It has been some time since I haven't post on my blog... well, I've been busy especially with the end of SATE, and oh well! had vacation :)

Anyway, at the next Static Analysis Workshop this Thursday, we're gonna talk about the SATE experiment and the observations/results we could get from this. I am then gonna talk about a tool I wrote in order to probe if a reported weakness is a false-positive: this is the Automated Evaluation.

The main idea of the Automated Evaluation, is to get some information on the source code and, under some assumptions, try to make a conclusion on the correctness of the piece of code. Behind all the reasoning from that particular tool, my approach had to be radically different than a classical SCA otherwise this would have been like creating a new SCA and this would have been obviously useless. The context of this automated evaluation is limited to the buffer overflows and this can only work for proving false-positive only!

So basically, I am reading the source code from the reported sink to the possibles sources and grabbing the actions that possibly affect the variable which have a role in the code.

These actions are like:

  • Allocation of a destination buffer
  • Computing the size of the source buffer(s)
  • Test for NULL
  • Test that involves the size of the buffers...
  • ... and some others

Then, once these actions are detected, the tool increments a global score of false-positiveness to this reported weakness. We then only have to set a threshold in order to know what correctness we want to have; this is really tied to the source code and how the program is developed.

Even though this evaluation method is not perfect, this was adapted to the C test cases we had in SATE 2008 since the global code quality was good. We can even say that the software were well written; it was then okay to make some assumption on the code such as:

  • If the size of the destination buffer is computed with the size of the source buffer, the size is good (basically: no off-by-one)

Also, the tool itself needs some information on the source code such since it uses regular expression to match the "actions"...



Here we are for a quick explanation and here are the slides: SAW: Automated Evaluation of SCA output