Hackable debug mode found in AMD CPUs

A security researcher has discovered that some AMD processors have a special debugging mechanism that allows crackers to take a peek at how the chips work – and potentially reverse engineer them.

The researcher, known as Czerno, discovered that the debugging features, which are found in Athlon XP chips and those further up, can be used to expand an AMD CPU’s performance over standard, architecturally defined capabilities of x86.

Breaking into the CPU is not easy. According to Softpedia, the researcher had to check out the undocumented machine specific registers (MSRs), where certain hints regarding its existence were found.

According to Czerno, there are at least four new machine specific registers (MSR) that are involved in the expanded debug facilities, all of which are password-protected against access.

The read/write access (RDMSR/WRMSR) to the registers is granted only if EDI holds the correct password value, viz. EDI=9C5A203A, because, otherwise, a GPF exception occurs.

Czerno posted an updated comment on his website where he said: “Hardly was this page released, the news somehow spread like a fire : as a result this page and the entire site disappeared from the web for a few hours, /slash-dotted/ :=) News and speculations about the hidden feature are all over the web now, and that is good in my book : hopefully we’ll see it applied in application and/or kernel debuggers – Linux anybody?”.