Some question about "security context of a task"

hi all:
I found there is a user_struct, user, located at struct cred.
is it possible be null?
My platform sometimes panic when using kernel_thread to creating a
thread for running.

below is the place it happen

if (atomic_read(&p->real_cred->user->processes) >=
             p->signal->rlim[RLIMIT_NPROC].rlim_cur) {
after checking the parameter, I found p->real_cred->user is NULL.

When and how we allocate/destroy this parameter?
Does that mean we should some basic check before calling kernel_thread?

-- 
Regards,