[PATCH v2] vfprintf stack overflow [BZ #16617]

[Florian Weimer <fweimer at redhat dot com>]

On 12/05/2014 05:43 PM, Florian Weimer wrote:
> This fell through the cracks.  I took Jeff Law's patch (which we carry
> as a local patch in Fedora and downstream), compressed the bug23-3.c
> test case, and added Joseph's test case from the bug as bug23-4.c.

Second patch, with overflow and NULL check, and a short NEWS entry.

--
Florian Weimer / Red Hat Product Security

>From 19e372f07ee0f31dc9585d0b806a8e6afef3d315 Mon Sep 17 00:00:00 2001
From: Jeff Law <law@redhat.com>
Date: Fri, 5 Dec 2014 15:49:34 +0100
Subject: [PATCH] CVE-2012-3406: Stack overflow in vfprintf [BZ #16617]

A larger number of format specifiers coudld cause a stack overflow,
potentially allowing to bypass _FORTIFY_SOURCE format string
protection.

2014-12-05  Jeff Law  <law@redhat.com>

	[BZ #16617]
	* stdio-common/vfprintf.c (vfprintf): Allocate large specs array
	on the heap.  (CVE-2012-3406)
	* stdio-common/bug23-2.c, stdio-common/bug23-3.c: New file.
	* stdio-common/bug23-4.c: New file.  Test case by Joseph Myers.
	* stdio-common/Makefile (tests): Add bug23-2, bug23-3, bug23-4.

diff --git a/NEWS b/NEWS
index 1b08d40..821892f 100644
--- a/NEWS
+++ b/NEWS
@@ -10,11 +10,11 @@ Version 2.21
 * The following bugs are resolved with this release:
 
   6652, 12926, 13862, 14132, 14138, 14171, 14498, 15215, 15884, 16469,
-  16619, 16740, 16857, 17192, 17266, 17344, 17363, 17370, 17371, 17411,
-  17460, 17475, 17485, 17501, 17506, 17508, 17522, 17555, 17570, 17571,
-  17572, 17573, 17574, 17581, 17582, 17583, 17584, 17585, 17589, 17594,
-  17601, 17608, 17616, 17625, 17633, 17647, 17653, 17664, 17665, 17668,
-  17682.
+  16617, 16619, 16740, 16857, 17192, 17266, 17344, 17363, 17370, 17371,
+  17411, 17460, 17475, 17485, 17501, 17506, 17508, 17522, 17555, 17570,
+  17571, 17572, 17573, 17574, 17581, 17582, 17583, 17584, 17585, 17589,
+  17594, 17601, 17608, 17616, 17625, 17633, 17647, 17653, 17664, 17665,
+  17668, 17682.
 
 * CVE-2104-7817 The wordexp function could ignore the WRDE_NOCMD flag
   under certain input conditions resulting in the execution of a shell for
@@ -22,6 +22,9 @@ Version 2.21
   implementation now checks WRDE_NOCMD immediately before executing the
   shell and returns the error WRDE_CMDSUB as expected.
 
+* CVE-2012-3406 printf-style functions could run into a stack overflow when
+  processing format strings with a large number of format specifiers.
+
 * The minimum GCC version that can be used to build this version of the GNU
   C Library is GCC 4.6.  Older GCC versions, and non-GNU compilers, can
   still be used to compile programs using the GNU C Library.
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 5f8e534..24e8496 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -57,7 +57,7 @@ tests := tstscanf test_rdwr test-popen tstgetln test-fseek \
 	 bug19 bug19a tst-popen2 scanf13 scanf14 scanf15 bug20 bug21 bug22 \
 	 scanf16 scanf17 tst-setvbuf1 tst-grouping bug23 bug24 \
 	 bug-vfprintf-nargs tst-long-dbl-fphex tst-fphex-wide tst-sprintf3 \
-	 bug25 tst-printf-round bug26
+	 bug25 tst-printf-round bug23-2 bug23-3 bug23-4
 
 test-srcs = tst-unbputc tst-printf
 
diff --git a/stdio-common/bug23-2.c b/stdio-common/bug23-2.c
new file mode 100644
index 0000000..9e0cfe6
--- /dev/null
+++ b/stdio-common/bug23-2.c
@@ -0,0 +1,70 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+static const char expected[] = "\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55\
+\n\
+a\n\
+abbcd55%%%%%%%%%%%%%%%%%%%%%%%%%%\n";
+
+static int
+do_test (void)
+{
+  char *buf = malloc (strlen (expected) + 1);
+  snprintf (buf, strlen (expected) + 1,
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+	    "%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\n",
+	    "a", "b", "c", "d", 5);
+  return strcmp (buf, expected) != 0;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/stdio-common/bug23-3.c b/stdio-common/bug23-3.c
new file mode 100644
index 0000000..57c8cef
--- /dev/null
+++ b/stdio-common/bug23-3.c
@@ -0,0 +1,50 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+int
+do_test (void)
+{
+  size_t instances = 16384;
+#define X0 "\n%1$s\n" "%1$s" "%2$s" "%2$s" "%3$s" "%4$s" "%5$d" "%5$d"
+  const char *item = "\na\nabbcd55";
+#define X3 X0 X0 X0 X0 X0 X0 X0 X0
+#define X6 X3 X3 X3 X3 X3 X3 X3 X3
+#define X9 X6 X6 X6 X6 X6 X6 X6 X6
+#define X12 X9 X9 X9 X9 X9 X9 X9 X9
+#define X14 X12 X12 X12 X12
+#define TRAILER "%%%%%%%%%%%%%%%%%%%%%%%%%%"
+#define TRAILER2 TRAILER TRAILER
+  size_t length = instances * strlen (item) + strlen (TRAILER) + 1;
+
+  char *buf = malloc (length + 1);
+  snprintf (buf, length + 1,
+	    X14 TRAILER2 "\n",
+	    "a", "b", "c", "d", 5);
+
+  const char *p = buf;
+  size_t i;
+  for (i = 0; i < instances; ++i)
+    {
+      const char *expected;
+      for (expected = item; *expected; ++expected)
+	{
+	  if (*p != *expected)
+	    {
+	      printf ("mismatch at offset %zu (%zu): expected %d, got %d\n",
+		      (size_t) (p - buf), i, *expected & 0xFF, *p & 0xFF);
+	      return 1;
+	    }
+	  ++p;
+	}
+    }
+  if (strcmp (p, TRAILER "\n") != 0)
+    {
+      printf ("mismatch at trailer: [%s]\n", p);
+      return 1;
+    }
+  free (buf);
+  return 0;
+}
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/stdio-common/bug23-4.c b/stdio-common/bug23-4.c
new file mode 100644
index 0000000..a478564
--- /dev/null
+++ b/stdio-common/bug23-4.c
@@ -0,0 +1,31 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/resource.h>
+
+#define LIMIT 1000000
+
+int
+main (void)
+{
+  struct rlimit lim;
+  getrlimit (RLIMIT_STACK, &lim);
+  lim.rlim_cur = 1048576;
+  setrlimit (RLIMIT_STACK, &lim);
+  char *fmtstr = malloc (4 * LIMIT + 1);
+  if (fmtstr == NULL)
+    abort ();
+  char *output = malloc (LIMIT + 1);
+  if (output == NULL)
+    abort ();
+  for (size_t i = 0; i < LIMIT; i++)
+    memcpy (fmtstr + 4 * i, "%1$d", 4);
+  fmtstr[4 * LIMIT] = '\0';
+  int ret = snprintf (output, LIMIT + 1, fmtstr, 0);
+  if (ret != LIMIT)
+    abort ();
+  for (size_t i = 0; i < LIMIT; i++)
+    if (output[i] != '0')
+      abort ();
+  return 0;
+}
diff --git a/stdio-common/vfprintf.c b/stdio-common/vfprintf.c
index c4ff833..768c170 100644
--- a/stdio-common/vfprintf.c
+++ b/stdio-common/vfprintf.c
@@ -263,6 +263,12 @@ vfprintf (FILE *s, const CHAR_T *format, va_list ap)
   /* For the argument descriptions, which may be allocated on the heap.  */
   void *args_malloced = NULL;
 
+  /* For positional argument handling.  */
+  struct printf_spec *specs;
+
+  /* Track if we malloced the SPECS array and thus must free it.  */
+  bool specs_malloced = false;
+
   /* This table maps a character into a number representing a
      class.  In each step there is a destination label for each
      class.  */
@@ -1679,8 +1685,8 @@ do_positional:
     size_t nspecs = 0;
     /* A more or less arbitrary start value.  */
     size_t nspecs_size = 32 * sizeof (struct printf_spec);
-    struct printf_spec *specs = alloca (nspecs_size);
 
+    specs = alloca (nspecs_size);
     /* The number of arguments the format string requests.  This will
        determine the size of the array needed to store the argument
        attributes.  */
@@ -1721,11 +1727,38 @@ do_positional:
 	if (nspecs * sizeof (*specs) >= nspecs_size)
 	  {
 	    /* Extend the array of format specifiers.  */
+	    if (nspecs_size * 2 < nspecs_size)
+	      {
+		__set_errno (ENOMEM);
+		done = -1;
+		goto all_done;
+	      }
 	    struct printf_spec *old = specs;
-	    specs = extend_alloca (specs, nspecs_size, 2 * nspecs_size);
+	    if (__libc_use_alloca (2 * nspecs_size))
+	      specs = extend_alloca (specs, nspecs_size, 2 * nspecs_size);
+	    else
+	      {
+		nspecs_size *= 2;
+		specs = malloc (nspecs_size);
+		if (specs == NULL)
+		  {
+		    __set_errno (ENOMEM);
+		    done = -1;
+		    goto all_done;
+		  }
+	      }
 
 	    /* Copy the old array's elements to the new space.  */
 	    memmove (specs, old, nspecs * sizeof (*specs));
+
+	    /* If we had previously malloc'd space for SPECS, then
+	       release it after the copy is complete.  */
+	    if (specs_malloced)
+	      free (old);
+
+	    /* Now set SPECS_MALLOCED if needed.  */
+	    if (!__libc_use_alloca (nspecs_size))
+	      specs_malloced = true;
 	  }
 
 	/* Parse the format specifier.  */
@@ -2046,6 +2079,8 @@ do_positional:
   }
 
 all_done:
+  if (specs_malloced)
+    free (specs);
   if (__glibc_unlikely (args_malloced != NULL))
     free (args_malloced);
   if (__glibc_unlikely (workstart != NULL))
-- 
1.9.3