Christopher Domas has
published a proof of concept with a description showing how to use AMD memory
controllers' bank swizzle mode to bypass memory protection and read or write
arbitrary data, including CPU microcode definitions and memory belonging to the 

platform security processor. Among other things, this allows code running at the
kernel level to directly manipulate the meaning of processor instructions,
potentially bypassing other security measures such as memory encryption and
virtual machine isolation.

This is not, strictly, unexpected behavior: it is 

documented in AMD's manual (on page 113 of that PDF). But the fact that it can
be used to access arbitrary memory and thereby rewrite supposedly immutable
parts of the computer's firmware without crashing the host machine seems like an
unintentional side-effect of the design. Fortunately, since enabling bank
swizzle mode requires kernel-level privileges, the vulnerability is not an
immediate problem for most software. Still, it seems likely that this technique
will end up being used for nefarious purposes eventually.

https://lwn.net/Articles/1088778/
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