To Add Capsicum or Not To Add Capsicum

David Drysdale noticed that FreeBSD had a new security feature called Capsicum that might work well in Linux. He gave a link to a paper from the 19th USENIX Security Symposium in 2010 describing the project [1].

The idea was to implement fine-grained security privileges so that applications could isolate their own abilities and prevent an attacker from forcing them to do the wrong thing. David gave the example of tcpdump constraining itself to read only from the network file descriptor and write only to standard output. An interesting aspect of this type of security is that the application must be aware of the security features provided by the operating system and include code to take advantage of them.

David posted some of his implementation ideas, but Eric W. Biederman felt that most of these were badly conceived. For example, Capsicum required that the kernel police the rights checks of file descriptors, and David thought that the best place to do that was in the code that converted a userspace file descriptor into a kernel space file pointer structure. This in turn, David said, would require implementing an extensive and invasive abstraction layer within the kernel code. However, Eric pointed out that the abstraction layer wasn't necessary, because filesystem "capabilities" had already existed for 20 years and (with some modifications) could perform a similar function.

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