On free Mac Anti-Virus

On Tuesday, my pals at my old stomping ground Sophos launched their Free home edition Mac product. I’ve been asked by several people what makes it tick, so here’s Mac Anti-Virus In A Nutshell.

What is the AV doing?

So anti-virus is basically a categorisation technology: you look at a file and decide whether it’s bad. The traditional view people have of an AV engine is that there’s a huge table of file checksums, and the AV product just compares every file it encounters to every checksum and warns you if it finds a match. That’s certainly how it used to work around a decade ago, but even low-end products like ClamAV don’t genuinely work this way any more.

Modern Anti-Virus starts its work by classifying the file it’s looking at. This basically means deciding what type of file it is: a Mac executable, a Word document, a ZIP etc. Some of these are actually containers for other file types: a ZIP obviously contains other files, but a Word document contains sections with macros in which might be interesting. A Mac fat file contains one or more executable files, which each contains various data and program segments. Even a text file might actually contain a shell script (which could contain a perl script as a here doc), and so on. But eventually the engine will have classified zero or more parts of the file that it wants to inspect.

Because the engine now knows the type of the data it’s looking at, it can be clever about what tests it applies. So the engine contains a whole barrage of different tests, but still runs very quickly because it knows when any test is necessary. For example, most AV products now including Sophos’ can actually run x86 code in an emulator or sandbox, to see whether it would try to do something naughty. But it doesn’t bother trying to do that to a JPEG.

That sounds slow.

And the figures seem to bear that out: running a scan via the GUI can take hours, or even a day. A large part of this is due to limitations on the hard drive’s throughput, exacerbated by the fact that there’s no way to ask a disk to come up with a file access strategy that minimises seek time (time that’s effectively wasted while the disk moves its heads and platters to the place where the file is stored). Such a thing would mean reading the whole drive catalogue (its table of contents), and thinking for a while about the best order to read all of the files. Besides, such strategies fall apart when one of the other applications needs to open a file, because the hard drive has to jump away and get that one. So as this approach can’t work, the OS doesn’t support it.

On a Mac with a solid state drive, you actually can get to the point where CPU availability, rather than storage throughput, is the limiting factor. But surely even solid state drives are far too slow compared with CPUs, and the Anti-Virus app must be quite inefficient to be CPU-limited? Not so. Of course, there is some work that Sophos Anti-Virus must be doing in order to get worthwhile results, so I can’t say that it uses no CPU at all. But having dealt with the problem of hard drive seeking, we now meet the UBC.

The Unified Buffer Cache is a place in memory where the kernel holds the content of recently accessed files. As new files are read, the kernel throws away the contents of old files and stores the new one in the cache. Poor kernel. It couldn’t possibly know that this scanner is just going to do some tests on the file then never look at it again, so it goes to a lot of effort swapping contents around in its cache that will never get used. This is where a lot of the time ends up.

On not wasting all that time

This is where the on-access scanner comes in. If you look at the Sopohs installation, you’ll see an application at /Library/Sophos Anti-Virus/InterCheck.app – this is a small UNIX tool that includes a kernel extension to intercept file requests and test the target files. If it finds an infected file, it stops the operating system from opening it.

To find out how to this interception, you can do worse than look at Professional Cocoa Application Security, where I talk about the KAUTH (Kernel AUTHorisation) mechanism in Chapter 11. But the main point is that this approach – checking files when you ask for them – is actually more efficient than doing the whole scan. For a start, you’re only looking at files that are going to be needed anyway, so you’re not asking the hard drive to go out of its way and prepare loads of content that isn’t otherwise being used. InterCheck can also be clever about what it does, for example there’s no need to scan the same file twice if it hasn’t changed in the meantime.

OK, so it’s not a resource hog. But I still don’t need anti-virus.

Not true. This can best be described as anecdotal, but all of the people who reported to me that they had run a scan since the free Sophos product had become available, around 75% reported that it had detected threats. These were mainly Windows executables attached to mail, but it’s still good to detect and destroy those so they don’t get onto your Boot Camp partition or somebody else’s PC.

There definitely is a small, but growing, pile of malware that really does target Macs. I was the tech reviewer for Enterprise Mac Security, for the chapter on malware my research turned up tens of different strains: mainly Trojan horses (as on Windows), some OpenOffice macros, and some web-based threats. And that was printed well before Koobface was ported to the Mac.

Alright, it’s free, I’ll give it a go. Wait, why is it free?

Well here I have to turn to speculation. If your reaction to my first paragraph was “hang on, who is Sophos?”, then you’re not alone. Sophos is still a company that only sells to other businesses, and that means that the inhabitants of the Clapham Omnibus typically haven’t heard of them. Windows users have usually heard of Symantec via their Norton brand, McAfee and even smaller outfits like Kaspersky, so those are names that come up in the board room.

That explains why they might release a free product, but not this one. Well, now you have to think about what makes AV vendors different from one another, and really the answer is “not much”. They all sell pretty much the same thing, occasionally one of them comes up with a new feature but that gap usually closes quite quickly.

Cross-platform support is one area that’s still open, surprisingly. Despite the fact that loads of the vendors (and I do mean loads: Symantec, McAfee, Trend Micro, Sophos, Kaspersky, F-Secure, Panda and Eset all spring to mind readily) support the Mac and some other UNIX platforms, most of these are just checkbox products that exist to prop up their feature matrix. My suspicion is that by raising the profile of their Mac offering Sophos hopes to become the cross-platform security vendor. And that makes giving their Mac product away for free more valuable than selling it.