reversed bias diode on GPIO
The diodes are protection devices against short transients and ESD.

If the pin is pushed higher than system voltage the diode clamps the pin to about 0.7v above system voltage. Same goes for the diode to ground. If the pin is pulled lower than ground then the diode clamps the pin to -0.7v. 

In both cases the internals of the processor can withstand 0.7v above VDD and 0.7v below VSS and are protected. This does not mean that you can apply 10v to a 3.3v processor and expect it to survive.