Patch "arm64: mm: abort uaccess retries upon fatal signal" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

arm64: mm: abort uaccess retries upon fatal signal

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-mm-abort-uaccess-retries-upon-fatal-signal.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

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

From: Mark Rutland <[email protected]>

commit 289d07a2dc6c6b6f3e4b8a62669320d99dbe6c3d upstream.

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.

---
 arch/arm64/mm/fault.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

@@ -373,8 +373,11 @@ retry:
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+		if (!user_mode(regs))
+			goto no_context;
 		return 0;
+	}
 
 	/*
 	 * Major/minor page fault accounting is only done on the initial

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/arm64-mm-abort-uaccess-retries-upon-fatal-signal.patch queue-4.9/kvm-arm-arm64-fix-race-in-resetting-stage2-pgd.patch