MIPS: mm: abort uaccess retries upon fatal signal

When there's a fatal signal pending, MIPS's do_page_fault()
implementation returns. The intent is that we'll return to the
faulting userspace instruction, delivering the signal on the way.

However, if we take a fatal signal during fixing up a uaccess, this
results in a return to the faulting kernel instruction, which will be
instantly retried, resulting in the same fault being taken forever. As
the task never reaches userspace, the signal is not delivered, and the
task is left unkillable. While the task is stuck in this state, it can
inhibit the forward progress of the system.

To avoid this, we must ensure that when a fatal signal is pending, we
apply any necessary fixup for a faulting kernel instruction. Thus we
will return to an error path, and it is up to that code to make forward
progress towards delivering the fatal signal.

[ Description taken from commit 746a272e ("ARM: 8692/1: mm: abort
   uaccess retries upon fatal signal") ]
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>

arch/mips/mm/fault.c

@@ -156,8 +156,11 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	 */
 	fault = handle_mm_fault(vma, address, flags, regs);
 
-	if (fault_signal_pending(fault, regs))
+	if (fault_signal_pending(fault, regs)) {
+		if (!user_mode(regs))
+			goto no_context;
 		return;
+	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)