libxc: range checks in xc_dom_p2m_host and _guest

These functions take guest pfns and look them up in the p2m.  They did
no range checking.

However, some callers, notably xc_dom_boot.c:setup_hypercall_page want
to pass untrusted guest-supplied value(s).  It is most convenient to
detect this here and return INVALID_MFN.

This is part of the fix to a security issue, XSA-55.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Tim Deegan <tim@xen.org>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>

v6: Check for underflow too (thanks to Andrew Cooper).

tools/libxc/xc_dom.h

@@ -342,6 +342,8 @@ static inline xen_pfn_t xc_dom_p2m_host(struct xc_dom_image *dom, xen_pfn_t pfn)
 {
     if (dom->shadow_enabled)
         return pfn;
+    if (pfn < dom->rambase_pfn || pfn >= dom->rambase_pfn + dom->total_pages)
+        return INVALID_MFN;
     return dom->p2m_host[pfn - dom->rambase_pfn];
 }
 
@@ -350,6 +352,8 @@ static inline xen_pfn_t xc_dom_p2m_guest(struct xc_dom_image *dom,
 {
     if (xc_dom_feature_translated(dom))
         return pfn;
+    if (pfn < dom->rambase_pfn || pfn >= dom->rambase_pfn + dom->total_pages)
+        return INVALID_MFN;
     return dom->p2m_host[pfn - dom->rambase_pfn];
 }