commit 0262507918cfad7223bf81b8f162b7adc7a2af01
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Jun 1 10:43:06 2018 +0200

    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)

Backported from the upstream release/2.27/master branch.

diff --git a/debug/vasprintf_chk.c b/debug/vasprintf_chk.c
index a8ca32bad57b4d13..113354749ccf8d9a 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 e18a7756b297c9f4..9a51331e525c3468 100644
--- a/libio/memstream.c
+++ b/libio/memstream.c
@@ -87,8 +87,8 @@ open_memstream (char **bufloc, _IO_size_t *sizeloc)
   _IO_JUMPS ((struct _IO_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 4ea7548f9fa92638..9cd8e7c466616b52 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 fdd113a60811e593..129a0f6aeca818fd 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;
@@ -317,7 +315,7 @@ 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 282c86fff0a7ae0e..867ef4fe4ca4ec56 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 bd6d1798b1685fe9..3a9a681c80a321a7 100644
--- a/libio/wmemstream.c
+++ b/libio/wmemstream.c
@@ -90,8 +90,8 @@ 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 7a9a33ab8763b8ff..a31d0e23341b2aad 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;
@@ -326,7 +322,7 @@ 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);