Date: Thu, 16 Feb 2012 08:16:13 -0800
From: Kees Cook <kees@outflux.net>
To: "Ryan S. Arnold" <ryan.arnold@gmail.com>
Cc: libc-alpha@sourceware.org, Paul Eggert <eggert@cs.ucla.edu>,
        Roland McGrath <roland@hack.frob.com>,
        Andreas Schwab <schwab@linux-m68k.org>
Subject: Re: [PATCH] vfprintf: validate nargs and maybe allocate from heap

The nargs value can overflow when doing allocations, allowing arbitrary
memory writes via format strings, bypassing _FORTIFY_SOURCE:
http://www.phrack.org/issues.html?issue=67&id=9

This checks for nargs overflow and possibly allocates from heap instead of
stack, and adds a regression test for the situation.

I have FSF assignment via Google. (Sent from @outflux since that's how I'm
subscribed here, but CL shows @chromium.org as part of my Google work.)

This version disables the useless test on non-32-bit platforms.

2012-02-16  Kees Cook  <keescook@chromium.org>

        [BZ #13656]
        * stdio-common/vfprintf.c (vfprintf): Check for nargs overflow and
        possibly allocate from heap instead of stack.
        * stdio-common/bug-vfprintf-nargs.c: New file.
        * stdio-common/Makefile (tests): Add nargs overflow test.

--- glibc-2.11.1/stdio-common/bug-vfprintf-nargs.c	1970-01-01 01:00:00.000000000 +0100
+++ glibc-2.11.1/stdio-common/bug-vfprintf-nargs.c	2012-03-13 17:20:26.896916603 +0100
@@ -0,0 +1,78 @@
+/* Test for vfprintf nargs allocation overflow (BZ #13656).
+   Copyright (C) 2012 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Kees Cook <keescook@chromium.org>, 2012.
+
+   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, write to the Free
+   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+   02111-1307 USA.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <inttypes.h>
+#include <string.h>
+#include <signal.h>
+
+static int
+format_failed (const char *fmt, const char *expected)
+{
+  char output[80];
+
+  printf ("%s : ", fmt);
+
+  memset (output, 0, sizeof output);
+  /* Having sprintf itself detect a failure is good.  */
+  if (sprintf (output, fmt, 1, 2, 3, "test") > 0
+      && strcmp (output, expected) != 0)
+    {
+      printf ("FAIL (output '%s' != expected '%s')\n", output, expected);
+      return 1;
+    }
+  puts ("ok");
+  return 0;
+}
+
+static int
+do_test (void)
+{
+  int rc = 0;
+  char buf[64];
+
+  /* Regular positionals work.  */
+  if (format_failed ("%1$d", "1") != 0)
+    rc = 1;
+
+  /* Regular width positionals work.  */
+  if (format_failed ("%1$*2$d", " 1") != 0)
+    rc = 1;
+
+  /* Positional arguments are constructed via read_int, so nargs can only
+     overflow on 32-bit systems.  On 64-bit systems, it will attempt to
+     allocate a giant amount of memory and possibly crash, which is the
+     expected situation.  Since the 64-bit behavior is arch-specific, only
+     test this on 32-bit systems.  */
+  if (sizeof (long int) == 4)
+    {
+      sprintf (buf, "%%1$d %%%" PRIdPTR "$d", UINT32_MAX / sizeof (int));
+      if (format_failed (buf, "1 %$d") != 0)
+        rc = 1;
+    }
+
+  return rc;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
--- glibc-2.11.1/stdio-common/Makefile	2010-01-18 18:01:41.000000000 +0100
+++ glibc-2.11.1/stdio-common/Makefile	2012-03-13 17:20:46.423737906 +0100
@@ -60,7 +60,7 @@
 	 tst-popen tst-unlockedio tst-fmemopen2 tst-put-error tst-fgets \
 	 tst-fwrite bug16 bug17 tst-swscanf tst-sprintf2 bug18 bug18a \
 	 bug19 bug19a tst-popen2 scanf13 scanf14 scanf15 bug20 bug21 bug22 \
-	 scanf16 scanf17 tst-setvbuf1
+	 scanf16 scanf17 tst-setvbuf1 bug-vfprintf-nargs
 
 test-srcs = tst-unbputc tst-printf
 
--- glibc-2.11.1/stdio-common/vfprintf.c	2010-01-18 18:01:41.000000000 +0100
+++ glibc-2.11.1/stdio-common/vfprintf.c	2012-03-13 17:20:26.897916594 +0100
@@ -235,6 +235,9 @@
      0 if unknown.  */
   int readonly_format = 0;
 
+  /* For the argument descriptions, which may be allocated on the heap.  */
+  void *args_malloced = NULL;
+
   /* This table maps a character into a number representing a
      class.  In each step there is a destination label for each
      class.  */
@@ -1641,9 +1644,10 @@
        determine the size of the array needed to store the argument
        attributes.  */
     size_t nargs = 0;
-    int *args_type;
-    union printf_arg *args_value = NULL;
+    size_t bytes_per_arg;
+    union printf_arg *args_value;
     int *args_size;
+    int *args_type;
 
     /* Positional parameters refer to arguments directly.  This could
        also determine the maximum number of arguments.  Track the
@@ -1692,13 +1696,33 @@
 
     /* Determine the number of arguments the format string consumes.  */
     nargs = MAX (nargs, max_ref_arg);
+    bytes_per_arg = sizeof (*args_value) + sizeof (*args_size)
+                    + sizeof (*args_type);
+
+    /* Check for potential integer overflow.  */
+    if (nargs > SIZE_MAX / bytes_per_arg)
+      {
+         done = -1;
+         goto all_done;
+      }
 
     /* Allocate memory for the argument descriptions.  */
-    args_type = alloca (nargs * sizeof (int));
+    if (__libc_use_alloca (nargs * bytes_per_arg))
+        args_value = alloca (nargs * bytes_per_arg);
+    else
+      {
+        args_value = args_malloced = malloc (nargs * bytes_per_arg);
+        if (args_value == NULL)
+          {
+            done = -1;
+            goto all_done;
+          }
+      }
+
+    args_size = &args_value[nargs].pa_int;
+    args_type = &args_size[nargs];
     memset (args_type, s->_flags2 & _IO_FLAGS2_FORTIFY ? '\xff' : '\0',
-	    nargs * sizeof (int));
-    args_value = alloca (nargs * sizeof (union printf_arg));
-    args_size = alloca (nargs * sizeof (int));
+	    nargs * sizeof (*args_type));
 
     /* XXX Could do sanity check here: If any element in ARGS_TYPE is
        still zero after this loop, format is invalid.  For now we
@@ -1967,8 +1991,8 @@
   }
 
 all_done:
-  if (__builtin_expect (workstart != NULL, 0))
-    free (workstart);
+  free (args_malloced);
+  free (workstart);
   /* Unlock the stream.  */
   _IO_funlockfile (s);
   _IO_cleanup_region_end (0);
