[PATCH] vfprintf: validate nargs and maybe allocate from heap

[Kees Cook <kees at outflux dot net>]

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.)


2012-02-05  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.



diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 006f546..593f5d4 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -60,7 +60,8 @@ tests := tstscanf test_rdwr test-popen tstgetln test-fseek \
 	 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 tst-grouping bug23 bug24
+	 scanf16 scanf17 tst-setvbuf1 tst-grouping bug23 bug24 \
+	 bug-vfprintf-nargs
 
 test-srcs = tst-unbputc tst-printf
 
diff --git a/stdio-common/bug-vfprintf-nargs.c b/stdio-common/bug-vfprintf-nargs.c
new file mode 100644
index 0000000..ad82713
--- /dev/null
+++ b/stdio-common/bug-vfprintf-nargs.c
@@ -0,0 +1,81 @@
+/* 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$d", " 1") != 0)
+    rc = 1;
+
+  /* Check behavior of 32-bit positional overflow.  */
+  sprintf (buf, "%%1$d %%%" PRIdPTR "$d", UINT32_MAX / sizeof (int));
+  if (format_failed (buf, "1 %$d") != 0)
+    rc = 1;
+
+  return rc;
+}
+
+/* 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 stack memory and crash, which is the
+   expected situation.  */
+#if __WORDSIZE == 32
+# define EXPECTED_STATUS 0
+#else
+# define EXPECTED_SIGNAL SIGSEGV
+#endif
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/stdio-common/vfprintf.c b/stdio-common/vfprintf.c
index 952886b..dbd97b5 100644
--- a/stdio-common/vfprintf.c
+++ b/stdio-common/vfprintf.c
@@ -236,6 +236,10 @@ vfprintf (FILE *s, const CHAR_T *format, va_list ap)
      0 if unknown.  */
   int readonly_format = 0;
 
+  /* For the argument descriptions, which may be allocated on the heap.  */
+  int args_malloced = 0;
+  int *args_type;
+
   /* This table maps a character into a number representing a
      class.  In each step there is a destination label for each
      class.  */
@@ -1648,9 +1652,9 @@ do_positional:
        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;
     int *args_size;
+    union printf_arg *args_value;
 
     /* Positional parameters refer to arguments directly.  This could
        also determine the maximum number of arguments.  Track the
@@ -1699,13 +1703,31 @@ do_positional:
 
     /* Determine the number of arguments the format string consumes.  */
     nargs = MAX (nargs, max_ref_arg);
+    bytes_per_arg = sizeof (*args_type) + sizeof (*args_size)
+                    + sizeof (*args_value);
+
+    /* 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_type = alloca (nargs * bytes_per_arg);
+    else if ((args_type = (int *) malloc (nargs * bytes_per_arg)) == NULL)
+      {
+        done = -1;
+        goto all_done;
+      }
+    else
+      args_malloced = 1;
+
     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));
+    args_size = args_type + nargs;
+    args_value = (union printf_arg *) (args_size + nargs);
 
     /* XXX Could do sanity check here: If any element in ARGS_TYPE is
        still zero after this loop, format is invalid.  For now we
@@ -1974,6 +1996,8 @@ do_positional:
   }
 
 all_done:
+  if (__builtin_expect (args_malloced, 0))
+    free (args_type);
   if (__builtin_expect (workstart != NULL, 0))
     free (workstart);
   /* Unlock the stream.  */
-- 
1.7.5.4


-- 
Kees Cook                                            @outflux.net