Symptoms

Unable to access Plesk: 502 Bad Gateway error is shown.

In /var/log/sw-cp-server/error_log the following error is shown:

2017/02/25 11:21:58 [error] 29398#0: *11 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: xxx.xxx.xxx.xxx, server: , request: "POST /login_up.php3 HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock:", host: "xxx.xxx.xxx.xxx:8443", referrer: "https://xxx.xxx.xxx.xxx:8443/login_up.php3"

In /var/log/syslog or in /var/log/messages the following message can be found:

Feb 25 12:55:21 server kernel: grsec: From xxx.xxx.xxx.xxx: process /usr/bin/sw-engine(sw-engine:3782) attached to via ptrace by /usr/bin/sw-engine[sw-engine:3785] uid/euid:0/0 gid/egid:0/0, parent /usr/bin/sw-engine[sw-engine:3782] uid/euid:0/0 gid/egid:0/0
Feb 25 12:55:22 server kernel: grsec: From xxx.xxx.xxx.xxx: denied ptrace of /usr/bin/sw-engine(sw-engine:3793) by /usr/bin/sw-engine[sw-engine:3796] uid/euid:999/0 gid/egid:1000/1000, parent /usr/bin/sw-engine[sw-engine:3793] uid/euid:999/0 gid/egid:1000/1000

OR

Unable to install any application, it hangs on 0% or 1%.

Unable to update plugins.

Cause

OS uses grsec kernel with enabled ptrace which does not allow Plesk work normally

Resolution

Disable ptrace by adding the following line into /etc/sysctl.conf :

kernel.grsecurity.harden_ptrace = 0

Reload system variables:

# sysctl -p