Unable to access Plesk panel: 502 Bad Gateway. [pool plesk] child exited on signal 9 (SIGKILL)

Symptoms

• Unable to access Plesk panel with the following error:

  Server Error
  502
  Bad Gateway
  Web server received an invalid response while acting as a gateway or proxy server.

• The following error can be found in /var/log/sw-cp-server/error_log:

  [error] 7832#0: *1 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 203.0.113.2, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock:", host: "example.com:8443"

  And in /var/log/sw-cp-server/sw-engine.log:

  WARNING: [pool plesk] child 7905 exited on signal 9 (SIGKILL) after 7.914415 seconds from start
  NOTICE: [pool plesk] child 8028 started

• The following entries are found in /var/log/messages:

  Nov 20 21:42:53 server kernel: PAX: execution attempt in: <anonymous mapping>, 37a606be000-37a606ce000 37a606be000
  Nov 20 21:42:53 server kernel: PAX: terminating task: /usr/sbin/sw-engine-fpm(sw-engine-fpm):5973, uid/euid: 998/998, PC: 0000037a606be010, SP: 000003ba76df64d8

Cause

3rd party Atomic Secured Linux Installation (ASL) is interfering with the Plesk sw-engine

Resolution

1. Connect to the server via SSH

2. Stop the sw-engine service:

   # systemctl stop sw-engine

3. Find the sw-engine processes and kill them:

   # ps aux | grep sw-engine | grep -v grep
   psaadm 8133 0.0 0.1 369784 43364 ? Ss 21:50 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini -dauto_prepend_file=sdk.php /usr/local/psa/admin/plib/modules/plesk-mobile/scripts/push_worker.php
   psaadm 31554 0.0 0.1 373880 48088 ? S 21:27 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/bin/extension –exec revisium-antivirus ra_executor.php
   psaadm 31557 0.0 0.1 369788 45508 ? S 21:27 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini -dauto_prepend_file=sdk.php /usr/local/psa/admin/plib/modules/revisium-antivirus/scripts/ra_executor.php

   # kill 8133 31554 31557

4. Set MPROTECT flag to false for sw-engine:

   # paxctl -cm /usr/sbin/sw-engine-fpm
   # paxctl -cm /usr/bin/sw-engine

5. Start the sw-engine service:

   # systemctl start sw-engine