FAILED: patch "[PATCH] arm64: mm: abort uaccess retries upon fatal signal" failed to apply to 4.12-stable tree

The patch below does not apply to the 4.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Mark Rutland <[email protected]>
Date: Tue, 11 Jul 2017 15:19:22 +0100
Subject: [PATCH] arm64: mm: abort uaccess retries upon fatal signal

When there's a fatal signal pending, arm64's do_page_fault() implementation returns 0. 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.

index 2509e4fe6992..1f22a41565a3 100644
@@ -435,8 +435,11 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 		 * the mmap_sem because it would already be released
 		 * in __lock_page_or_retry in mm/filemap.c.
 		 */
-		if (fatal_signal_pending(current))
+		if (fatal_signal_pending(current)) {
+			if (!user_mode(regs))
+				goto no_context;
 			return 0;
+		}
 
 		/*
 		 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk of