Custom kernel syscall module & data validation

[Kentucky Mandeloid Mo.]

Hi, hackers!
I wrote a small kernel module whch impliments some syscall.
Syscall takes a char * as a parameter.
Theoriticaly its posible to pass a bad pointer from userland.
What should I do in orde to check the pointer from userland?
I use copyout(9) to store data from kernel to userland but it doesn't seams to 
be enough - systems got panic if I pass a bad pounter.
I think I'm missing something important about kernel programming.