CVE-2015-1472: wscanf allocates too little memory

BZ #16618

Under certain conditions wscanf can allocate too little memory for the
to-be-scanned arguments and overflow the allocated buffer.  The
implementation now correctly computes the required buffer size when
using malloc.

A regression test was added to tst-sscanf.

ChangeLog

@@ -1,3 +1,11 @@
+2015-02-05  Paul Pluzhnikov  <ppluzhnikov@google.com>
+
+	[BZ #16618]
+	* stdio-common/tst-sscanf.c (main): Test for buffer overflow.
+	* stdio-common/vfscanf.c (_IO_vfscanf_internal): Compute needed
+	size in bytes. Store needed elements in wpmax. Use needed size
+	in bytes for extend_alloca.
+
 2015-02-05  Carlos O'Donell  <carlos@systemhalted.org>
 
 	* manual/install.texi: Latest tested versions are GCC 4.9.2,

NEWS

@@ -10,15 +10,21 @@ Version 2.21
 * The following bugs are resolved with this release:
 
   6652, 10672, 12674, 12847, 12926, 13862, 14132, 14138, 14171, 14498,
-  15215, 15378, 15884, 16009, 16418, 16191, 16469, 16576, 16617, 16619,
+  15215, 15378, 15884, 16009, 16418, 16191, 16469, 16576, 16617, 16618,
-  16657, 16740, 16857, 17192, 17266, 17273, 17344, 17363, 17370, 17371,
+  16619, 16657, 16740, 16857, 17192, 17266, 17273, 17344, 17363, 17370,
-  17411, 17460, 17475, 17485, 17501, 17506, 17508, 17522, 17555, 17570,
+  17371, 17411, 17460, 17475, 17485, 17501, 17506, 17508, 17522, 17555,
-  17571, 17572, 17573, 17574, 17582, 17583, 17584, 17585, 17589, 17594,
+  17570, 17571, 17572, 17573, 17574, 17582, 17583, 17584, 17585, 17589,
-  17601, 17608, 17616, 17625, 17630, 17633, 17634, 17635, 17647, 17653,
+  17594, 17601, 17608, 17616, 17625, 17630, 17633, 17634, 17635, 17647,
-  17657, 17658, 17664, 17665, 17668, 17682, 17702, 17717, 17719, 17722,
+  17653, 17657, 17658, 17664, 17665, 17668, 17682, 17702, 17717, 17719,
-  17723, 17724, 17725, 17732, 17733, 17744, 17745, 17746, 17747, 17748,
+  17722, 17723, 17724, 17725, 17732, 17733, 17744, 17745, 17746, 17747,
-  17775, 17777, 17780, 17781, 17782, 17791, 17793, 17796, 17797, 17801,
+  17748, 17775, 17777, 17780, 17781, 17782, 17791, 17793, 17796, 17797,
-  17803, 17806, 17834, 17844, 17848, 17868, 17869, 17870, 17885, 17892.
+  17801, 17803, 17806, 17834, 17844, 17848, 17868, 17869, 17870, 17885,
+  17892.
+
+* CVE-2015-1472 Under certain conditions wscanf can allocate too little
+  memory for the to-be-scanned arguments and overflow the allocated
+  buffer.  The implementation now correctly computes the required buffer
+  size when using malloc.
 
 * A new semaphore algorithm has been implemented in generic C code for all
   machines. Previous custom assembly implementations of semaphore were

stdio-common/tst-sscanf.c

@@ -233,5 +233,38 @@ main (void)
 	}
     }
 
+  /* BZ #16618
+     The test will segfault during SSCANF if the buffer overflow
+     is not fixed.  The size of `s` is such that it forces the use
+     of malloc internally and this triggers the incorrect computation.
+     Thus the value for SIZE is arbitrariy high enough that malloc
+     is used.  */
+  {
+#define SIZE 131072
+    CHAR *s = malloc ((SIZE + 1) * sizeof (*s));
+    if (s == NULL)
+      abort ();
+    for (size_t i = 0; i < SIZE; i++)
+      s[i] = L('0');
+    s[SIZE] = L('\0');
+    int i = 42;
+    /* Scan multi-digit zero into `i`.  */
+    if (SSCANF (s, L("%d"), &i) != 1)
+      {
+	printf ("FAIL: bug16618: SSCANF did not read one input item.\n");
+	result = 1;
+      }
+    if (i != 0)
+      {
+	printf ("FAIL: bug16618: Value of `i` was not zero as expected.\n");
+	result = 1;
+      }
+    free (s);
+    if (result != 1)
+      printf ("PASS: bug16618: Did not crash.\n");
+#undef SIZE
+  }
+
+
   return result;
 }

stdio-common/vfscanf.c

@@ -272,9 +272,10 @@ _IO_vfscanf_internal (_IO_FILE *s, const char *format, _IO_va_list argptr,
       if (__glibc_unlikely (wpsize == wpmax))				      \
 	{								    \
 	  CHAR_T *old = wp;						    \
-	  size_t newsize = (UCHAR_MAX + 1 > 2 * wpmax			    \
+	  bool fits = __glibc_likely (wpmax <= SIZE_MAX / sizeof (CHAR_T) / 2); \
-			    ? UCHAR_MAX + 1 : 2 * wpmax);		    \
+	  size_t wpneed = MAX (UCHAR_MAX + 1, 2 * wpmax);		    \
-	  if (use_malloc || !__libc_use_alloca (newsize))		    \
+	  size_t newsize = fits ? wpneed * sizeof (CHAR_T) : SIZE_MAX;	    \
+	  if (!__libc_use_alloca (newsize))				    \
 	    {								    \
 	      wp = realloc (use_malloc ? wp : NULL, newsize);		    \
 	      if (wp == NULL)						    \
@@ -286,14 +287,13 @@ _IO_vfscanf_internal (_IO_FILE *s, const char *format, _IO_va_list argptr,
 		}							    \
 	      if (! use_malloc)						    \
 		MEMCPY (wp, old, wpsize);				    \
-	      wpmax = newsize;						    \
+	      wpmax = wpneed;						    \
 	      use_malloc = true;					    \
 	    }								    \
 	  else								    \
 	    {								    \
 	      size_t s = wpmax * sizeof (CHAR_T);			    \
-	      wp = (CHAR_T *) extend_alloca (wp, s,			    \
+	      wp = (CHAR_T *) extend_alloca (wp, s, newsize);		    \
-					     newsize * sizeof (CHAR_T));    \
 	      wpmax = s / sizeof (CHAR_T);				    \
 	      if (old != NULL)						    \
 		MEMCPY (wp, old, wpsize);				    \