libxc: Fix range checking in xc_dom_pfn_to_ptr etc.

* Ensure that xc_dom_pfn_to_ptr (when called with count==0) does not
  return a previously-allocated block which is entirely before the
  requested pfn (!)

* Provide a version of xc_dom_pfn_to_ptr, xc_dom_pfn_to_ptr_retcount,
  which provides the length of the mapped region via an out parameter.

* Change xc_dom_vaddr_to_ptr to always provide the length of the
  mapped region and change the call site in xc_dom_binloader.c to
  check it.  The call site in xc_dom_load_elf_symtab will be corrected
  in a forthcoming patch, and for now ignores the returned length.

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

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

v5: This patch is new in v5 of the series.

1 files changed, 13 insertions, 3 deletions

diff --git a/tools/libxc/xc_dom.h b/tools/libxc/xc_dom.h
index 316c5cbefc..ad6fdd49e2 100644
--- a/tools/libxc/xc_dom.h
+++ b/tools/libxc/xc_dom.h
@@ -291,6 +291,8 @@ int xc_dom_alloc_segment(struct xc_dom_image *dom,
 
 void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t first,
                         xen_pfn_t count);
+void *xc_dom_pfn_to_ptr_retcount(struct xc_dom_image *dom, xen_pfn_t first,
+                                 xen_pfn_t count, xen_pfn_t *count_out);
 void xc_dom_unmap_one(struct xc_dom_image *dom, xen_pfn_t pfn);
 void xc_dom_unmap_all(struct xc_dom_image *dom);
 
@@ -318,13 +320,21 @@ static inline void *xc_dom_seg_to_ptr(struct xc_dom_image *dom,
 }
 
 static inline void *xc_dom_vaddr_to_ptr(struct xc_dom_image *dom,
-                                        xen_vaddr_t vaddr)
+                                        xen_vaddr_t vaddr,
+                                        size_t *safe_region_out)
 {
     unsigned int page_size = XC_DOM_PAGE_SIZE(dom);
     xen_pfn_t page = (vaddr - dom->parms.virt_base) / page_size;
     unsigned int offset = (vaddr - dom->parms.virt_base) % page_size;
-    void *ptr = xc_dom_pfn_to_ptr(dom, page, 0);
-    return (ptr ? (ptr + offset) : NULL);
+    xen_pfn_t safe_region_count;
+    void *ptr;
+
+    *safe_region_out = 0;
+    ptr = xc_dom_pfn_to_ptr_retcount(dom, page, 0, &safe_region_count);
+    if ( ptr == NULL )
+        return ptr;
+    *safe_region_out = (safe_region_count << XC_DOM_PAGE_SHIFT(dom)) - offset;
+    return ptr;
 }
 
 static inline xen_pfn_t xc_dom_p2m_host(struct xc_dom_image *dom, xen_pfn_t pfn)