+==== capabilities support ====
 
-On Linux (only?), you can use POSIX capabilities to reduce a
+On Linux (only?), you can compile sslh with USELIBCAP=1 to
-server's capabilities to the minimum it needs (see
+make use of POSIX capabilities; this will save the required
-capabilities(8).  For sslh, this is CAP_NET_ADMIN (to
+capabilities needed for transparent proxying for unprivileged
-perform transparent proxy-ing) and CAP_NET_BIND_SERVICE (to
+processes.
-bind to port 443 without being root).
 
-The simplest way to use capabilities is to give them to the
+Alternatively, you may use filesystem capabilities instead
-executable as root:
+of starting sslh as root and asking it to drop privileges.
+You will need CAP_NET_BIND_SERVICE for listening on port 443
+and CAP_NET_ADMIN for transparent proxying (see
+capabilities(7)).
+
+You can use the setcap(8) utility to give these capabilities
+to the executable:
 
 # setcap cap_net_bind_service,cap_net_admin+pe sslh-select
 
@@ -174,13 +179,9 @@ Then you can run sslh-select as an unpriviledged user, e.g.:
 
 $ sslh-select -p myname:443 --ssh localhost:22 --ssl localhost:443
 
-This has 2 advantages over starting as root with -u:
-- You no longer start as root (duh)
-- This enables transparent proxying.
-
 Caveat: CAP_NET_ADMIN does give sslh too many rights, e.g.
 configuring the interface. If you're not going to use
-transparent proxying, just don't use it.
+transparent proxying, just don't use it (or use the libcap method).
 
 ==== Transparent proxy support ====
 
@@ -192,8 +193,8 @@ simplifies IP-based access control (or makes it possible at
 all).
 
 sslh needs extended rights to perform this: you'll need to
-give it cap_net_admin capabilities (see appropriate chapter)
+give it cap_net_admin capabilities manually or enable libcap
-or run it as root (but don't do that).
+support (see appropriate chapter).
 
 The firewalling tables also need to be adjusted as follow
 (example to connect to https on 4443 -- adapt to your needs

common.c

@@ -37,6 +37,10 @@ struct addrinfo *addr_listen = NULL; /* what addresses do we listen to? */
 int allow_severity =0, deny_severity = 0;
 #endif
 
+#ifdef LIBCAP
+#include <sys/prctl.h>
+#include <sys/capability.h>
+#endif
 
 /* check result and die, printing the offending address and error */
 void check_res_dumpdie(int res, struct addrinfo *addr, char* syscall)
@@ -509,6 +513,56 @@ void setup_syslog(const char* bin_name) {
     log_message(LOG_INFO, "%s %s started\n", server_type, VERSION);
 }
 
+/* Ask OS to keep capabilities over a setuid(nonzero) */
+void set_keepcaps(int val) {
+#ifdef LIBCAP
+    int res;
+    res = prctl(PR_SET_KEEPCAPS, val, 0, 0, 0);
+    if (res) {
+        perror("prctl");
+        exit(1);
+    }
+#endif
+}
+
+/* set needed capabilities for effective and permitted, clear rest */
+void set_capabilities(void) {
+#ifdef LIBCAP
+    int res;
+    cap_t caps;
+    cap_value_t cap_list[10];
+    int ncap = 0;
+
+    if (transparent)
+        cap_list[ncap++] = CAP_NET_ADMIN;
+
+    caps = cap_init();
+
+#define _cap_set_flag(flag) do { \
+        res = cap_clear_flag(caps, flag); \
+        CHECK_RES_DIE(res, "cap_clear_flag(" #flag ")"); \
+        if (ncap > 0) { \
+            res = cap_set_flag(caps, flag, ncap, cap_list, CAP_SET); \
+            CHECK_RES_DIE(res, "cap_set_flag(" #flag ")"); \
+        } \
+    } while(0)
+
+    _cap_set_flag(CAP_EFFECTIVE);
+    _cap_set_flag(CAP_PERMITTED);
+
+#undef _cap_set_flag
+
+    res = cap_set_proc(caps);
+    CHECK_RES_DIE(res, "cap_set_proc");
+
+    res = cap_free(caps);
+    if (res) {
+        perror("cap_free");
+        exit(1);
+    }
+#endif
+}
+
 /* We don't want to run as root -- drop privileges if required */
 void drop_privileges(const char* user_name)
 {
@@ -521,10 +575,15 @@ void drop_privileges(const char* user_name)
     if (verbose)
         fprintf(stderr, "turning into %s\n", user_name);
 
+    set_keepcaps(1);
+
     res = setgid(pw->pw_gid);
     CHECK_RES_DIE(res, "setgid");
     res = setuid(pw->pw_uid);
     CHECK_RES_DIE(res, "setuid");
+
+    set_capabilities();
+    set_keepcaps(0);
 }
 
 /* Writes my PID */