uvm: prevent TLB invalidation races during COW resolution

When a thread takes a page fault which results in COW resolution,
other threads in the same process can be concurrently accessing that
same mapping on other CPUs.  When the faulting thread updates the pmap
entry at the end of COW processing, the resulting TLB invalidations to
other CPUs are not done atomically, so another thread can write to the
new writable page and then a third thread might still read from the
old read-only page, resulting in inconsistent views of the page by the
latter two threads.  Fix this by removing the pmap entry entirely for
the original page before we install the new pmap entry for the new
page, so that the new page can only be modified after the old page is
no longer accessible.

This fixes PR 56535 as well as the netbsd versions of problems
described in various bug trackers:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225584
https://reviews.freebsd.org/D14347
golang/go#34988

Author: chs

sys/uvm/uvm_fault.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: uvm_fault.c,v 1.233 2023/07/17 12:55:37 riastradh Exp $	*/
+/*	$NetBSD: uvm_fault.c,v 1.234 2023/08/13 23:06:07 chs Exp $	*/
 
 /*
  * Copyright (c) 1997 Charles D. Cranor and Washington University.
@@ -32,7 +32,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: uvm_fault.c,v 1.233 2023/07/17 12:55:37 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: uvm_fault.c,v 1.234 2023/08/13 23:06:07 chs Exp $");
 
 #include "opt_uvmhist.h"
 
@@ -634,8 +634,17 @@ uvmfault_promote(struct uvm_faultinfo *ufi,
 		goto done;
 	}
 
-	/* copy page [pg now dirty] */
+	/*
+	 * copy the page [pg now dirty]
+	 *
+	 * Remove the pmap entry now for the old page at this address
+	 * so that no thread can modify the new page while any thread
+	 * might still see the old page.
+	 */
 	if (opg) {
+		pmap_remove(vm_map_pmap(ufi->orig_map), ufi->orig_rvaddr,
+			     ufi->orig_rvaddr + PAGE_SIZE);
+		pmap_update(vm_map_pmap(ufi->orig_map));
 		uvm_pagecopy(opg, pg);
 	}
 	KASSERT(uvm_pagegetdirty(pg) == UVM_PAGE_STATUS_DIRTY);