Monday, April 6, 2009

Linux kernel 2.6.27 exploits

I found two exploits of the Linux kernel back in January.

The first one is a crash in inotify, where a locking imbalance would unlock the inotify mutex twice before returning to userspace. It happens only if the buffer passed to read() was an invalid userspace address. It's fixed in mainline and went into -stable last month (if I recall correctly), though I can't say the fix has showed up in the Fedora kernel yet. The result is usually a BUG in the kernel log with list corruption. The system is mostly unusable afterwards, probably because the program is holding some lock at the time that it is terminated by the kernel. So it's a local DOS.

The second one is a deadlock in splice() code, which happens because splice needs to lock two mutexes, and it needs to drop them at one point too. So with two or more threads executing concurrently, they may grab them in opposite order. Oops. The result is that the process hangs as an unkillable zombie. This may or may not be so bad. If you run the exploit several times, you can end up with any number of zombies, and for some reason I don't know, zombies count in the load average, so with, say, 20 zombies, the load will rise up to around 20 (even though these processes don't actually use the CPU). I guess this makes for a nice local DOS, though. I tried to submit a fix, but it didn't make it to mainline (or -stable, or any other distribution that I know of) yet. It doesn't seem that anybody else is working on a fix either.

I guess the moral of the story is to review existing kernel code (as well as new patches), because it isn't actually perfect. I found these exploits by reading the source code and writing some test programs to test my theories, and I can testify that this method works well for finding errors.

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