looping in bfd_mach_o_fat_openr_next_archived_file

[Alan Modra <amodra@gmail.com>]

mach-o.c doesn't sanity check mach-o-fat archives, making it easy for
fuzzers to create an archive with mach_o_fat_archentry headers that
point to the same offset.  bfd_mach_o_fat_openr_next_archived_file
uses the previous element offset to find its header, and thus the next
element.  If two offsets are the same, any tool reading the archive
will get stuck.  This patch rejects such archives, and any with
overlapping elements.

	* mach-o.c (overlap_previous): New function.
	(bfd_mach_o_fat_archive_p): Sanity check that elements do not
	overlap each other or the file and archive headers.

diff --git a/bfd/mach-o.c b/bfd/mach-o.c
index eb325236454..acb35e7f0c6 100644
--- a/bfd/mach-o.c
+++ b/bfd/mach-o.c
@@ -5497,6 +5497,22 @@ typedef struct mach_o_fat_data_struct
   mach_o_fat_archentry *archentries;
 } mach_o_fat_data_struct;
 
+/* Check for overlapping archive elements.  Note that we can't allow
+   multiple elements at the same offset even if one is empty, because
+   bfd_mach_o_fat_openr_next_archived_file assume distinct offsets.  */
+static bool
+overlap_previous (const mach_o_fat_archentry *elt, unsigned long i)
+{
+  unsigned long j = i;
+  while (j-- != 0)
+    if (elt[i].offset == elt[j].offset
+	|| (elt[i].offset > elt[j].offset
+	    ? elt[i].offset - elt[j].offset < elt[j].size
+	    : elt[j].offset - elt[i].offset < elt[i].size))
+      return true;
+  return false;
+}
+
 bfd_cleanup
 bfd_mach_o_fat_archive_p (bfd *abfd)
 {
@@ -5545,10 +5561,13 @@ bfd_mach_o_fat_archive_p (bfd *abfd)
       adata->archentries[i].offset = bfd_getb32 (arch.offset);
       adata->archentries[i].size = bfd_getb32 (arch.size);
       adata->archentries[i].align = bfd_getb32 (arch.align);
-      if (filesize != 0
-	  && (adata->archentries[i].offset > filesize
-	      || (adata->archentries[i].size
-		  > filesize - adata->archentries[i].offset)))
+      if ((filesize != 0
+	   && (adata->archentries[i].offset > filesize
+	       || (adata->archentries[i].size
+		   > filesize - adata->archentries[i].offset)))
+	  || (adata->archentries[i].offset
+	      < sizeof (hdr) + adata->nfat_arch * sizeof (arch))
+	  || overlap_previous (adata->archentries, i))
 	{
 	  bfd_release (abfd, adata);
 	  bfd_set_error (bfd_error_malformed_archive);

-- 
Alan Modra
Australia Development Lab, IBM