[PATCH] libxc: x86: ensure that the initial mapping fits into the guest's memory

[Ian Campbell <Ian.Campbell@citrix.com>]

On Fri, 2013-01-04 at 16:01 +0000, Xen.org security team wrote:
> 	     Xen Security Advisory CVE-2013-0154 / XSA-37
> 
>      Hypervisor crash due to incorrect ASSERT (debug build only)

This issue was exposed/triggered by a libxc bug, the fix for which is
below.

8<---------------------------------

# HG changeset patch
# User Ian Campbell <ijc@hellion.org.uk>
# Date 1357318325 0
# Node ID 84dea8458c36e6ee7d9347d32455cbeb5595825b
# Parent  7a61269a0c1ab33cc93b47f88d760f0d1f88eaab
libxc: x86: ensure that the initial mapping fits into the guest's memory

In particular we need to check that adding 512KB of slack and
rounding up to a 4MB boundary do not overflow the guest's memory
allocation. Otherwise we run off the end of the p2m when building the
guest's initial page tables and populate them with garbage.

Wei noticed this when build tiny (2MB) mini-os domains.

Reported-by: Wei Liu <Wei.Liu2@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff -r 7a61269a0c1a -r 84dea8458c36 tools/libxc/xc_dom_core.c
--- a/tools/libxc/xc_dom_core.c	Thu Jan 03 16:31:44 2013 +0000
+++ b/tools/libxc/xc_dom_core.c	Fri Jan 04 16:52:05 2013 +0000
@@ -873,7 +873,8 @@ int xc_dom_build_image(struct xc_dom_ima
         goto err;
     if ( dom->arch_hooks->count_pgtables )
     {
-        dom->arch_hooks->count_pgtables(dom);
+        if ( dom->arch_hooks->count_pgtables(dom) != 0 )
+            goto err;
         if ( (dom->pgtables > 0) &&
              (xc_dom_alloc_segment(dom, &dom->pgtables_seg, "page tables", 0,
                                    dom->pgtables * page_size) != 0) )
diff -r 7a61269a0c1a -r 84dea8458c36 tools/libxc/xc_dom_x86.c
--- a/tools/libxc/xc_dom_x86.c	Thu Jan 03 16:31:44 2013 +0000
+++ b/tools/libxc/xc_dom_x86.c	Fri Jan 04 16:52:05 2013 +0000
@@ -91,6 +91,15 @@ static int count_pgtables(struct xc_dom_
     {
         try_virt_end = round_up(dom->virt_alloc_end + pages * PAGE_SIZE_X86,
                                 bits_to_mask(22)); /* 4MB alignment */
+
+        if ( (try_virt_end >> PAGE_SHIFT_X86) > dom->total_pages )
+        {
+            xc_dom_panic(dom->xch, XC_OUT_OF_MEMORY,
+                         "%s: not enough memory for initial mapping",
+                         __FUNCTION__);
+            return -ENOMEM;
+        }
+
         dom->pg_l4 =
             nr_page_tables(dom, dom->parms.virt_base, try_virt_end, l4_bits);
         dom->pg_l3 =