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/20210121123140.GD48431@C02TD0UTHF1T.local/

Cc: stable@vger.kernel.org
Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index 7666408..95a7489 100644 (file)
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -112,8 +112,11 @@ good_area:
         */
        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)