Reverse bias I-V characteristics of mosfet
Basic threshold model-- 

It is in the triode region for Vgs > Vth, regardless of the direction (and |Vds| low). 

There is also a body diode in parallel so if -Vds is more than than a few hundred mV that must be taken into account as the diode will conduct. 

If you want more accurate subthreshold modeling, someone has probably done it, but this suffices for many or most applications where the transistor is reverse biased.