gdb attach fails with ptrace: Operation not permitted

Today I ran into a weird problem. I could not attach to my own process with gdb. The process ran under my UID, but gdb refused to attach. This is a problem of wrong permissions, although /proc/[pid]/status looked ok:

Uid:    1000    1000    1000    1000
Gid:    1000    1000    1000    1000

I am the owner but cannot attach? Well, I launched gdb as root and could attach. Strange. Without digging deeper into this, my dirty workaround was this:

sudo chmod +s /usr/bin/gdb

Update: Thanks to Mario, who pointed out, that the reason is the Kernel hardening stuff build into the Ubuntu kernel. See his comment how to fix the problem permanently.