[src/trunk]: src/sys/kern Kernel Heap Hardening: put the pool header at the b...

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/8845ae93e019
branches:  trunk
changeset: 449714:8845ae93e019
user:      maxv <maxv%NetBSD.org@localhost>
date:      Sun Mar 17 15:33:50 2019 +0000

description:
Kernel Heap Hardening: put the pool header at the beginning of the backing
page, not at the end of it.

This makes it harder to exploit buffer overflows, because it eliminates the
certainty that sensitive kernel data is located after the item space and is
therefore overwritable.

The pr_itemoffset field is recycled, and holds the (aligned) offset of the
item space. The pr_phoffset field becomes unused. We align 'itemspace' for
clarity, but it's not strictly necessary.

This comes with no performance cost or increase in memory usage, in
particular the potential padding consumed by roundup(PHSIZE, align) was
already implicitly consumed before, because of the (necessary) truncations
in the divisions. Now it's just more explicit, but not bigger.

diffstat:

 sys/kern/subr_pool.c |  25 ++++++++++++++-----------
 1 files changed, 14 insertions(+), 11 deletions(-)

diffs (80 lines):

diff -r 360a70f18262 -r 8845ae93e019 sys/kern/subr_pool.c
--- a/sys/kern/subr_pool.c      Sun Mar 17 14:52:25 2019 +0000
+++ b/sys/kern/subr_pool.c      Sun Mar 17 15:33:50 2019 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: subr_pool.c,v 1.240 2019/03/17 14:52:25 maxv Exp $     */
+/*     $NetBSD: subr_pool.c,v 1.241 2019/03/17 15:33:50 maxv Exp $     */
 
 /*
  * Copyright (c) 1997, 1999, 2000, 2002, 2007, 2008, 2010, 2014, 2015, 2018
@@ -33,7 +33,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_pool.c,v 1.240 2019/03/17 14:52:25 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_pool.c,v 1.241 2019/03/17 15:33:50 maxv Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_ddb.h"
@@ -440,8 +440,7 @@
                    (void *)((uintptr_t)v & pp->pr_alloc->pa_pagemask);
 
                if ((pp->pr_roflags & PR_PHINPAGE) != 0) {
-                       ph = (struct pool_item_header *)
-                           ((char *)page + pp->pr_phoffset);
+                       ph = (struct pool_item_header *)page;
                        if (__predict_false((void *)ph->ph_page != page)) {
                                panic("%s: [%s] item %p not part of pool",
                                    __func__, pp->pr_wchan, v);
@@ -700,7 +699,6 @@
        pp->pr_drain_hook_arg = NULL;
        pp->pr_freecheck = NULL;
        pool_redzone_init(pp, size);
-       pp->pr_itemoffset = 0;
 
        /*
         * Decide whether to put the page header off-page to avoid wasting too
@@ -709,14 +707,14 @@
         * based on the page address.
         */
        if (pool_init_is_phinpage(pp)) {
-               /* Use the end of the page for the page header */
-               itemspace = palloc->pa_pagesz - PHSIZE;
-               pp->pr_phoffset = itemspace;
+               /* Use the beginning of the page for the page header */
+               itemspace = palloc->pa_pagesz - roundup(PHSIZE, align);
+               pp->pr_itemoffset = roundup(PHSIZE, align);
                pp->pr_roflags |= PR_PHINPAGE;
        } else {
                /* The page header will be taken from our page header pool */
                itemspace = palloc->pa_pagesz;
-               pp->pr_phoffset = 0;
+               pp->pr_itemoffset = 0;
                SPLAY_INIT(&pp->pr_phtree);
        }
 
@@ -856,7 +854,7 @@
        struct pool_item_header *ph;
 
        if ((pp->pr_roflags & PR_PHINPAGE) != 0)
-               ph = (void *)((char *)storage + pp->pr_phoffset);
+               ph = storage;
        else
                ph = pool_get(pp->pr_phpool, flags);
 
@@ -1284,9 +1282,14 @@
        pp->pr_nidle++;
 
        /*
+        * The item space starts after the on-page header, if any.
+        */
+       ph->ph_off = pp->pr_itemoffset;
+
+       /*
         * Color this page.
         */
-       ph->ph_off = pp->pr_curcolor;
+       ph->ph_off += pp->pr_curcolor;
        cp = (char *)cp + ph->ph_off;
        if ((pp->pr_curcolor += align) > pp->pr_maxcolor)
                pp->pr_curcolor = 0;