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TUCoPS :: General Information :: mult5602.htm

Flaw in calloc and similar routines



6th Aug 2002 [SBWID-5602]
COMMAND

	Flaw in calloc and similar routines

SYSTEMS AFFECTED

	RUS-CERT has verified that the following products are affected  by  this
	defect:
	

	     * C run-time libraries:

	          + GNU libc 2.2.5

	          + dietlibc CVS as of 2002-08-01

	          + Microsoft Visual C++ 4.0

	          + Microsoft Visual C++ 6.0

	

	     * language-specific allocators:

	          + GNU C++ Compiler (GCC 2.95, 3.0, 3.1.1)

	          + GNU Ada Compiler (GNAT 3.14p, GCC 3.1.1)

	          + Microsoft Visual C++ 6.0 (C++ new allocator)

	

	Probably many more products are affected. (If a product  is  not  listed
	above, it has not been examined.)

PROBLEM

	Florian    Weimer     [Weimer@CERT.Uni-Stuttgart.DE]     of     RUS-CERT
	[http://CERT.Uni-Stuttgart.DE/]               published               in
	[http://CERT.Uni-Stuttgart.DE/advisories/calloc.php] :
	

	Many memory allocation interfaces exhibit the same erratic  behavior  as
	the xdr_array Sun RPC function which has recently been published by  ISS
	(CAN-2002-0391).  All  these  interfaces  take  a  (somtimes   implicit)
	argument, the storage size of  the  element  type,  and  the  number  of
	elements. To compute the size of the memory area which is  needed,  both
	numbers are multipliedA. If  the  result  cannot  be  represented  in  a
	machine word, it can  happen  that  the  allocation  routine  returns  a
	pointer to an allocated area which is too small (instead  of  signalling
	an error condition  using  the  appropriate  mechanism  defined  by  the
	programming language). As a result, the application might overflow  this
	buffer. The defect discovered in the xdr_array function mentioned  above
	shows that errors in this class do have security implications.
	

	Typical code fragments which might lead to vulnerable  applications  are
	listed below.
	

	     * C: pointer = calloc(sizeof(element_t), count);

	     * C++: pointer = new ElementType[count];

	     * Ada: Array_Access := new Element_Type (1 .. Count);

	

	How To Detect The Defect
	

	In the calloc case, the source code should be examined. Constructs  like
	"size  =  count  *  element_size;"  without  any  overflow  checks   are
	problematic (and, similarly, expressions like "size *= nelems").
	

	In  the  C++  and  Ada  cases,  the  compiler  can  emit  machine   code
	instructions to calculate the total size in place. A small test  program
	like the following can be compiled:
	

	typedef struct {

	  char data[0x10];

	} DATA10;

	

	void allocate(unsigned size)

	{

	  DATA10 *x = new DATA10[size];

	}

	

	After that, the generated machine code has to be examined  for  overflow
	checking. In the Ada case, the following procedure can be used:
	

	procedure Test (Size : Positive) is

	

	   subtype Data10 is String (1 .. 16#0000_0010#);

	   type Data10_Array is array (Positive range <>) of Data10;

	

	   type Pointer is access Data10_Array;

	

	   V : Pointer;

	

	begin

	   V := new Data10_Array (1 .. Size);

	end;

	

	For both languages, the size might be computed using a library  routine.
	In this case,  the  source  code  of  the  library  routine  has  to  be
	examined.

SOLUTION

	The GNU libc CVS repository contains a patch to add  overflow  detection
	to calloc.
	

	Although in most cases, the present  overflows  can  be  detected  after
	they occured because  only  unsigned  types  are  involved,  programmers
	should be aware that overflow checking in C is not straightforward  (see
	RUS-CERT                      Advisory                       2002-08:01,
	http://cert.uni-stuttgart.de/advisories/c-integer-overflow.php).


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