libio: Implement vtable verification [BZ #20191]

This commit puts all libio vtables in a dedicated, read-only ELF
section, so that they are consecutive in memory.  Before any indirect
jump, the vtable pointer is checked against the section boundaries,
and the process is terminated if the vtable pointer does not fall into
the special ELF section.

To enable backwards compatibility, a special flag variable
(_IO_accept_foreign_vtables), protected by the pointer guard, avoids
process termination if libio stream object constructor functions have
been called earlier.  Such constructor functions are called by the GCC
2.95 libstdc++ library, and this mechanism ensures compatibility with
old binaries.  Existing callers inside glibc of these functions are
adjusted to call the original functions, not the wrappers which enable
vtable compatiblity.

The compatibility mechanism is used to enable passing FILE * objects
across a static dlopen boundary, too.

22 files changed, 195 insertions, 53 deletions

diff --git a/libio/Makefile b/libio/Makefile
index 4189bc4..12589f2 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -46,7 +46,7 @@ routines	:=							      \
 	__fbufsize __freading __fwriting __freadable __fwritable __flbf	      \
 	__fpurge __fpending __fsetlocking				      \
 									      \
-	libc_fatal fmemopen oldfmemopen
+	libc_fatal fmemopen oldfmemopen vtables
 
 tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst_wprintf2 tst-widetext test-fmemopen tst-ext tst-ext2 \
diff --git a/libio/fileops.c b/libio/fileops.c
index 8e83b1c..1315735 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -140,7 +140,7 @@ extern struct __gconv_trans_data __libio_translit attribute_hidden;
 
 
 void
-_IO_new_file_init (struct _IO_FILE_plus *fp)
+_IO_new_file_init_internal (struct _IO_FILE_plus *fp)
 {
   /* POSIX.1 allows another file handle to be used to change the position
      of our file descriptor.  Hence we actually don't know the actual
@@ -151,7 +151,15 @@ _IO_new_file_init (struct _IO_FILE_plus *fp)
   _IO_link_in (fp);
   fp->file._fileno = -1;
 }
-libc_hidden_ver (_IO_new_file_init, _IO_file_init)
+
+/* External version of _IO_new_file_init_internal which switches off
+   vtable validation.  */
+void
+_IO_new_file_init (struct _IO_FILE_plus *fp)
+{
+  IO_set_accept_foreign_vtables (&_IO_vtable_check);
+  _IO_new_file_init_internal (fp);
+}
 
 int
 _IO_new_file_close_it (_IO_FILE *fp)
@@ -1534,7 +1542,7 @@ versioned_symbol (libc, _IO_new_file_write, _IO_file_write, GLIBC_2_1);
 versioned_symbol (libc, _IO_new_file_xsputn, _IO_file_xsputn, GLIBC_2_1);
 #endif
 
-const struct _IO_jump_t _IO_file_jumps =
+const struct _IO_jump_t _IO_file_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_file_finish),
@@ -1559,7 +1567,7 @@ const struct _IO_jump_t _IO_file_jumps =
 };
 libc_hidden_data_def (_IO_file_jumps)
 
-const struct _IO_jump_t _IO_file_jumps_mmap =
+const struct _IO_jump_t _IO_file_jumps_mmap libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_file_finish),
@@ -1583,7 +1591,7 @@ const struct _IO_jump_t _IO_file_jumps_mmap =
   JUMP_INIT(imbue, _IO_default_imbue)
 };
 
-const struct _IO_jump_t _IO_file_jumps_maybe_mmap =
+const struct _IO_jump_t _IO_file_jumps_maybe_mmap libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_file_finish),
diff --git a/libio/genops.c b/libio/genops.c
index 5803cbf..6234bf9 100644
--- a/libio/genops.c
+++ b/libio/genops.c
@@ -558,11 +558,17 @@ _IO_default_doallocate (_IO_FILE *fp)
 libc_hidden_def (_IO_default_doallocate)
 
 void
-_IO_init (_IO_FILE *fp, int flags)
+_IO_init_internal (_IO_FILE *fp, int flags)
 {
   _IO_no_init (fp, flags, -1, NULL, NULL);
 }
-libc_hidden_def (_IO_init)
+
+void
+_IO_init (_IO_FILE *fp, int flags)
+{
+  IO_set_accept_foreign_vtables (&_IO_vtable_check);
+  _IO_init_internal (fp, flags);
+}
 
 void
 _IO_old_init (_IO_FILE *fp, int flags)
diff --git a/libio/iofdopen.c b/libio/iofdopen.c
index e00f337..a4b6757 100644
--- a/libio/iofdopen.c
+++ b/libio/iofdopen.c
@@ -153,15 +153,15 @@ _IO_new_fdopen (int fd, const char *mode)
     (use_mmap && (read_write & _IO_NO_WRITES)) ? &_IO_file_jumps_maybe_mmap :
 #endif
       &_IO_file_jumps;
-  _IO_file_init (&new_f->fp);
+  _IO_new_file_init_internal (&new_f->fp);
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fp.vtable = NULL;
 #endif
-  /* We only need to record the fd because _IO_file_init will have unset the
-     offset.  It is important to unset the cached offset because the real
-     offset in the file could change between now and when the handle is
-     activated and we would then mislead ftell into believing that we have a
-     valid offset.  */
+  /* We only need to record the fd because _IO_file_init_internal will
+     have unset the offset.  It is important to unset the cached
+     offset because the real offset in the file could change between
+     now and when the handle is activated and we would then mislead
+     ftell into believing that we have a valid offset.  */
   new_f->fp.file._fileno = fd;
   new_f->fp.file._flags &= ~_IO_DELETE_DONT_CLOSE;
 
diff --git a/libio/iofopen.c b/libio/iofopen.c
index 13e3910..855fe2f 100644
--- a/libio/iofopen.c
+++ b/libio/iofopen.c
@@ -79,7 +79,7 @@ __fopen_internal (const char *filename, const char *mode, int is32)
   _IO_no_init (&new_f->fp.file, 1, 0, NULL, NULL);
 #endif
   _IO_JUMPS (&new_f->fp) = &_IO_file_jumps;
-  _IO_file_init (&new_f->fp);
+  _IO_new_file_init_internal (&new_f->fp);
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fp.vtable = NULL;
 #endif
diff --git a/libio/iofopncook.c b/libio/iofopncook.c
index ceb444a..ae5df17 100644
--- a/libio/iofopncook.c
+++ b/libio/iofopncook.c
@@ -110,7 +110,7 @@ _IO_cookie_seekoff (_IO_FILE *fp, _IO_off64_t offset, int dir, int mode)
 }
 
 
-static const struct _IO_jump_t _IO_cookie_jumps = {
+static const struct _IO_jump_t _IO_cookie_jumps libio_vtable = {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_file_finish),
   JUMP_INIT(overflow, _IO_file_overflow),
@@ -151,13 +151,13 @@ void
 _IO_cookie_init (struct _IO_cookie_file *cfile, int read_write,
 		 void *cookie, _IO_cookie_io_functions_t io_functions)
 {
-  _IO_init (&cfile->__fp.file, 0);
+  _IO_init_internal (&cfile->__fp.file, 0);
   _IO_JUMPS (&cfile->__fp) = &_IO_cookie_jumps;
 
   cfile->__cookie = cookie;
   set_callbacks (&cfile->__io_functions, io_functions);
 
-  _IO_file_init (&cfile->__fp);
+  _IO_new_file_init_internal (&cfile->__fp);
 
   _IO_mask_flags (&cfile->__fp.file, read_write,
 		  _IO_NO_READS+_IO_NO_WRITES+_IO_IS_APPENDING);
@@ -238,7 +238,7 @@ _IO_old_cookie_seek (_IO_FILE *fp, _IO_off64_t offset, int dir)
   return (ret == -1) ? _IO_pos_BAD : ret;
 }
 
-static const struct _IO_jump_t _IO_old_cookie_jumps = {
+static const struct _IO_jump_t _IO_old_cookie_jumps libio_vtable = {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_file_finish),
   JUMP_INIT(overflow, _IO_file_overflow),
diff --git a/libio/iopopen.c b/libio/iopopen.c
index 9ddde23..d85370c 100644
--- a/libio/iopopen.c
+++ b/libio/iopopen.c
@@ -287,9 +287,9 @@ _IO_new_popen (const char *command, const char *mode)
   new_f->fpx.file.file._lock = &new_f->lock;
 #endif
   fp = &new_f->fpx.file.file;
-  _IO_init (fp, 0);
+  _IO_init_internal (fp, 0);
   _IO_JUMPS (&new_f->fpx.file) = &_IO_proc_jumps;
-  _IO_new_file_init (&new_f->fpx.file);
+  _IO_new_file_init_internal (&new_f->fpx.file);
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fpx.file.vtable = NULL;
 #endif
@@ -344,7 +344,7 @@ _IO_new_proc_close (_IO_FILE *fp)
   return wstatus;
 }
 
-static const struct _IO_jump_t _IO_proc_jumps = {
+static const struct _IO_jump_t _IO_proc_jumps libio_vtable = {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_new_file_finish),
   JUMP_INIT(overflow, _IO_new_file_overflow),
diff --git a/libio/iovdprintf.c b/libio/iovdprintf.c
index 8ca55fc..d279e34 100644
--- a/libio/iovdprintf.c
+++ b/libio/iovdprintf.c
@@ -39,7 +39,7 @@ _IO_vdprintf (int d, const char *format, _IO_va_list arg)
 #endif
   _IO_no_init (&tmpfil.file, _IO_USER_LOCK, 0, &wd, &_IO_wfile_jumps);
   _IO_JUMPS (&tmpfil) = &_IO_file_jumps;
-  _IO_file_init (&tmpfil);
+  _IO_new_file_init_internal (&tmpfil);
 #if  !_IO_UNIFIED_JUMPTABLES
   tmpfil.vtable = NULL;
 #endif
diff --git a/libio/libioP.h b/libio/libioP.h
index 8706af2..54dc35c 100644
--- a/libio/libioP.h
+++ b/libio/libioP.h
@@ -125,11 +125,12 @@ extern "C" {
 
 #if _IO_JUMPS_OFFSET
 # define _IO_JUMPS_FUNC(THIS) \
- (*(struct _IO_jump_t **) ((void *) &_IO_JUMPS_FILE_plus (THIS) \
-			   + (THIS)->_vtable_offset))
+  (IO_validate_vtable                                                   \
+   (*(struct _IO_jump_t **) ((void *) &_IO_JUMPS_FILE_plus (THIS)	\
+			     + (THIS)->_vtable_offset)))
 # define _IO_vtable_offset(THIS) (THIS)->_vtable_offset
 #else
-# define _IO_JUMPS_FUNC(THIS) _IO_JUMPS_FILE_plus (THIS)
+# define _IO_JUMPS_FUNC(THIS) (IO_validate_vtable (_IO_JUMPS_FILE_plus (THIS)))
 # define _IO_vtable_offset(THIS) 0
 #endif
 #define _IO_WIDE_JUMPS_FUNC(THIS) _IO_WIDE_JUMPS(THIS)
@@ -378,8 +379,7 @@ extern void _IO_switch_to_main_get_area (_IO_FILE *) __THROW;
 extern void _IO_switch_to_backup_area (_IO_FILE *) __THROW;
 extern int _IO_switch_to_get_mode (_IO_FILE *);
 libc_hidden_proto (_IO_switch_to_get_mode)
-extern void _IO_init (_IO_FILE *, int) __THROW;
-libc_hidden_proto (_IO_init)
+extern void _IO_init_internal (_IO_FILE *, int) attribute_hidden;
 extern int _IO_sputbackc (_IO_FILE *, int) __THROW;
 libc_hidden_proto (_IO_sputbackc)
 extern int _IO_sungetc (_IO_FILE *) __THROW;
@@ -587,8 +587,6 @@ extern int _IO_file_underflow_maybe_mmap (_IO_FILE *);
 extern int _IO_file_overflow (_IO_FILE *, int);
 libc_hidden_proto (_IO_file_overflow)
 #define _IO_file_is_open(__fp) ((__fp)->_fileno != -1)
-extern void _IO_file_init (struct _IO_FILE_plus *) __THROW;
-libc_hidden_proto (_IO_file_init)
 extern _IO_FILE* _IO_file_attach (_IO_FILE *, int);
 libc_hidden_proto (_IO_file_attach)
 extern _IO_FILE* _IO_file_open (_IO_FILE *, const char *, int, int, int, int);
@@ -614,7 +612,8 @@ extern _IO_FILE* _IO_new_file_fopen (_IO_FILE *, const char *, const char *,
 				     int);
 extern void _IO_no_init (_IO_FILE *, int, int, struct _IO_wide_data *,
 			 const struct _IO_jump_t *) __THROW;
-extern void _IO_new_file_init (struct _IO_FILE_plus *) __THROW;
+extern void _IO_new_file_init_internal (struct _IO_FILE_plus *)
+  __THROW attribute_hidden;
 extern _IO_FILE* _IO_new_file_setbuf (_IO_FILE *, char *, _IO_ssize_t);
 extern _IO_FILE* _IO_file_setbuf_mmap (_IO_FILE *, char *, _IO_ssize_t);
 extern int _IO_new_file_sync (_IO_FILE *);
@@ -629,7 +628,8 @@ extern _IO_off64_t _IO_old_file_seekoff (_IO_FILE *, _IO_off64_t, int, int);
 extern _IO_size_t _IO_old_file_xsputn (_IO_FILE *, const void *, _IO_size_t);
 extern int _IO_old_file_underflow (_IO_FILE *);
 extern int _IO_old_file_overflow (_IO_FILE *, int);
-extern void _IO_old_file_init (struct _IO_FILE_plus *) __THROW;
+extern void _IO_old_file_init_internal (struct _IO_FILE_plus *)
+  __THROW attribute_hidden;
 extern _IO_FILE* _IO_old_file_attach (_IO_FILE *, int);
 extern _IO_FILE* _IO_old_file_fopen (_IO_FILE *, const char *, const char *);
 extern _IO_ssize_t _IO_old_file_write (_IO_FILE *, const void *, _IO_ssize_t);
@@ -673,10 +673,6 @@ extern void _IO_str_finish (_IO_FILE *, int) __THROW;
 
 /* Other strfile functions */
 struct _IO_strfile_;
-extern void _IO_str_init_static (struct _IO_strfile_ *, char *, int, char *)
-     __THROW;
-extern void _IO_str_init_readonly (struct _IO_strfile_ *, const char *, int)
-     __THROW;
 extern _IO_ssize_t _IO_str_count (_IO_FILE *) __THROW;
 
 /* And the wide character versions.  */
@@ -890,3 +886,57 @@ _IO_acquire_lock_clear_flags2_fct (_IO_FILE **p)
                                           | _IO_FLAGS2_SCANF_STD);	      \
   } while (0)
 #endif
+
+/* Collect all vtables in a special section for vtable verification.
+   These symbols cover the extent of this section.  */
+symbol_set_declare (__libc_IO_vtables)
+
+/* libio vtables need to carry this attribute so that they pass
+   validation.  */
+#define libio_vtable __attribute__ ((section ("__libc_IO_vtables")))
+
+#ifdef SHARED
+/* If equal to &_IO_vtable_check (with pointer guard protection),
+   unknown vtable pointers are valid.  This function pointer is solely
+   used as a flag.  */
+extern void (*IO_accept_foreign_vtables) (void) attribute_hidden;
+
+/* Assigns the passed function pointer (either NULL or
+   &_IO_vtable_check) to IO_accept_foreign_vtables.  */
+static inline void
+IO_set_accept_foreign_vtables (void (*flag) (void))
+{
+  PTR_MANGLE (flag);
+  atomic_store_relaxed (&IO_accept_foreign_vtables, flag);
+}
+
+#else  /* !SHARED */
+
+/* The statically-linked version does nothing. */
+static inline void
+IO_set_accept_foreign_vtables (void (*flag) (void))
+{
+}
+
+#endif
+
+/* Check if unknown vtable pointers are permitted; otherwise,
+   terminate the process.  */
+void _IO_vtable_check (void) attribute_hidden;
+
+/* Perform vtable pointer validation.  If validation fails, terminate
+   the process.  */
+static inline const struct _IO_jump_t *
+IO_validate_vtable (const struct _IO_jump_t *vtable)
+{
+  /* Fast path: The vtable pointer is within the __libc_IO_vtables
+     section.  */
+  uintptr_t section_length = __stop___libc_IO_vtables - __start___libc_IO_vtables;
+  const char *ptr = (const char *) vtable;
+  uintptr_t offset = ptr - __start___libc_IO_vtables;
+  if (__glibc_unlikely (offset >= section_length))
+    /* The vtable pointer is not in the expected section.  Use the
+       slow path, which will terminate the process if necessary.  */
+    _IO_vtable_check ();
+  return vtable;
+}
diff --git a/libio/memstream.c b/libio/memstream.c
index 7fa5245..e20b9c2 100644
--- a/libio/memstream.c
+++ b/libio/memstream.c
@@ -33,7 +33,7 @@ static int _IO_mem_sync (_IO_FILE* fp) __THROW;
 static void _IO_mem_finish (_IO_FILE* fp, int) __THROW;
 
 
-static const struct _IO_jump_t _IO_mem_jumps =
+static const struct _IO_jump_t _IO_mem_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT (finish, _IO_mem_finish),
@@ -86,7 +86,7 @@ __open_memstream (char **bufloc, _IO_size_t *sizeloc)
       free (new_f);
       return NULL;
     }
-  _IO_init (&new_f->fp._sf._sbf._f, 0);
+  _IO_init_internal (&new_f->fp._sf._sbf._f, 0);
   _IO_JUMPS_FILE_plus (&new_f->fp._sf._sbf) = &_IO_mem_jumps;
   _IO_str_init_static_internal (&new_f->fp._sf, buf, _IO_BUFSIZ, buf);
   new_f->fp._sf._sbf._f._flags &= ~_IO_USER_BUF;
diff --git a/libio/obprintf.c b/libio/obprintf.c
index aa17b46..d7144e8 100644
--- a/libio/obprintf.c
+++ b/libio/obprintf.c
@@ -91,7 +91,7 @@ _IO_obstack_xsputn (_IO_FILE *fp, const void *data, _IO_size_t n)
 
 
 /* the jump table.  */
-const struct _IO_jump_t _IO_obstack_jumps attribute_hidden =
+const struct _IO_jump_t _IO_obstack_jumps libio_vtable attribute_hidden =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, NULL),
diff --git a/libio/oldfileops.c b/libio/oldfileops.c
index 4f3bdfe..26b2bf5 100644
--- a/libio/oldfileops.c
+++ b/libio/oldfileops.c
@@ -114,7 +114,7 @@ extern int errno;
 
 void
 attribute_compat_text_section
-_IO_old_file_init (struct _IO_FILE_plus *fp)
+_IO_old_file_init_internal (struct _IO_FILE_plus *fp)
 {
   /* POSIX.1 allows another file handle to be used to change the position
      of our file descriptor.  Hence we actually don't know the actual
@@ -138,6 +138,14 @@ _IO_old_file_init (struct _IO_FILE_plus *fp)
 #endif
 }
 
+void
+attribute_compat_text_section
+_IO_old_file_init (struct _IO_FILE_plus *fp)
+{
+  IO_set_accept_foreign_vtables (&_IO_vtable_check);
+  _IO_old_file_init_internal (fp);
+}
+
 int
 attribute_compat_text_section
 _IO_old_file_close_it (_IO_FILE *fp)
@@ -745,7 +753,7 @@ _IO_old_file_xsputn (_IO_FILE *f, const void *data, _IO_size_t n)
 }
 
 
-const struct _IO_jump_t _IO_old_file_jumps =
+const struct _IO_jump_t _IO_old_file_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_old_file_finish),
diff --git a/libio/oldiofdopen.c b/libio/oldiofdopen.c
index 33406ff..59fcfa4 100644
--- a/libio/oldiofdopen.c
+++ b/libio/oldiofdopen.c
@@ -112,7 +112,7 @@ _IO_old_fdopen (int fd, const char *mode)
 #endif
   _IO_old_init (&new_f->fp.file._file, 0);
   _IO_JUMPS_FILE_plus (&new_f->fp) = &_IO_old_file_jumps;
-  _IO_old_file_init ((struct _IO_FILE_plus *) &new_f->fp);
+  _IO_old_file_init_internal ((struct _IO_FILE_plus *) &new_f->fp);
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fp.vtable = NULL;
 #endif
diff --git a/libio/oldiofopen.c b/libio/oldiofopen.c
index cc7c342..ce6a33d 100644
--- a/libio/oldiofopen.c
+++ b/libio/oldiofopen.c
@@ -51,7 +51,7 @@ _IO_old_fopen (const char *filename, const char *mode)
 #endif
   _IO_old_init (&new_f->fp.file._file, 0);
   _IO_JUMPS_FILE_plus (&new_f->fp) = &_IO_old_file_jumps;
-  _IO_old_file_init ((struct _IO_FILE_plus *) &new_f->fp);
+  _IO_old_file_init_internal ((struct _IO_FILE_plus *) &new_f->fp);
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fp.vtable = NULL;
 #endif
diff --git a/libio/oldiopopen.c b/libio/oldiopopen.c
index ea75b4f..bce74b1 100644
--- a/libio/oldiopopen.c
+++ b/libio/oldiopopen.c
@@ -211,7 +211,7 @@ _IO_old_popen (const char *command, const char *mode)
   fp = &new_f->fpx.file.file._file;
   _IO_old_init (fp, 0);
   _IO_JUMPS_FILE_plus (&new_f->fpx.file) = &_IO_old_proc_jumps;
-  _IO_old_file_init ((struct _IO_FILE_plus *) &new_f->fpx.file);
+  _IO_old_file_init_internal ((struct _IO_FILE_plus *) &new_f->fpx.file);
 #if  !_IO_UNIFIED_JUMPTABLES
   new_f->fpx.file.vtable = NULL;
 #endif
@@ -267,7 +267,7 @@ _IO_old_proc_close (_IO_FILE *fp)
   return wstatus;
 }
 
-const struct _IO_jump_t _IO_old_proc_jumps = {
+const struct _IO_jump_t _IO_old_proc_jumps libio_vtable = {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_old_file_finish),
   JUMP_INIT(overflow, _IO_old_file_overflow),
diff --git a/libio/strops.c b/libio/strops.c
index 0932d4c..2ba3704 100644
--- a/libio/strops.c
+++ b/libio/strops.c
@@ -323,7 +323,7 @@ _IO_str_finish (_IO_FILE *fp, int dummy)
   _IO_default_finish (fp, 0);
 }
 
-const struct _IO_jump_t _IO_str_jumps =
+const struct _IO_jump_t _IO_str_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_str_finish),
diff --git a/libio/vsnprintf.c b/libio/vsnprintf.c
index f1063a1..ee7f752 100644
--- a/libio/vsnprintf.c
+++ b/libio/vsnprintf.c
@@ -64,7 +64,7 @@ _IO_strn_overflow (_IO_FILE *fp, int c)
 }
 
 
-const struct _IO_jump_t _IO_strn_jumps attribute_hidden =
+const struct _IO_jump_t _IO_strn_jumps libio_vtable attribute_hidden =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_str_finish),
diff --git a/libio/vswprintf.c b/libio/vswprintf.c
index b90441a..df7b719 100644
--- a/libio/vswprintf.c
+++ b/libio/vswprintf.c
@@ -63,7 +63,7 @@ _IO_wstrn_overflow (_IO_FILE *fp, wint_t c)
 }
 
 
-const struct _IO_jump_t _IO_wstrn_jumps attribute_hidden =
+const struct _IO_jump_t _IO_wstrn_jumps libio_vtable attribute_hidden =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_wstr_finish),
diff --git a/libio/vtables.c b/libio/vtables.c
new file mode 100644
index 0000000..e364ea0
--- /dev/null
+++ b/libio/vtables.c
@@ -0,0 +1,70 @@
+/* libio vtable validation.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <dlfcn.h>
+#include <libioP.h>
+#include <stdio.h>
+
+#ifdef SHARED
+
+void (*IO_accept_foreign_vtables) (void) attribute_hidden;
+
+/* Used to detected multiple libcs.  */
+extern struct dl_open_hook *_dl_open_hook;
+libc_hidden_proto (_dl_open_hook);
+
+#else  /* !SHARED */
+
+/* Used to check whether static dlopen support is needed.  */
+# pragma weak __dlopen
+
+#endif
+
+void attribute_hidden
+_IO_vtable_check (void)
+{
+#ifdef SHARED
+  /* Honor the compatibility flag.  */
+  void (*flag) (void) = atomic_load_relaxed (&IO_accept_foreign_vtables);
+  PTR_DEMANGLE (flag);
+  if (flag == &_IO_vtable_check)
+    return;
+
+  /* In case this libc copy is in a non-default namespace, we always
+     need to accept foreign vtables because there is always a
+     possibility that FILE * objects are passed across the linking
+     boundary.  */
+  {
+    Dl_info di;
+    struct link_map *l;
+    if (_dl_open_hook != NULL
+        || (_dl_addr (_IO_vtable_check, &di, &l, NULL) != 0
+            && l->l_ns != LM_ID_BASE))
+      return;
+  }
+
+#else /* !SHARED */
+  /* We cannot perform vtable validation in the static dlopen case
+     because FILE * handles might be passed back and forth across the
+     boundary.  Therefore, we disable checking in this case.  */
+  if (__dlopen != NULL)
+    return;
+#endif
+
+  __libc_fatal ("Fatal error: glibc detected an invalid stdio handle\n");
+}
diff --git a/libio/wfileops.c b/libio/wfileops.c
index df1fbda..d88d08a 100644
--- a/libio/wfileops.c
+++ b/libio/wfileops.c
@@ -1042,7 +1042,7 @@ _IO_wfile_xsputn (_IO_FILE *f, const void *data, _IO_size_t n)
 libc_hidden_def (_IO_wfile_xsputn)
 
 
-const struct _IO_jump_t _IO_wfile_jumps =
+const struct _IO_jump_t _IO_wfile_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_new_file_finish),
@@ -1068,7 +1068,7 @@ const struct _IO_jump_t _IO_wfile_jumps =
 libc_hidden_data_def (_IO_wfile_jumps)
 
 
-const struct _IO_jump_t _IO_wfile_jumps_mmap =
+const struct _IO_jump_t _IO_wfile_jumps_mmap libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_new_file_finish),
@@ -1092,7 +1092,7 @@ const struct _IO_jump_t _IO_wfile_jumps_mmap =
   JUMP_INIT(imbue, _IO_default_imbue)
 };
 
-const struct _IO_jump_t _IO_wfile_jumps_maybe_mmap =
+const struct _IO_jump_t _IO_wfile_jumps_maybe_mmap libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_new_file_finish),
diff --git a/libio/wmemstream.c b/libio/wmemstream.c
index 1bdbae9..bf2a50b 100644
--- a/libio/wmemstream.c
+++ b/libio/wmemstream.c
@@ -34,7 +34,7 @@ static int _IO_wmem_sync (_IO_FILE* fp) __THROW;
 static void _IO_wmem_finish (_IO_FILE* fp, int) __THROW;
 
 
-static const struct _IO_jump_t _IO_wmem_jumps =
+static const struct _IO_jump_t _IO_wmem_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT (finish, _IO_wmem_finish),
diff --git a/libio/wstrops.c b/libio/wstrops.c
index 8d0e1cb..09fa543 100644
--- a/libio/wstrops.c
+++ b/libio/wstrops.c
@@ -332,7 +332,7 @@ _IO_wstr_finish (_IO_FILE *fp, int dummy)
   _IO_wdefault_finish (fp, 0);
 }
 
-const struct _IO_jump_t _IO_wstr_jumps =
+const struct _IO_jump_t _IO_wstr_jumps libio_vtable =
 {
   JUMP_INIT_DUMMY,
   JUMP_INIT(finish, _IO_wstr_finish),