MEDIUM: init: prevent process and thread creation at runtime

Some concerns are regularly raised about the risk to inherit some Lua
files which make use of a fork (e.g. via os.execute()) as well as
whether or not some of bugs we fix might or not be exploitable to run
some code. Given that haproxy is event-driven, any foreground activity
completely stops processing and is easy to detect, but background
activity is a different story. A Lua script could very well discretely
fork a sub-process connecting to a remote location and taking commands,
and some injected code could also try to hide its activity by creating
a process or a thread without blocking the rest of the processing. While
such activities should be extremely limited when run in an empty chroot
without any permission, it would be better to get a higher assurance
they cannot happen.

This patch introduces something very simple: it limits the number of
processes and threads to zero in the workers after the last thread was
created. By doing so, it effectively instructs the system to fail on
any fork() or clone() syscall. Thus any undesired activity has to happen
in the foreground and is way easier to detect.

This will obviously break external checks (whose concept is already
totally insecure), and for this reason a new option
"insecure-fork-wanted" was added to disable this protection, and it
is suggested in the fork() error report from the checks. It is
obviously recommended not to use it and to reconsider the reasons
leading to it being enabled in the first place.

If for any reason we fail to disable forks, we still start because it
could be imaginable that some operating systems refuse to set this
limit to zero, but in this case we emit a warning, that may or may not
be reported since we're after the fork point. Ideally over the long
term it should be conditionned by strict-limits and cause a hard fail.

doc/configuration.txt

@@ -594,6 +594,7 @@ The following keywords are supported in the "global" section :
    - hard-stop-after
    - h1-case-adjust
    - h1-case-adjust-file
+   - insecure-fork-wanted
    - log
    - log-tag
    - log-send-hostname
@@ -822,9 +823,10 @@ deviceatlas-properties-cookie <name>
   and set to DAPROPS by default if not set.
 
 external-check
-  Allows the use of an external agent to perform health checks.
-  This is disabled by default as a security precaution.
-  See "option external-check".
+  Allows the use of an external agent to perform health checks. This is
+  disabled by default as a security precaution, and even when enabled, checks
+  may still fail unless "insecure-fork-wanted" is enabled as well.
+  See "option external-check", and "insecure-fork-wanted".
 
 gid <number>
   Changes the process' group ID to <number>. It is recommended that the group
@@ -903,6 +905,24 @@ h1-case-adjust-file <hdrs-file>
   See "h1-case-adjust", "option h1-case-adjust-bogus-client" and
   "option h1-case-adjust-bogus-server".
 
+insecure-fork-wanted
+  By default haproxy tries hard to prevent any thread and process creation
+  after it starts. Doing so is particularly important when using Lua files of
+  uncertain origin, and when experimenting with development versions which may
+  still contain bugs whose exploitability is uncertain. And generally speaking
+  it's good hygiene to make sure that no unexpected background activity can be
+  triggered by traffic. But this prevents external checks from working, and may
+  break some very specific Lua scripts which actively rely on the ability to
+  fork. This option is there to disable this protection. Note that it is a bad
+  idea to disable it, as a vulnerability in a library or within haproxy itself
+  will be easier to exploit once disabled. In addition, forking from Lua or
+  anywhere else is not reliable as the forked process may randomly embed a lock
+  set by another thread and never manage to finish an operation. As such it is
+  highly recommended that this option is never used and that any workload
+  requiring such a fork be reconsidered and moved to a safer solution (such as
+  agents instead of external checks). This option supports the "no" prefix to
+  disable it.
+
 log <address> [len <length>] [format <format>] [sample <ranges>:<smp_size>]
     <facility> [max level [min level]]
   Adds a global syslog server. Several global servers can be defined. They

include/types/global.h

@@ -74,6 +74,7 @@
 #define GTUNE_SET_DUMPABLE       (1<<13)
 #define GTUNE_USE_EVPORTS        (1<<14)
 #define GTUNE_STRICT_LIMITS      (1<<15)
+#define GTUNE_INSECURE_FORK      (1<<16)
 
 /* SSL server verify mode */
 enum {

src/cfgparse-global.c

@@ -94,6 +94,14 @@ int cfg_parse_global(const char *file, int linenum, char **args, int kwm)
 		else
 			global.tune.options |=  GTUNE_SET_DUMPABLE;
 	}
+	else if (!strcmp(args[0], "insecure-fork-wanted")) { /* "no insecure-fork-wanted" or "insecure-fork-wanted" */
+		if (alertif_too_many_args(0, file, linenum, args, &err_code))
+			goto out;
+		if (kwm == KWM_NO)
+			global.tune.options &= ~GTUNE_INSECURE_FORK;
+		else
+			global.tune.options |=  GTUNE_INSECURE_FORK;
+	}
 	else if (!strcmp(args[0], "nosplice")) {
 		if (alertif_too_many_args(0, file, linenum, args, &err_code))
 			goto out;

src/cfgparse.c

@@ -2160,10 +2160,11 @@ next_line:
 
 		if (kwm != KWM_STD && strcmp(args[0], "option") != 0 &&
 		    strcmp(args[0], "log") != 0 && strcmp(args[0], "busy-polling") != 0 &&
-		    strcmp(args[0], "set-dumpable") != 0 && strcmp(args[0], "strict-limits") != 0) {
+		    strcmp(args[0], "set-dumpable") != 0 && strcmp(args[0], "strict-limits") != 0 &&
+		    strcmp(args[0], "insecure-fork-wanted") != 0) {
 			ha_alert("parsing [%s:%d]: negation/default currently "
 				 "supported only for options, log, busy-polling, "
-				 "set-dumpable and strict-limits.\n", file, linenum);
+				 "set-dumpable, strict-limits, and insecure-fork-wanted.\n", file, linenum);
 			err_code |= ERR_ALERT | ERR_FATAL;
 		}
 
@@ -2549,6 +2550,11 @@ int check_config_validity()
 					 curproxy->id, "option external-check");
 				cfgerr++;
 			}
+			if (!(global.tune.options & GTUNE_INSECURE_FORK)) {
+				ha_warning("Proxy '%s' : 'insecure-fork-wanted' not enabled in the global section, '%s' will likely fail.\n",
+					 curproxy->id, "option external-check");
+				err_code |= ERR_WARN;
+			}
 		}
 
 		if (curproxy->email_alert.set) {

src/checks.c

@@ -2012,7 +2012,9 @@ static int connect_proc_chk(struct task *t)
 
 	pid = fork();
 	if (pid < 0) {
-		ha_alert("Failed to fork process for external health check: %s. Aborting.\n",
+		ha_alert("Failed to fork process for external health check%s: %s. Aborting.\n",
+			 (global.tune.options & GTUNE_INSECURE_FORK) ?
+			 "" : " (likely caused by missing 'insecure-fork-wanted')",
 			 strerror(errno));
 		set_server_check_status(check, HCHK_STATUS_SOCKERR, strerror(errno));
 		goto out;

src/haproxy.c

@@ -2749,6 +2749,23 @@ static void *run_thread_poll_loop(void *data)
 	pthread_mutex_unlock(&init_mutex);
 #endif
 
+#if defined(RLIMIT_NPROC)
+	/* all threads have started, it's now time to prevent any new thread
+	 * or process from starting. Obviously we do this in workers only. We
+	 * can't hard-fail on this one as it really is implementation dependent
+	 * though we're interested in feedback, hence the warning.
+	 */
+	if (!(global.tune.options & GTUNE_INSECURE_FORK) && !master) {
+		struct rlimit limit = { .rlim_cur = 0, .rlim_max = 0 };
+		static int warn_fail;
+
+		if (setrlimit(RLIMIT_NPROC, &limit) == -1 && !_HA_ATOMIC_XADD(&warn_fail, 1)) {
+			ha_warning("Failed to disable forks, please report to developers with detailed "
+				   "information about your operating system. You can silence this warning "
+				   "by adding 'insecure-fork-wanted' in the 'global' section.\n");
+		}
+	}
+#endif
 	run_poll_loop();
 
 	list_for_each_entry(ptdf, &per_thread_deinit_list, list)