xtensa: fix uaccess-related livelock in do_page_fault

If a uaccess (e.g. get_user()) triggers a fault and there's a fault signal pending, the handler will return to the uaccess without having performed a uaccess fault fixup, and so the CPU will immediately execute the uaccess instruction again, whereupon it will livelock bouncing between that instruction and the fault handler.

https://lore.kernel.org/lkml/[email protected]/

---
 arch/xtensa/mm/fault.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 7666408ce12a..95a74890c7e9 100644
@@ -112,8 +112,11 @@ void do_page_fault(struct pt_regs *regs)
 	 */
 	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 bad_page_fault;
 		return;
+	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)

2.20.1