When performing pointer arithmetic, the size of the value to add to a pointer is automatically scaled to the size of the pointer's type. For instance, when adding a value to a pointer to a four-byte integer, the value will be scaled by a factor of four. Improper use of pointer arithmetic can lead to miscalculations that result in unintended program behavior.
Non-Compliant Code Example
In this example taken from Dowd, buf_ptr is used to insert new integers into buf, which is an array of 1024 integers. If there is data to be inserted into buf (which is indicated by havedata()) and buf_ptr has not been incremented past buf + sizeof(buf), then an integer is inserted into buf via buf_ptr. However, the sizeof operator returns the total number of bytes in buf, which, assuming four-byte integers, is 4096 bytes. This value is then scaled to the size of an integer and added to buf. As a result, the check to make sure integers are not written past the end of buf is incorrect and a buffer overflow occurs.
int buf[1024];
int *buf_ptr = buf;
while (havedata() && buf_ptr < buf + sizeof(buf))
{
*buf_ptr++ = parseint(getdata());
}
Compliant Code Example
To correct this example, the size of buf can be directly added to buf and used as an upper bound. The integer literal is scaled to the size of an integer and the upper bound of buf is correctly checked.
int buf[1024];
int *b = buf;
while (havedata() && b < buf+1024)
{
*b++ = parseint(getdata());
}
Risk Analysis
Failure to understand and properly use pointer arithmetic can allow an attacker to execute arbitrary code.
Reference
[[Dowd]] Chapter 6, "C Language Issues" (Vulnerabilities)