I previously discussed the kernel self protection project. As part of this, I’ve been looking at free memory sanitization. This task is a great example about how features actually get into the kernel.
First, a description of sanitization. Under typical operation, when memory is freed back to the heap nothing happens to the data left in the memory. What’s there will remain there. If an attacker manages to find a use-after-free or arbitrary read bug, the attacker may be able to read whatever is sitting around in free memory. This can be harmless or very bad depending on what’s there. If what’s sitting around in free memory is encryption keys, someone is in for a very bad day. An easy way to reduce risk here is to clear the memory (sanitize) on free. This way, any sensitive data is no longer available for an attacker to get. This is a great example of what the self protection project is trying to do: issues such as arbitrary read and use-after-free are still bugs that need to be fixed but their risk can be reduced somewhat with sanitization.
Sanitization is not a new idea. Grsecurity has had it for some time. I have some background in working with kernel memory management already so it was a good match of my skills to a missing feature. Given Grsecurity had a working implementation of this already, I elected to use that as a starting point for the first submission. Typically, the upstream community likes features as small separate patches which can be reviewed individually. The Grsecurity patch is not structured this way so getting it in a form which could be submitted involved picking pieces out of the mega patch and turning those pieces into smaller patches. This is similar to doing a backport of a patch and much of the same thought processes apply here as well (i.e. blindly copy pasting will lead to trouble).
Once the patches were completed and sent out, the reviewers had a lot to say. The first thing that popped up was “why is this necessary, we have slub_debug already”. This is a very common occurrence when having patches reviewed: as the patch author you know why you want the patch submitted but the reviewers can’t read your mind. Well written commit text and cover letters go a long way towards understanding but questions will still come up. My thought was there was too much other ‘debug’ work being done with slub debug to make it useful as a security feature even though it did provide the necessary functionality. The slub maintainer disagreed and requested I make use of the existing infrastructure and fix up issues instead of adding new code. In general if a maintainer says no, you have to be really convinced that you are right to continue arguing and win. Maintainers have to be looking out for more than just your feature so they usually have good reason to say no. I had considered the exact suggestion when I was starting out so I didn’t see a good reason to continue arguing my case.
One of the biggest concerns with sanitization is performance. Doing an extra memset on every free is going to be an expensive operation. The existing slub_debug infrastructure only used the slow path of allocation as well which has a notable impact on performance. The next version of my patch series added an option to let the debugging happen on the fast path instead of just the slow path. The slub allocator is well tuned and very performance sensitive so adding anything extra could impact benchmarks. I set up the change behind a Kconfig option so those who wanted the old behavior could turn it off. When I measured some benchmarks they didn’t seem to be affected much. I considered the minor difference a trade off of performance vs. a security feature.
The slub maintainer had a different opinion though. The difference while small was still an increase which was considered unacceptable. The request was to work on making the slow path faster instead of impacting the fast path. This, again, goes back to the point that maintainers have to be looking out for the entire code base and not just one feature. It’s frustrating to hear but ultimately it makes the overall kernel better. The maintainer also had good feedback on a few other parts of the series. This is another advantage of breaking down the patches into smaller parts: it’s easier to indicate which parts are okay with a little bit of work and which parts need a new approach.
Most of the existing optimization in the slub allocator has occurred on the fast path. Nobody has really looked at the optimization of the debug path at all. It was pretty easy to find several spots where performance could be improved. The slub maintainer agreed and Acked the patches. This was a nice milestone for the sanitization feature. There’s still more work to be done to get it to match the level of Grsecurity in terms of coverage and performance.
So what are the lessons here?
- Kernel development is iterative. It generally takes more than one patch version to get anything beyond the simples features accepted.
- Be persistent. It’s easy to let a feature fall off because a maintainer says no but if you really want it to go in you have to keep working with the feedback.
- Small steps are much easier to deal with than huge changes
- What you end up with may look completely different than what you started with. The larger goal is more important than the individual patches.
- Take the bigger picture into account when working on features. Think about what else might be affected. How might the maintainers react?
- Make a choice about a design and then go with it. It’s easy to get bogged down trying to figure out what’s the best design. Sometimes you won’t actually know until you ask others so do the best you can.
- This entire process only works when there is positive communication. It’s important to say that an approach won’t work and it’s important that the message come across respectfully. You do not have to compromise on either part.