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, 0 deletions

diff --git a/tools/libxc/xc_dom_core.c b/tools/libxc/xc_dom_core.c
index b92e4a9405..cf96bfa99b 100644
--- a/tools/libxc/xc_dom_core.c
+++ b/tools/libxc/xc_dom_core.c
@@ -351,11 +351,20 @@ int xc_dom_try_gunzip(struct xc_dom_image *dom, void **blob, size_t * size)
 void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
                         xen_pfn_t count)
 {
+    xen_pfn_t count_out_dummy;
+    return xc_dom_pfn_to_ptr_retcount(dom, pfn, count, &count_out_dummy);
+}
+
+void *xc_dom_pfn_to_ptr_retcount(struct xc_dom_image *dom, xen_pfn_t pfn,
+                                 xen_pfn_t count, xen_pfn_t *count_out)
+{
     struct xc_dom_phys *phys;
     xen_pfn_t offset;
     unsigned int page_shift = XC_DOM_PAGE_SHIFT(dom);
     char *mode = "unset";
 
+    *count_out = 0;
+
     offset = pfn - dom->rambase_pfn;
     if ( offset > dom->total_pages || /* multiple checks to avoid overflows */
          count > dom->total_pages ||
@@ -386,6 +395,7 @@ void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
                           phys->count);
                 return NULL;
             }
+            *count_out = count;
         }
         else
         {
@@ -393,6 +403,9 @@ void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
                just hand out a pointer to it */
             if ( pfn < phys->first )
                 continue;
+            if ( pfn >= phys->first + phys->count )
+                continue;
+            *count_out = phys->count - (pfn - phys->first);
         }
         return phys->ptr + ((pfn - phys->first) << page_shift);
     }