fping: retain privileges until after privileged setsockopt

On Linux, one needs privileges to setsockopt(SO_BINDTODEVICE), and the
current code would drop setuid root privileges too soon.

Temporarily drop privileges instead, and raise them back to issue the
privileged setsockopt().  Once we know we won't need to do any further
privileged setsockopt(), permanently drop privileges.

For now, assume SO_BINDTODEVICE is the only setsockopt that needs this.

src/fping.c

@@ -395,6 +395,42 @@ struct event *host_get_timeout_event(HOST_ENTRY *h, int index);
 void stats_add(HOST_ENTRY *h, int index, int success, int64_t latency);
 void update_current_time();
 
+/************************************************************
+
+  Function: p_setsockopt
+
+*************************************************************
+
+  Inputs:  p_uid: privileged uid. Others as per setsockopt(2)
+
+  Description:
+
+  Elevates privileges to p_uid when required, calls
+  setsockopt, and drops privileges back.
+
+************************************************************/
+
+int p_setsockopt(uid_t p_uid, int sockfd, int level, int optname,
+	const void *optval, socklen_t optlen)
+{
+    const uid_t saved_uid = geteuid();
+    int res;
+
+    if (p_uid != saved_uid && seteuid(p_uid)) {
+	perror("cannot elevate privileges for setsockopt");
+    }
+
+    res = setsockopt(sockfd, level, optname, optval, optlen);
+
+    if (p_uid != saved_uid && seteuid(saved_uid)) {
+	perror("fatal error: could not drop privileges after setsockopt");
+	/* continuing would be a security hole */
+	exit(4);
+    }
+
+    return res;
+}
+
 /************************************************************
 
   Function: main
@@ -412,7 +448,7 @@ void update_current_time();
 int main(int argc, char** argv)
 {
     int c;
-    uid_t uid;
+    const uid_t suid = geteuid();
     int tos = 0;
     struct optparse optparse_state;
 #ifdef USE_SIGACTION
@@ -448,9 +484,9 @@ int main(int argc, char** argv)
     memset(&src_addr6, 0, sizeof(src_addr6));
 #endif
 
-    if ((uid = getuid())) {
-        /* drop privileges */
-        if (setuid(getuid()) == -1)
+    if (!suid && suid != getuid()) {
+        /* *temporarily* drop privileges */
+        if (seteuid(getuid()) == -1)
             perror("cannot setuid");
     }
 
@@ -740,14 +776,14 @@ int main(int argc, char** argv)
         case 'I':
 #ifdef SO_BINDTODEVICE
             if (socket4 >= 0) {
-                if (setsockopt(socket4, SOL_SOCKET, SO_BINDTODEVICE, optparse_state.optarg, strlen(optparse_state.optarg))) {
+                if (p_setsockopt(suid, socket4, SOL_SOCKET, SO_BINDTODEVICE, optparse_state.optarg, strlen(optparse_state.optarg))) {
                     perror("binding to specific interface (SO_BINTODEVICE)");
                     exit(1);
                 }
             }
 #ifdef IPV6
             if (socket6 >= 0) {
-                if (setsockopt(socket6, SOL_SOCKET, SO_BINDTODEVICE, optparse_state.optarg, strlen(optparse_state.optarg))) {
+                if (p_setsockopt(suid, socket6, SOL_SOCKET, SO_BINDTODEVICE, optparse_state.optarg, strlen(optparse_state.optarg))) {
                     perror("binding to specific interface (SO_BINTODEVICE), IPV6");
                     exit(1);
                 }
@@ -796,6 +832,13 @@ int main(int argc, char** argv)
         }
     }
 
+    /* permanetly drop privileges */
+    if (suid != getuid() && setuid(getuid())) {
+	perror("fatal: failed to permanently drop privileges");
+	/* continuing would be a security hole */
+	exit(4);
+    }
+
     /* validate various option settings */
 
 #ifndef IPV6