webui: rate-limit POST /login via nginx limit_req

Adds an nginx-side rate limit on POST /login so a brute-force or DoS
attempt can't pin the box on the bcrypt-shaped credential check inside
rousette/PAM.  Configuration:

- Keyed on $binary_remote_addr (per source IP), 5 requests/minute
  sustained with a burst of 3 nodelay, a real user fumbling their
  password three times in a row sails through; the 4th attempt in
  the same window returns 429.
- GET /login is unmetered (the keyed map yields the empty string,
  which limit_req treats as "no key, no limit"), so a 401-driven
  HX-Redirect bouncing the user back to the login page doesn't
  eat into the budget.
- 32k zone (~500 IPs, the minimum nginx accepts).  At cache
  saturation an attacker rotating addresses still tops out around
  ~2500 attempts/min, all serialised through bcrypt one at a time.
- 429 instead of the default 503 keeps the response out of the
  /50x.html error_page rewrite (which auto-refreshes and would
  loop a throttled client straight back to /login).

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

package/webui/default.conf

@@ -1,3 +1,23 @@
+# Throttle POST /login so a brute-force attempt can't pin the box on
+# the bcrypt-shaped credential check inside rousette/PAM. GET is left
+# unmetered: page loads after a 401-driven HX-Redirect shouldn't eat
+# into the budget.
+#
+# Zone is 32k (~500 IPs) — the minimum nginx accepts via shared-memory
+# allocation. With the cache full an attacker rotating source addresses
+# still tops out at ~2500 attempts/min total, all serialised through
+# bcrypt one at a time.
+#
+# 429 instead of the default 503 keeps the rate-limit response out of
+# the /50x.html error_page rewrite below (which auto-refreshes — would
+# loop a throttled client straight back to /login).
+map $request_method $webui_login_key {
+    POST    $binary_remote_addr;
+    default "";
+}
+limit_req_zone   $webui_login_key zone=webui_login:32k rate=5r/m;
+limit_req_status 429;
+
 server {
     listen       80;
     listen       [::]:80;

package/webui/webui.conf

@@ -8,6 +8,13 @@ location / {
     include /etc/nginx/webui-proxy.conf;
 }
 
+# burst=3 nodelay: three POSTs land back-to-back; the next needs a
+# fresh token (~12 s at 5r/m) or 429s.
+location = /login {
+    limit_req zone=webui_login burst=3 nodelay;
+    include /etc/nginx/webui-proxy.conf;
+}
+
 # Keep proxy_request_buffering on (the default). Streaming the body races
 # the Go handler's response close, producing RST instead of FIN at
 # Content-Length and a client-visible 502. nginx spools the body to