I love to read about the Human brain and yesterday I was feeling weird about this thing. As far as I know, everyone (publicly) was trying to search sysent in one way or another after Apple removed the sysent symbols but not bruteforcing it. It seems no one bothered to question the original method (Landon Fuller?) and just kept using it. Are there any historical reasons for this? I can’t remember any. Sometimes, we are just blind to the simple things and solutions and don’t question them. It’s very probable that someone already posed the same question about the known methods (this is not cold fusion :P). Let’s continue…
This is a simple method to bruteforce 32 and 64bits kernels (tested with Snow Leopard and Lion) and retrieve sysent address. The __DATA segment where the sysent symbol is located is around 250kbytes in size, which is pretty a small space to search. My first method required the kernel base address to be configured, which isn’t a great issue – historically, it is quite stable. Computers exist to automate tasks and @snare “complained” about hardcoding that address. I was trying to fix checkidt to 64bits and the solution to overcome the hardcoded address just came to mind. The IDT can be used for this! Just retrieve the IDT address, then the address of interrupt 80 (or some other implemented interrupt handler) and you know where the kernel is loaded. Now it is just a matter of finding the start of the kernel mach-o address, reading the __DATA segment to know its address and size, and then bruteforce search sysent array.
This can be even easier if we just bruteforce beyond and before the int 80 handler – not a sexy approach, we want elegant bruteforcing ;-).
The PoC code is for an userland util that retrieves the information through /dev/kmem. I did a kernel port, which works without any problems (it’s even easier to implement). On my Mac it takes 0m0.035s to find sysent (0m0.012s on a 64bits Mac Mini server – thanks to Saure for all tests). That is very good for what should be a future-proof method to retrieve sysent. Unfortunately, hijacking sysent isn’t sexy anymore!
In other news, Snare finally created his own blog, available at http://ho.ax. He started with a nice article about kernel debugging in VMWare, updating my old one. Good work!
Of course this is competition so he should expect a very evil EFI rootkit one of these days with total destruction of his computer ;-). Give it a look while it lasts!
Well, time to move forward to the next project. Ideas continue to abound…
If you like to read and are looking for great books, I highly recommend “Thinking, Fast and Slow” by Daniel Kahneman, Dan Ariely’s two books, and “Being Wrong: Adventures in the Margin of Error” by Kathryn Schulz. The world would probably be a better place if everyone knew their potential “shortcomings”. There’s more behind the scenes in our brains that we consciously know and like to admit.
P.S.: If you are a github fan, I have been uploading stuff there.
P.S.2: Don’t forget the IRC channel, more active lately, irc.freenode.net, #osxre !