Replace alien FD's with /dev/null rather than just closing them

When we fork the worker process, we close all filedescriptors we
have not explictly marked for it to inherit, for security reasons.

Operating system libraries may have open filedescriptors (see
end*ent(3)) and there is no way to chase these down.

At least on OSX something related to DNS lookups leaves such
a FD around, and when that code later discovers the FD doesn't
work, it closes it, even though it no longer owns it.

In ticket 1841, that happens to be FD7 which is one of our kqueue FDs.

Normally such library code should set 'close-on-exec' status with
fcntl(2) but that doesn't seem to be the case here, and this bit
of wisdom seems neglegted about 50/50, so it probably wouldn't
help us to examine this.

The fix here is to close the FDs, and replace them with a FD open
to /dev/null, so that there is no risk of information leak, but we
don't reuse the FD for something else until the library has properly
closed it.

Fixes #1841

bin/varnishd/mgt/mgt_child.c

@@ -295,7 +295,7 @@ mgt_launch_child(struct cli *cli)
 	unsigned u;
 	char *p;
 	struct vev *e;
-	int i, cp[2];
+	int i, j, k, cp[2];
 	struct sigaction sa;
 
 	if (child_state != CH_STOPPED && child_state != CH_DIED)
@@ -351,13 +351,29 @@ mgt_launch_child(struct cli *cli)
 		assert(dup2(heritage.std_fd, STDOUT_FILENO) == STDOUT_FILENO);
 		assert(dup2(heritage.std_fd, STDERR_FILENO) == STDERR_FILENO);
 
-		/* Close anything we shouldn't know about */
+		/*
+		 * Close all FDs the child shouldn't know about
+		 *
+		 * We cannot just close these filedescriptors, some random
+		 * library routine might miss it later on and wantonly close
+		 * a FD we use at that point in time. (See bug #1841).
+		 * We close the FD and replace it with /dev/null instead,
+		 * That prevents security leakage, and gives the library
+		 * code a valid FD to close when it discovers the changed
+		 * circumstances.
+		 */
 		closelog();
 
 		for (i = STDERR_FILENO + 1; i < CLOSE_FD_UP_TO; i++) {
 			if (vbit_test(fd_map, i))
 				continue;
-			(void)(close(i) == 0);
+			if (close(i) == 0) {
+				k = open("/dev/null", O_RDONLY);
+				assert(k >= 0);
+				j = dup2(k, i);
+				assert(j == i);
+				AZ(close(k));
+			}
 		}
 #ifdef HAVE_SETPROCTITLE
 		setproctitle("Varnish-Chld %s", heritage.name);