ARC: mm: SIGSEGV userspace trying to access kernel virtual memory

commit a8c715b4 upstream.

As of today if userspace process tries to access a kernel virtual addres
(0x7000_0000 to 0x7ffff_ffff) such that a legit kernel mapping already
exists, that process hangs instead of being killed with SIGSEGV

Fix that by ensuring that do_page_fault() handles kenrel vaddr only if
in kernel mode.

And given this, we can also simplify the code a bit. Now a vmalloc fault
implies kernel mode so its failure (for some reason) can reuse the
@no_context label and we can remove @bad_area_nosemaphore.

Reproduce user test for original problem:

------------------------>8-----------------
 #include <stdlib.h>
 #include <stdint.h>

 int main(int argc, char *argv[])
 {
 	volatile uint32_t temp;

 	temp = *(uint32_t *)(0x70000000);
 }
------------------------>8-----------------

Cc: <stable@vger.kernel.org>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arc/mm/fault.c

@@ -66,7 +66,7 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 	struct vm_area_struct *vma = NULL;
 	struct task_struct *tsk = current;
 	struct mm_struct *mm = tsk->mm;
-	int si_code = 0;
+	int si_code = SEGV_MAPERR;
 	int ret;
 	vm_fault_t fault;
 	int write = regs->ecr_cause & ECR_C_PROTV_STORE;  /* ST/EX */
@@ -81,16 +81,14 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 	 * only copy the information from the master page table,
 	 * nothing more.
 	 */
-	if (address >= VMALLOC_START) {
+	if (address >= VMALLOC_START && !user_mode(regs)) {
 		ret = handle_kernel_vaddr_fault(address);
 		if (unlikely(ret))
-			goto bad_area_nosemaphore;
+			goto no_context;
 		else
 			return;
 	}
 
-	si_code = SEGV_MAPERR;
-
 	/*
 	 * If we're in an interrupt or have no user
 	 * context, we must not take the fault..
@@ -198,7 +196,6 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 bad_area:
 	up_read(&mm->mmap_sem);
 
-bad_area_nosemaphore:
 	/* User mode accesses just cause a SIGSEGV */
 	if (user_mode(regs)) {
 		tsk->thread.fault_address = address;