riscv/mm: Prevent kernel module to access user memory without uaccess routines

We found this issue in an legacy out-of-tree kernel module
which didn't properly access user space pointer by get/put_user().
Such an illegal access loops in the page fault handler.
To resolve this, let it die here.

Signed-off-by: Eric Lin <[e-mail]>
Reviewed-by: Pekka Enberg <[e-mail]>
Signed-off-by: Palmer Dabbelt <[e-mail]>

arch/riscv/mm/fault.c

···
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 
+	if (!user_mode(regs) && addr < TASK_SIZE &&
+			unlikely(!(regs->status & SR_SUM)))
+		die_kernel_fault("access to user memory without uaccess routines",
+				addr, regs);
+
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
 
 	if (cause == EXC_STORE_PAGE_FAULT)