CERT C: Rule ARR38-C

Mismatch between data length and size looks for memory copying functions such as memcpy, memset, or memmove. If you do not control the length argument and data buffer argument properly, Bug Finder raises a defect.

If an attacker can manipulate the data buffer or length argument, the attacker can cause buffer overflow by making the actual data size smaller than the length.

This mismatch in length allows the attacker to copy memory past the data buffer to a new location. If the extra memory contains sensitive information, the attacker can now access that data.

This defect is similar to the SSL Heartbleed bug.

When copying or manipulating memory, compute the length argument directly from the data so that the sizes match.

#include <stdlib.h>
#include <string.h>

typedef struct buf_mem_st {
    char *data;
    size_t max;     /* size of buffer */
} BUF_MEM;

extern BUF_MEM beta;

int cpy_data(BUF_MEM *alpha)
{
    BUF_MEM *os = alpha;
    int num, length;

    if (alpha == 0x0) return 0;
    num = 0;

    length = *(unsigned short *)os->data;
    memcpy(&(beta.data[num]), os->data + 2, length); //Noncompliant

    return(1);
}

This function copies the buffer alpha into a buffer beta. However, the length variable is not related to data+2.

One possible correction is to check the length of your buffer against the maximum value minus 2. This check ensures that you have enough space to copy the data to the beta structure.

#include <stdlib.h>
#include <string.h>

typedef struct buf_mem_st {
    char *data;
    size_t max;     /* size of buffer */
} BUF_MEM;

extern BUF_MEM beta;

int cpy_data(BUF_MEM *alpha)
{
    BUF_MEM *os = alpha;
    int num, length;

    if (alpha == 0x0) return 0;
    num = 0;

    length = *(unsigned short *)os->data;
    if (length<(os->max -2)) {
        memcpy(&(beta.data[num]), os->data + 2, length); 
    }

    return(1);

}

Invalid use of standard library memory routine occurs when a memory library function is called with invalid arguments. For instance, the memcpy function copies to an array that cannot accommodate the number of bytes copied.

Use of a memory library function with invalid arguments can result in issues such as buffer overflow.

The fix depends on the root cause of the defect. Often the result details show a sequence of events that led to the defect. You can implement the fix on any event in the sequence. If the result details do not show the event history, you can trace back using right-click options in the source code and see previous related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface.

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

#include <string.h>
#include <stdio.h>

char* Copy_First_Six_Letters(void)
 {
  char str1[10],str2[5];

  printf("Enter string:\n");
  scanf("%9s",str1);

  memcpy(str2,str1,6);  //Noncompliant
  /* Defect: Arguments of memcpy invalid: str2 has size < 6 */

  return str2;
 }

The size of string str2 is 5, but six characters of string str1 are copied into str2 using the memcpy function.

One possible correction is to adjust the size of str2 so that it accommodates the characters copied with the memcpy function.

#include <string.h>
#include <stdio.h>

char* Copy_First_Six_Letters(void)
 {
  /* Fix: Declare str2 with size 6 */
  char str1[10],str2[6]; 

  printf("Enter string:\n");
  scanf("%9s",str1);

  memcpy(str2,str1,6);
  return str2;
 }

Possible misuse of sizeof occurs when Polyspace^® Bug Finder™ detects possibly unintended results from the use of sizeof operator. For instance:

Incorrect use of the sizeof operator can cause the following issues:

Possible fixes are:

#define MAX_SIZE 1024

void func(int a[MAX_SIZE]) {
    int i;

    for (i = 0; i < sizeof(a)/sizeof(int); i++)    { //Noncompliant
        a[i] = i + 1;
    }
}

In this example, sizeof(a) returns the size of the pointer a and not the array size.

One possible correction is to use another means to determine the array size.

#define MAX_SIZE 1024

void func(int a[MAX_SIZE]) {
    int i;

    for (i = 0; i < MAX_SIZE; i++)    {
        a[i] = i + 1;
    }
}

Buffer overflow from incorrect string format specifier occurs when the format specifier argument for functions such as sscanf leads to an overflow or underflow in the memory buffer argument.

If the format specifier specifies a precision that is greater than the memory buffer size, an overflow occurs. Overflows can cause unexpected behavior such as memory corruption.

Use a format specifier that is compatible with the memory buffer size.

#include <stdio.h>

void func (char *str[]) {
    char buf[32];
    sscanf(str[1], "%33c", buf); //Noncompliant
}

In this example, buf can contain 32 char elements. Therefore, the format specifier %33c causes a buffer overflow.

One possible correction is to use a smaller precision in the format specifier.

#include <stdio.h>

void func (char *str[]) {
    char buf[32];
    sscanf(str[1], "%32c", buf);
}

Invalid use of standard library string routine occurs when a string library function is called with invalid arguments.

The risk depends on the type of invalid arguments. For instance, using the strcpy function with a source argument larger than the destination argument can result in buffer overflows.

The fix depends on the standard library function involved in the defect. In some cases, you can constrain the function arguments before the function call. For instance, if the strcpy function:

char * strcpy(char * destination, const char* source);

See examples of fixes below.

If you do not want to fix the issue, add comments to your result or code to avoid another review. See:

 #include <string.h>
 #include <stdio.h>
 
 char* Copy_String(void)
 {
  char *res;
  char gbuffer[5],text[20]="ABCDEFGHIJKL";

  res=strcpy(gbuffer,text);  //Noncompliant
  /* Error: Size of text is less than gbuffer */

  return(res);
 }

The string text is larger in size than gbuffer. Therefore, the function strcpy cannot copy text into gbuffer.

One possible correction is to declare the destination string gbuffer with equal or larger size than the source string text.

#include <string.h>
 #include <stdio.h>
 
 char* Copy_String(void)
 {
  char *res;
  /*Fix: gbuffer has equal or larger size than text */
  char gbuffer[20],text[20]="ABCDEFGHIJKL";

  res=strcpy(gbuffer,text);

  return(res);
 }

Destination buffer overflow in string manipulation occurs when certain string manipulation functions write to their destination buffer argument at an offset greater than the buffer size.

For instance, when calling the function sprintf(char* buffer, const char* format), you use a constant string format of greater size than buffer.

Buffer overflow can cause unexpected behavior such as memory corruption or stopping your system. Buffer overflow also introduces the risk of code injection.

One possible solution is to use alternative functions to constrain the number of characters written. For instance:

Another possible solution is to increase the buffer size.

#include <stdio.h>

void func(void) {
    char buffer[20];
    char *fmt_string = "This is a very long string, it does not fit in the buffer";

    sprintf(buffer, fmt_string); //Noncompliant
}

In this example, buffer can contain 20 char elements but fmt_string has a greater size.

One possible correction is to use the snprintf function to enforce length control.

#include <stdio.h>

void func(void) {
    char buffer[20];
    char *fmt_string = "This is a very long string, it does not fit in the buffer";

    snprintf(buffer, 20, fmt_string);
}

Destination buffer underflow in string manipulation occurs when certain string manipulation functions write to their destination buffer argument at a negative offset from the beginning of the buffer.

For instance, for the function sprintf(char* buffer, const char* format), you obtain the buffer from an operation buffer = (char*)arr; ... buffer += offset;. arr is an array and offset is a negative value.

Buffer underflow can cause unexpected behavior such as memory corruption or stopping your system. Buffer underflow also introduces the risk of code injection.

If the destination buffer argument results from pointer arithmetic, see if you are decrementing a pointer. Fix the pointer decrement by modifying either the original value before decrement or the decrement value.

#include <stdio.h>
#define offset -2

void func(void) {
    char buffer[20];
    char *fmt_string ="Text";

    sprintf(&buffer[offset], fmt_string);      //Noncompliant
}

In this example, &buffer[offset] is at a negative offset from the memory allocated to buffer.

One possible correction is to change the value of offset.

#include <stdio.h>
#define offset 2

void func(void) {
    char buffer[20];
    char *fmt_string ="Text";

    sprintf(&buffer[offset], fmt_string);     
}