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>

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

tools/libxc/xc_dom.h

@@ -259,6 +259,8 @@ 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);
 
@@ -286,13 +288,21 @@ 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 int xc_dom_feature_translated(struct xc_dom_image *dom)

tools/libxc/xc_dom_binloader.c

@@ -249,6 +249,7 @@ static int xc_dom_load_bin_kernel(struct xc_dom_image *dom)
     char *image = dom->kernel_blob;
     char *dest;
     size_t image_size = dom->kernel_size;
+    size_t dest_size;
     uint32_t start_addr;
     uint32_t load_end_addr;
     uint32_t bss_end_addr;
@@ -272,7 +273,15 @@ static int xc_dom_load_bin_kernel(struct xc_dom_image *dom)
     DOMPRINTF("  text_size: 0x%" PRIx32 "", text_size);
     DOMPRINTF("  bss_size:  0x%" PRIx32 "", bss_size);
 
-    dest = xc_dom_vaddr_to_ptr(dom, dom->kernel_seg.vstart);
+    dest = xc_dom_vaddr_to_ptr(dom, dom->kernel_seg.vstart, &dest_size);
+
+    if ( dest_size < text_size ||
+         dest_size - text_size < bss_size )
+    {
+        DOMPRINTF("%s: mapped region is too small for image", __FUNCTION__);
+        return -EINVAL;
+    }
+
     memcpy(dest, image + skip, text_size);
     memset(dest + text_size, 0, bss_size);
 

tools/libxc/xc_dom_core.c

@@ -350,11 +350,20 @@ 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;
     unsigned int page_shift = XC_DOM_PAGE_SHIFT(dom);
     char *mode = "unset";
 
+    *count_out = 0;
+
     if ( pfn > dom->total_pages ||    /* multiple checks to avoid overflows */
          count > dom->total_pages ||
          pfn > dom->total_pages - count )
@@ -384,6 +393,7 @@ void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
                           phys->count);
                 return NULL;
             }
+            *count_out = count;
         }
         else
         {
@@ -391,6 +401,9 @@ 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);
     }

tools/libxc/xc_dom_elfloader.c

@@ -130,10 +130,11 @@ static int xc_dom_load_elf_symtab(struct xc_dom_image *dom,
 
     if ( load )
     {
+        size_t allow_size; /* will be used in a forthcoming XSA-55 patch */
         if ( !dom->bsd_symtab_start )
             return 0;
         size = dom->kernel_seg.vend - dom->bsd_symtab_start;
-        hdr  = xc_dom_vaddr_to_ptr(dom, dom->bsd_symtab_start);
+        hdr  = xc_dom_vaddr_to_ptr(dom, dom->bsd_symtab_start, &allow_size);
         *(int *)hdr = size - sizeof(int);
     }
     else