Hello. So I just competed in the HITCON competition with UTDCSG. I was personally able to solve only two problems: abyss I and II. I was going to move to abyss III and solve that, but that required reading up on kvm documentation, and I wasn't in the mood to doing that (but I think I had a good idea).
This was also my first real CTF competition trying out a recently obtained IDA 7 Pro with HexRays, which added a huge boost to my performance.
Unfortunately, not a lot of other teammates had time to contribute to HITCON CTF, so it was basically just me competing in it.
Before diving into the problem, I first looked at how the three binaries worked
together. I saw the
hypervisor.elf program essentially invoked KVM to prepare
a virtual machine loaded with a custom
kernel.bin kernel. This kernel is a
watered down version of linux with only certain syscalls implemented, and then
this kernel in turn invokes a statically linked executable.
In the hypervisor, we prepared an argument to
ld.so.2, which is the
ld.so.2 binary would load
user.elf and invoke it. After a bit of
reversing, I found that
user.elf is just a reverse polish calculator, but to
separate different operands I separated each of them with a ` operator. (It was
technically an invalid operator, but I noticed that the program just skips over
invalid operators so I just used that ;)
After I got a pretty good picture of how the infrasture worked for the
user.elf I found that there was an interesting exploit with the
unsigned int *swap()
Apparently, unlike other code, this function did not check the stack index to whether if it would be out of bounds. Intrestingly, the stack pointer happens to reside directly before the operand stack, so I can change the stack pointer to anything I want. Boom! Exploit found. (Though in my weary state, I naively chose to change it with -1, so I ended up doing a LOT of pivots to get to where I need).
So now I needed to figure out how to get code control. Instinctively, I thought of ret-to-libc to get shell. In hindsight, I'm glad I didn't go down that route, because it was until later I found out that a lot of the syscalls (include execve) were actually NOT implemented. At that point, I saw that a hint was released saying if there was NX. I thought for a moment, yeah duhh... but as I thought for a moment, maybe the kernel may not have enabled NX.
So I tried just simply modifying the GOT entry for printf, which was the logical
choice since it was only used when printing a number. Now the only remaining
issue was with trying to figure out a way around ASLR. This involved doing a
little bit more calculations within the reverse-polish calculator-esque program,
which was quite interesting. Then after that I modified the GOT entry to jump to
a shell code, which I prepared at the beginning of the stack. To do this I ended
up writing an assembler to both assemble the shellcode in a way that would
prepare my shellcode, and then run the operations to overwrite GOT entry. Here
is my special "assembly" code. I used a
ENDASM block to denote a
block of assembly code.
# Stack = 0x2020A0
In the end I ran this command:
python assembler.py exp1.asm | nc 18.104.22.168 31733
And the flag is:
So after digging around in the kernel a bit more, I found that the kernel syscalls actually were just wrappers that used IO ports to defer to the hypervisor. Before that I found that there were some IO ports that were servicing certain syscalls for the kernel:
int __fastcall process_io(unsigned __int16 port, kernel_state *state)
Hmm... okay so that's actually pretty damn cool to see how the kernel and user program interacts with the hypervisor.
Speaking of the kernel, originally, I thought the kernel was actually just a stock linux kernel. After looking into the hypervisor, I realized that the kernel was actually custom built (amazing!).
I was too lazy to find if there was a bug in the kernel code. So I thought, what if I just directly communicate to the IO ports to the hypervisor, circumventing the kernel.
I first tried it by doing the injecting the assembly code:
mov dx, 0xffff
mov eax, 0
out dx, eax
Which should cause a "PANIC" message within the hypervisor. To my surprise, this
actually worked when I connected to the network! This was a very exciting
moment... :) So it came down to trying to opening this
flag2 file and then
reading from it. Hmm... a little issue here...
I'm in a user program, which has virtual addresses.
I need pointers to some real address space.
The problem is, I could be running a program that can access some memory located at 0x100000000 without necessarily having the address space from 0-0xffffffff accessible. This allows every program to "believe" it owns the entire address space... yay!
But this results in a huge problem because the hypervisor expects pointers to
real address values... correction, offsets from the beginning of this memory
block. Though I noticed something perculiar. First, the kernel is actually
loaded at real address ZERO, and then I noticed that the kernel is doing some
strange XOR operation, xoring
0x8000000000 with all of its addresses before
passing them to the hypervisor.
Perhaps, in virtual memory the kernel is located at a static address 0x8000000000, so maybe I could just write somewhere in Kernel, and pretend to the hypervisor that I actually own that memory. I ended up first issuing a "read" command first to write to kernel memory via user input, then issuing an "open" command, passing the pathname to address zero. This turned out to be quite nice, especially for the next stage, which I did not finish on time.
But wait... how do I pass arguments to read? See the code to read in the kernel
__int64 __fastcall hp_read(int fd, char *buff, __int64 size)
__int64 v3; // r12
signed __int64 args; // rax
void **args_1; // rbx
void **args_; // rax
unsigned int res; // ST0C_4
v3 = size;
args = kmalloc(0x18uLL, 0);
args = (void*)fd;
args = buff;
args = (void*)v3;
args_1 = (_QWORD *)args;
LODWORD(args_) = (unsigned __int64)kernel_to_hyper((char *)args);
res = serial_comm(0x8001u, (unsigned int)args_);
which means that the kernel actually passes a memory address of the argument
array to the hyperviser, rather than just the arguments directly. See my
requirements were very simple, for the first argument,
fd it should be 0, or
stdin. The second argument,
buff, should be 0 as well. It was the third
size that should preferably be nonzero, but I didn't need a specific
value, it just had to be big enough. Actually, after a bit of thinking I used
part of the syscall table as an array to pass into the read hyperviser IO port.
It was a perfect match because it had a lot of zero's, with a few sparse values,
not too large because they were addresses corresponding to the beginning of the
So what I ended up doing was a two step input process. First, I
would input a similar "reverse-polish notation" program as the first exploit,
but change the assembly payload a bit so to open up
flag2. Then I would pass
in the name of the file to read. Here is my python exploit 2 that does that:
from pwn import *
And my (abridged) assembly code:
0 # padding
Okay so the flag for this one was
Okay so I haven't totally finished it yet. So basically what I have done so far, I want to be able to get kernel shellcode to run so that I can easily access kernel memory.
So first, I can simply copy the code from part 2, but instead of writing a flag,
I overwrite the first part of the kernel. Since I know that there is a function
that is called when the
syscall instruction is invoked, I can just overwrite
the first part of that syscall handler function, replacing it with a jump to a
part of kernel that I control, let's just use the address 0 to begin my
shellcode. I had a bit of trouble with the machine mysteriously shutting down
right after the IO read (not a syscall) when I tried replacing the 103'th
byte with some value smaller than 21. No idea why it's happening.
To be continued...