Malloc improvements in 3.8-beta, testers needed!

This release will bring a lot of new ideas from us. One of them in particular is somewhat risky. I think it is time to talk about that one, and let people know what is ahead on our road.

Traditionally, Unix malloc(3) has always just "extended the brk", which means extending the traditional Unix process data segment to allocate more memory. malloc(3) would simply extend the data segment, and then calve off little pieces to requesting callers as needed. It also remembered which pieces were which, so that free(3) could do it's job.

The way this was always done in Unix has had a number of consequences, some of which we wanted to get rid of. In particular, malloc & free have not been able to provide strong protection against overflows or other corruption.

Our malloc implementation is a lot more resistant (than Linux) to "heap overflows in the malloc arena", but we wanted to improve things even more.

Starting a few months ago, the following changes were made:

- We made the mmap(2) system call return random memory addresses. As well the kernel ensures that two objects are not mapped next to each other; in effect, this creates unallocated memory which we call a "guard page".

- We have changed malloc(3) to use mmap(2) instead of extending the data segment via brk()

- We also changed free(3) to return memory to the kernel, un-allocating them out of the process.

- As before, objects smaller than a page are allocated within shared pages that malloc(3) maintains. But their allocation is now somewhat randomized as well.

- A number of other similar changes which are too dangerous for normal software or cause too much of a slowdown are available as malloc options as described in the manual page. These are very powerful for debugging buggy applications.

Other results:

- When you free an object that is >= 1 page in size, it is actually returned to the system. Attempting to read or write to it after you free is no longer acceptable. That memory is unmapped. You get a SIGSEGV.

- For a decade and a bit, we have been fixing software for buffer overflows. Now we are finding a lot of software that reads before the start of the buffer, or reads too far off the end of the buffer. You get a SIGSEGV.

To some of you, this will sound like what the Electric Fence toolkit used to be for. But these features are enabled by default. Electric Fence was also very slow. It took nearly 3 years to write these OpenBSD changes since performance was a serious consideration. (Early versions caused a nearly 50% slowdown).

Our changes have tremendous benefits, but until some bugs in external packages are found and fixed, there are some risks as well. Some software making incorrect assumptions will be running into these new security technologies.

I discussed this in talks I have given before: I said that we were afraid to go ahead with guard pages, because a lot of software is just written to such low standards. Applications over-read memory all the time, go 1 byte too far, read 1 byte too early, access memory after free, etc etc etc.

Oh well -- we've decided that we will try to ship with this protection mechanism in any case, and try to solve the problems as we run into them.

Two examples:

Over the last two months, some OpenBSD users noticed that the X server was crashing occasionally. Two bugs have been diagnosed and fixed by us. One was a use-after-free bug in the X shared library linker. The other was a buffer-over-read bug deep down in the very lowest level fb* pixmap compositing routines. The latter bug in particular was very difficult to diagnose and fix, and is about 10 years old. We have found other bugs like this in other external software, and even a few in the base OpenBSD tree (though those were found a while back, even as we started experimenting with the new malloc code).

I would bet money that the X fb* bug has crashed Linux (and other) X servers before. It is just that it was very rare, and noone ever chased it. The new malloc we have just makes code get lucky less often, which lets us get to the source of a bug easier. As a programmer, I appreciate anything which makes bugs easier to reproduce.

We expect that our malloc will find more bugs in software, and this might hurt our user community in the short term. We know that what this new malloc is doing is perfectly legal, but that realistically some open source software is of such low quality that it is just not ready for these things to happen.