This separate processor contains closed source, proprietary binaries that have complete and unrestricted access to the host. Despite a large petition for AMD to opensource this, they refused in the end. [2][3]
Anyone running AMD chipsets have a completely separate and unaccessible operating system running on their computer that they can not control nor know exactly what it's doing. [4]
Intel has the same sort of system with it's Intel Management Engine (known as Intel ME) that even the NSA didn't want to have running on their own computers. [5]
[1] http://www.amd.com/en-us/innovations/software-technologies/s...
[2] https://www.reddit.com/r/privacy/comments/5z4phx/petition_fo...
[3] https://www.reddit.com/r/Amd/comments/6msujx/what_happened_t...
[4] https://libreboot.org/faq.html#amd
[5] https://www.theregister.co.uk/2017/08/29/intel_management_en...