gpg: Limit the nesting level of I/O filters.

* common/iobuf.c (MAX_NESTING_FILTER): New.
(iobuf_push_filter2): Limit the nesting level.

* g10/mainproc.c (mainproc_context): New field ANY.  Change HAVE_DATA
and ANY_SIG_SIGN to bit fields of ANY.  Add bit field
UNCOMPRESS_FAILED.
(proc_compressed): Avoid printing multiple Bad Data messages.
(check_nesting): Return GPG_ERR_BAD_DATA instead of UNEXPECTED_DATA.
--

This is a more general fix for the nested compression packet bug.  In
particular this helps g10/import.c:read_block to stop pushing
compression filters onto an iobuf stream.  This patch also reduces the
number of error messages for the non-import case.

Signed-off-by: Werner Koch <wk@gnupg.org>

common/iobuf.c

@@ -55,6 +55,10 @@
    be aware that there is no fsync support for the stdio backend.  */
 #undef FILE_FILTER_USES_STDIO
 
+/* To avoid a potential DoS with compression packets we better limit
+   the number of filters in a chain.  */
+#define MAX_NESTING_FILTER 64
+
 /*-- End configurable part.  --*/
 
 
@@ -1615,6 +1619,13 @@ iobuf_push_filter2 (iobuf_t a,
 
   if (a->use == 2 && (rc = iobuf_flush (a)))
     return rc;
+
+  if (a->subno >= MAX_NESTING_FILTER)
+    {
+      log_error ("i/o filter too deeply nested - corrupted data?\n");
+      return GPG_ERR_BAD_DATA;
+    }
+
   /* make a copy of the current stream, so that
    * A is the new stream and B the original one.
    * The contents of the buffers are transferred to the