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>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxc/xc_dom_core.c

@@ -873,7 +873,8 @@ int xc_dom_build_image(struct xc_dom_image *dom)
         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) )

tools/libxc/xc_dom_x86.c

@@ -82,6 +82,7 @@ static int count_pgtables(struct xc_dom_image *dom, int pae,
 {
     int pages, extra_pages;
     xen_vaddr_t try_virt_end;
+    xen_pfn_t try_pfn_end;
 
     extra_pages = dom->alloc_bootstack ? 1 : 0;
     extra_pages += dom->extra_pages;
@@ -91,6 +92,17 @@ static int count_pgtables(struct xc_dom_image *dom, int pae,
     {
         try_virt_end = round_up(dom->virt_alloc_end + pages * PAGE_SIZE_X86,
                                 bits_to_mask(22)); /* 4MB alignment */
+
+        try_pfn_end = (try_virt_end - dom->parms.virt_base) >> PAGE_SHIFT_X86;
+
+        if ( try_pfn_end > dom->total_pages )
+        {
+            xc_dom_panic(dom->xch, XC_OUT_OF_MEMORY,
+                         "%s: not enough memory for initial mapping (%#"PRIpfn" > %#"PRIpfn")",
+                         __FUNCTION__, try_pfn_end, dom->total_pages);
+            return -ENOMEM;
+        }
+
         dom->pg_l4 =
             nr_page_tables(dom, dom->parms.virt_base, try_virt_end, l4_bits);
         dom->pg_l3 =