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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.
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index a847b28..080badc 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -59,7 +59,8 @@ 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..13c66c0
--- /dev/null
+++ b/stdio-common/bug-vfprintf-nargs.c
@@ -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"
diff --git a/stdio-common/vfprintf.c b/stdio-common/vfprintf.c
index 863cd5d..022e72b 100644
--- a/stdio-common/vfprintf.c
+++ b/stdio-common/vfprintf.c
@@ -235,6 +235,9 @@ 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. */
+ 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. */
@@ -1647,9 +1650,10 @@ 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;
+ 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
@@ -1698,13 +1702,33 @@ do_positional:
/* 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
@@ -1973,8 +1997,8 @@ do_positional:
}
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);
--
1.7.5.4
--
Kees Cook @outflux.net