libio: Avoid _allocate_buffer, _free_buffer function pointers [BZ #23236]

These unmangled function pointers reside on the heap and could
be targeted by exploit writers, effectively bypassing libio vtable
validation.  Instead, we ignore these pointers and always call
malloc or free.

In theory, this is a backwards-incompatible change, but using the
global heap instead of the user-supplied callback functions should
have little application impact.  (The old libstdc++ implementation
exposed this functionality via a public, undocumented constructor
in its strstreambuf class.)

(cherry picked from commit 4e8a6346cd3da2d88bbad745a1769260d36f2783)

9 files changed, 50 insertions, 32 deletions

diff --git a/ChangeLog b/ChangeLog
index e45fa8e6d0..0126fb391b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,27 @@
+2018-06-01  Florian Weimer  <fweimer@redhat.com>
+
+	[BZ #23236]
+	* libio/strfile.h (struct _IO_str_fields): Rename members to
+	discourage their use and add comment.
+	(_IO_STR_DYNAMIC): Remove unused macro.
+	* libio/strops.c (_IO_str_init_static_internal): Do not use
+	callback pointers.  Call malloc and free.
+	(_IO_str_overflow): Do not use callback pointers.  Call malloc
+	and free.
+	(enlarge_userbuf): Likewise.
+	(_IO_str_finish): Call free.
+	* libio/wstrops.c (_IO_wstr_init_static): Initialize
+	_allocate_buffer_unused.
+	(_IO_wstr_overflow): Do not use callback pointers.  Call malloc
+	and free.
+	(enlarge_userbuf): Likewise.
+	(_IO_wstr_finish): Call free.
+	* debug/vasprintf_chk.c (__vasprintf_chk): Initialize
+	_allocate_buffer_unused, _free_buffer_unused.
+	* libio/memstream.c (__open_memstream): Likewise.
+	* libio/vasprintf.c (_IO_vasprintf): Likewise.
+	* libio/wmemstream.c (open_wmemstream): Likewise.
+
 2018-05-23  H.J. Lu  <hongjiu.lu@intel.com>
 
 	[BZ #23196]
diff --git a/NEWS b/NEWS
index c6c5538192..b8bd101f6c 100644
--- a/NEWS
+++ b/NEWS
@@ -140,6 +140,7 @@ The following bugs are resolved with this release:
   [23037] resolv: Fully initialize struct mmsghdr in send_dg
   [23137] s390: Fix blocking pthread_join
   [23196] __mempcpy_avx512_no_vzeroupper mishandles large copies
+  [23236] Harden function pointers in _IO_str_fields
 
 Version 2.26
 
diff --git a/debug/vasprintf_chk.c b/debug/vasprintf_chk.c
index 5ebfeed239..1d989dd707 100644
--- a/debug/vasprintf_chk.c
+++ b/debug/vasprintf_chk.c
@@ -55,8 +55,8 @@ __vasprintf_chk (char **result_ptr, int flags, const char *format,
   _IO_JUMPS (&sf._sbf) = &_IO_str_jumps;
   _IO_str_init_static_internal (&sf, string, init_string_size, string);
   sf._sbf._f._flags &= ~_IO_USER_BUF;
-  sf._s._allocate_buffer = (_IO_alloc_type) malloc;
-  sf._s._free_buffer = (_IO_free_type) free;
+  sf._s._allocate_buffer_unused = (_IO_alloc_type) malloc;
+  sf._s._free_buffer_unused = (_IO_free_type) free;
 
   /* For flags > 0 (i.e. __USE_FORTIFY_LEVEL > 1) request that %n
      can only come from read-only format strings.  */
diff --git a/libio/memstream.c b/libio/memstream.c
index e391efd48a..c26239968d 100644
--- a/libio/memstream.c
+++ b/libio/memstream.c
@@ -90,8 +90,8 @@ __open_memstream (char **bufloc, _IO_size_t *sizeloc)
   _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;
-  new_f->fp._sf._s._allocate_buffer = (_IO_alloc_type) malloc;
-  new_f->fp._sf._s._free_buffer = (_IO_free_type) free;
+  new_f->fp._sf._s._allocate_buffer_unused = (_IO_alloc_type) malloc;
+  new_f->fp._sf._s._free_buffer_unused = (_IO_free_type) free;
 
   new_f->fp.bufloc = bufloc;
   new_f->fp.sizeloc = sizeloc;
diff --git a/libio/strfile.h b/libio/strfile.h
index 74bd4eb269..7ce1008516 100644
--- a/libio/strfile.h
+++ b/libio/strfile.h
@@ -34,8 +34,11 @@ typedef void (*_IO_free_type) (void*);
 
 struct _IO_str_fields
 {
-  _IO_alloc_type _allocate_buffer;
-  _IO_free_type _free_buffer;
+  /* These members are preserved for ABI compatibility.  The glibc
+     implementation always calls malloc/free for user buffers if
+     _IO_USER_BUF or _IO_FLAGS2_USER_WBUF are not set.  */
+  _IO_alloc_type _allocate_buffer_unused;
+  _IO_free_type _free_buffer_unused;
 };
 
 /* This is needed for the Irix6 N32 ABI, which has a 64 bit off_t type,
@@ -55,10 +58,6 @@ typedef struct _IO_strfile_
   struct _IO_str_fields _s;
 } _IO_strfile;
 
-/* dynamic: set when the array object is allocated (or reallocated)  as
-   necessary to hold a character sequence that can change in length. */
-#define _IO_STR_DYNAMIC(FP) ((FP)->_s._allocate_buffer != (_IO_alloc_type)0)
-
 /* frozen: set when the program has requested that the array object not
    be altered, reallocated, or freed. */
 #define _IO_STR_FROZEN(FP) ((FP)->_f._IO_file_flags & _IO_USER_BUF)
diff --git a/libio/strops.c b/libio/strops.c
index d72a47c649..625c09416c 100644
--- a/libio/strops.c
+++ b/libio/strops.c
@@ -61,7 +61,7 @@ _IO_str_init_static_internal (_IO_strfile *sf, char *ptr, _IO_size_t size,
       fp->_IO_read_end = end;
     }
   /* A null _allocate_buffer function flags the strfile as being static. */
-  sf->_s._allocate_buffer = (_IO_alloc_type) 0;
+  sf->_s._allocate_buffer_unused = (_IO_alloc_type) 0;
 }
 
 void
@@ -103,8 +103,7 @@ _IO_str_overflow (_IO_FILE *fp, int c)
 	  _IO_size_t new_size = 2 * old_blen + 100;
 	  if (new_size < old_blen)
 	    return EOF;
-	  new_buf
-	    = (char *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size);
+	  new_buf = malloc (new_size);
 	  if (new_buf == NULL)
 	    {
 	      /*	  __ferror(fp) = 1; */
@@ -113,7 +112,7 @@ _IO_str_overflow (_IO_FILE *fp, int c)
 	  if (old_buf)
 	    {
 	      memcpy (new_buf, old_buf, old_blen);
-	      (*((_IO_strfile *) fp)->_s._free_buffer) (old_buf);
+	      free (old_buf);
 	      /* Make sure _IO_setb won't try to delete _IO_buf_base. */
 	      fp->_IO_buf_base = NULL;
 	    }
@@ -182,15 +181,14 @@ enlarge_userbuf (_IO_FILE *fp, _IO_off64_t offset, int reading)
 
   _IO_size_t newsize = offset + 100;
   char *oldbuf = fp->_IO_buf_base;
-  char *newbuf
-    = (char *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (newsize);
+  char *newbuf = malloc (newsize);
   if (newbuf == NULL)
     return 1;
 
   if (oldbuf != NULL)
     {
       memcpy (newbuf, oldbuf, _IO_blen (fp));
-      (*((_IO_strfile *) fp)->_s._free_buffer) (oldbuf);
+      free (oldbuf);
       /* Make sure _IO_setb won't try to delete
 	 _IO_buf_base. */
       fp->_IO_buf_base = NULL;
@@ -346,7 +344,7 @@ void
 _IO_str_finish (_IO_FILE *fp, int dummy)
 {
   if (fp->_IO_buf_base && !(fp->_flags & _IO_USER_BUF))
-    (((_IO_strfile *) fp)->_s._free_buffer) (fp->_IO_buf_base);
+    free (fp->_IO_buf_base);
   fp->_IO_buf_base = NULL;
 
   _IO_default_finish (fp, 0);
diff --git a/libio/vasprintf.c b/libio/vasprintf.c
index a9a21545a2..50e3559ab8 100644
--- a/libio/vasprintf.c
+++ b/libio/vasprintf.c
@@ -54,8 +54,8 @@ _IO_vasprintf (char **result_ptr, const char *format, _IO_va_list args)
   _IO_JUMPS (&sf._sbf) = &_IO_str_jumps;
   _IO_str_init_static_internal (&sf, string, init_string_size, string);
   sf._sbf._f._flags &= ~_IO_USER_BUF;
-  sf._s._allocate_buffer = (_IO_alloc_type) malloc;
-  sf._s._free_buffer = (_IO_free_type) free;
+  sf._s._allocate_buffer_unused = (_IO_alloc_type) malloc;
+  sf._s._free_buffer_unused = (_IO_free_type) free;
   ret = _IO_vfprintf (&sf._sbf._f, format, args);
   if (ret < 0)
     {
diff --git a/libio/wmemstream.c b/libio/wmemstream.c
index 103a760bf5..fdabaae6a9 100644
--- a/libio/wmemstream.c
+++ b/libio/wmemstream.c
@@ -92,8 +92,8 @@ open_wmemstream (wchar_t **bufloc, _IO_size_t *sizeloc)
   _IO_wstr_init_static (&new_f->fp._sf._sbf._f, buf,
 			_IO_BUFSIZ / sizeof (wchar_t), buf);
   new_f->fp._sf._sbf._f._flags2 &= ~_IO_FLAGS2_USER_WBUF;
-  new_f->fp._sf._s._allocate_buffer = (_IO_alloc_type) malloc;
-  new_f->fp._sf._s._free_buffer = (_IO_free_type) free;
+  new_f->fp._sf._s._allocate_buffer_unused = (_IO_alloc_type) malloc;
+  new_f->fp._sf._s._free_buffer_unused = (_IO_free_type) free;
 
   new_f->fp.bufloc = bufloc;
   new_f->fp.sizeloc = sizeloc;
diff --git a/libio/wstrops.c b/libio/wstrops.c
index bb62fd6702..8f5d45a9d1 100644
--- a/libio/wstrops.c
+++ b/libio/wstrops.c
@@ -63,7 +63,7 @@ _IO_wstr_init_static (_IO_FILE *fp, wchar_t *ptr, _IO_size_t size,
       fp->_wide_data->_IO_read_end = end;
     }
   /* A null _allocate_buffer function flags the strfile as being static. */
-  (((_IO_strfile *) fp)->_s._allocate_buffer) = (_IO_alloc_type)0;
+  (((_IO_strfile *) fp)->_s._allocate_buffer_unused) = (_IO_alloc_type)0;
 }
 
 _IO_wint_t
@@ -95,9 +95,7 @@ _IO_wstr_overflow (_IO_FILE *fp, _IO_wint_t c)
 	      || __glibc_unlikely (new_size > SIZE_MAX / sizeof (wchar_t)))
 	    return EOF;
 
-	  new_buf
-	    = (wchar_t *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size
-									* sizeof (wchar_t));
+	  new_buf = malloc (new_size * sizeof (wchar_t));
 	  if (new_buf == NULL)
 	    {
 	      /*	  __ferror(fp) = 1; */
@@ -106,7 +104,7 @@ _IO_wstr_overflow (_IO_FILE *fp, _IO_wint_t c)
 	  if (old_buf)
 	    {
 	      __wmemcpy (new_buf, old_buf, old_wblen);
-	      (*((_IO_strfile *) fp)->_s._free_buffer) (old_buf);
+	      free (old_buf);
 	      /* Make sure _IO_setb won't try to delete _IO_buf_base. */
 	      fp->_wide_data->_IO_buf_base = NULL;
 	    }
@@ -186,16 +184,14 @@ enlarge_userbuf (_IO_FILE *fp, _IO_off64_t offset, int reading)
     return 1;
 
   wchar_t *oldbuf = wd->_IO_buf_base;
-  wchar_t *newbuf
-    = (wchar_t *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (newsize
-								* sizeof (wchar_t));
+  wchar_t *newbuf = malloc (newsize * sizeof (wchar_t));
   if (newbuf == NULL)
     return 1;
 
   if (oldbuf != NULL)
     {
       __wmemcpy (newbuf, oldbuf, _IO_wblen (fp));
-      (*((_IO_strfile *) fp)->_s._free_buffer) (oldbuf);
+      free (oldbuf);
       /* Make sure _IO_setb won't try to delete
 	 _IO_buf_base. */
       wd->_IO_buf_base = NULL;
@@ -357,7 +353,7 @@ void
 _IO_wstr_finish (_IO_FILE *fp, int dummy)
 {
   if (fp->_wide_data->_IO_buf_base && !(fp->_flags2 & _IO_FLAGS2_USER_WBUF))
-    (((_IO_strfile *) fp)->_s._free_buffer) (fp->_wide_data->_IO_buf_base);
+    free (fp->_wide_data->_IO_buf_base);
   fp->_wide_data->_IO_buf_base = NULL;
 
   _IO_wdefault_finish (fp, 0);