autoconf(9): Avoid potential ABA bug in config_makeroom.

When we unlock alldevs_lock to allocate a new cd_devs array nsp,
other threads may have:

1. freed the old one (osp),
2. done some other memory allocation,
3. allocated a new _larger_ array whose address happens to concide
   with osp (e.g., in (2) the page was recycled for a different pool
   cache), and
4. updated cd_devs back to osp but increased cd_ndevs.

In that case, the memory may be corrupted: we try to copy the wrong
number of device_t pointers into nsp and we free osp with the wrong
(stale) length.

Avoid this by checking whether cd_ndevs has changed too -- if not,
osp might have been recycled but at least the lengths we're about to
copy and free are still correct so there's no harm in an ABA
situation.

XXX pullup-8
XXX pullup-9
XXX pullup-10


(riastradh)

@@ -1,4 +1,4 @@
-/* $NetBSD: subr_autoconf.c,v 1.307 2023/02/22 17:00:16 riastradh Exp $ */
+/* $NetBSD: subr_autoconf.c,v 1.308 2023/04/16 11:14:58 riastradh Exp $ */
 
 /*
  * Copyright (c) 1996, 2000 Christopher G. Demetriou
@@ -77,7 +77,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_autoconf.c,v 1.307 2023/02/22 17:00:16 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_autoconf.c,v 1.308 2023/04/16 11:14:58 riastradh Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_ddb.h"
@@ -1357,7 +1357,7 @@
 		 * If another thread moved the array while we did
 		 * not hold alldevs_lock, try again.
 		 */
-		if (cd->cd_devs != osp) {
+		if (cd->cd_devs != osp || cd->cd_ndevs != ondevs) {
 			mutex_exit(&alldevs_lock);
 			kmem_free(nsp, sizeof(device_t) * nndevs);
 			mutex_enter(&alldevs_lock);