Kernel OOPS "Unable to handle kernel NULL pointer dereference"
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I don't think your module is fine if it's causing that problem. If this error occurs after the module is unloaded, you probably want to check your __exit routine to make sure that it's not screwing up any data structures in kernel space. Just out of curiousity, why can't you use the kernel routines copy_to_user and copy_from_user to pass data back and forth? Particularly if you don't have a lot of data to copy, that would probably be a lot safer.
On kmalloc(), if I don't put in GFP_DMA, it gives me random Segmentation faults with a lot of registers & whatnot (I don't know what those are called...) but it doesn't crash. It also works normally when unloaded.