Patch "userfaultfd: don't block on the last VM updates at exit time" has been added to the 4.4-stable tree

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

userfaultfd: don't block on the last VM updates at exit time

to the 4.4-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:
userfaultfd-don-t-block-on-the-last-vm-updates-at-exit-time.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Linus Torvalds <[email protected]>
Date: Tue, 1 Mar 2016 11:56:22 -0800
Subject: userfaultfd: don't block on the last VM updates at exit time

From: Linus Torvalds <[email protected]>

commit 39680f50ae54cbbb6e72ac38b8329dd3eb9105f4 upstream.

The exit path will do some final updates to the VM of an exiting process to inform others of the fact that the process is going away.

That happens, for example, for robust futex state cleanup, but also if the parent has asked for a TID update when the process exits (we clear the child tid field in user space).

However, at the time we do those final VM accesses, we've already stopped accepting signals, so the usual "stop waiting for userfaults on signal" code in fs/userfaultfd.c no longer works, and the process can become an unkillable zombie waiting for something that will never happen.

To solve this, just make handle_userfault() abort any user fault handling if we're already in the exit path past the signal handling state being dead (marked by PF_EXITING).

This VM special case is pretty ugly, and it is possible that we should look at finalizing signals later (or move the VM final accesses earlier). But in the meantime this is a fairly minimally intrusive fix.

---
 fs/userfaultfd.c |    6 ++++++
 1 file changed, 6 insertions(+)

@@ -287,6 +287,12 @@ int handle_userfault(struct vm_area_stru
 		goto out;
 
 	/*
+	 * We don't do userfault handling for the final child pid update.
+	 */
+	if (current->flags & PF_EXITING)
+		goto out;
+
+	/*
 	 * Check that we can return VM_FAULT_RETRY.
 	 *
 	 * NOTE: it should become possible to return VM_FAULT_RETRY

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

queue-4.4/revert-drm-radeon-call-hpd_irq_event-on-resume.patch queue-4.4/userfaultfd-don-t-block-on-the-last-vm-updates-at-exit-time.patch