AnandTech: A bit about the NX bit; Virus Protection Woes

Introduction

NX protection seems great; it stops viruses dead in their tracks and eliminates those pesky buffer overflows we have been hearing so much about for the last 15 years. Well, maybe not. In fact it seems that NX provides several layers of false security, particularly since it only stops some buffer overflows and whether or not it stops any viruses has yet to be seen yet.

Although AMD and Intel both like to spin their newest NX/XD "Virus Protection" as a new feature on x86 technology, in reality NX behaves more like an emergency patch for an easily exploitable architecture. To really explore how NX benefits us, and where it provides a false sense of security, we must understand one of the scenarios that it supposedly prevents against.

The simplest explanation of a buffer overflow is when a program executes memory it was not intending to. Typically, this is done when a program writes something into memory that it has not properly allocated for; sometimes writing over pieces of the same program already in memory.

For example, let's take a look at the following program code:

int main(int argc, char **argv)
{
           char line[512];
           line[0] = 0;
           gets(line); 
           ...
           return 0;
}

Undoubtedly, even those who have taken only rudimentary programming classes are having a good chuckle with the monstrosity above. However, this code is actually nearly verbatim copied from the original UNIX fingerd program circa 1988. In UNIX (or any other operating system), the entire contents of the program are loaded into memory as machine code. A simple, although not necessarily proper, way to think about the above code is an array of several hundred bytes in memory; 512 bytes allocated for the array of characters, line, on line 3 of the code.

As the program starts flipping through the code in its array (which is actually called a stack), it reaches the function gets(). gets(), a horrible, ancient function, reads input and places it into the line array. But line has only enough memory allocated for 512 characters! It would only take a malicious user a few seconds to realize that writing more than 512 characters to the fingerd program would result in the ability to write data past the end of the line array. And since line exists inside a stack in memory, writing past the end of line actually starts to overwrite critical pieces of the program! If a user were to write 512 characters to fingerd followed by machine code, the user could essentially control any attribute of the entire machine. The example with fingerd above became the first real internet worm exploit, a buffer overflow. Worms like Nimda and Code Red work exactly the same as the original fingerd worm, so unfortunately 16 years later imprudent programming and exploitable hardware are still to blame.

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