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.
diff --git a/src/checks.c b/src/checks.c
index 247caf1..909bd52 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -2012,7 +2012,9 @@
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;