VMEmu uses [unicorn-engine](https://github.com/unicorn-engine/unicorn) to emulate `x86_64` instructions which make up the virtual machine handlers. This project is extremely simple in that it will check every executed instruction in order to find any `JMP` instruction which uses a register and jumps to a vm handler. When this JMP is executed all native registers, virtual scratch registers, and the virtual stack are saved into a trace entry. Emulation ends when a VMEXIT instruction is found. This project supports multi-code path virtual instruction code and will discover all code paths, this means virtual branching (JCC's) and switch cases (jmp tables & indirect jmp tables) are fully supported. You can continue the analysis using IDA outside of the virtual machine and then use VMEmu again once execution enters back into the virtual machine.
`vm_entry` is the code name for the vmp2 routine which is used to transition control flow from non-virtualized code to virtualized code. Prior to calling this function a value is pushed onto the stack.
#### encrypted opcode rva
```nasm
push 0xFFFFFFFF890001FA
jmp vm_entry
```
The value pushed onto the stack is an encrypted RVA to the vm instructions to execute. As you can see below, the value is decrypted inside of the `vm_entry` function and is loaded into RSI. The decryption is unique per-build, however it seems that there is only three transformations done, no more, no less.
#### decrypt opcode rva
```nasm
; pushed value on stack (0xFFFFFFFF890001FA)
mov esi, [rsp+0xA0]
; unique, per-build, decryption. there seems to obly be 3 of these.
not esi
neg esi
ror esi, 0x1A
; vmp2 image base address is always 0x14000000...
mov rax, 0x100000000
add rsi, rax
; add module base
add rsi, [rbp+0]
```
If you decrypt the encrypted stack value, you end up with `0x140007EE2`. This is the image based address of the virtual instructions which will be executed by the vm. These virtual instructions are encrypted. I talk more about those in the `virtual instructions` section of the readme.
### calc_jmp - transition from one vm handler to another
`calc_jmp` is a part of the `vm_entry` routine. vm handler jmp's to this section of code in order to calculate the address of the next vm handler. This code decrypts the first opcode of the next virtual instruction, advances vip, obtains and decrypts the next vm handler's linear virtual address, and finally jmps to it. `AL` contains the index into the vm handler table after the jmp, and `BL` contains the decryption key for the first opcode. `BL` is then used by vm handlers to decrypt immediate values.
```nasm
; get the first opcode of the next virtual instruction
mov al, [rsi]
; this is in every single vmp2 binary...
; BL contains a "rolling" encryption key...
xor al, bl
; again, three unique transformations...
neg al
rol al, 5
inc al
; this is in every single vmp2 binary...
; BL is a "rolling" encryption key...
; it gets updated here...
xor bl, al
; index into the vm handler table...
movzx rax, al
mov rdx, [r12+rax*8]
; decrypt the vm handler rva...
xor rdx, 7F3D2149h
; advance the virtual instruction pointer (++VIP)
inc rsi
; jmp to next vm handler
add rdx, r13
jmp rdx
```
# virtual instructions - opcode's, vm handler index, and immediate values
virtual instructions are per-build encrypted RISC based instructions. They consist of an encrypted vm handler index, and optionally an encrypted immediate value. They are located inside of `.vmp` and `.text` sections. When execution of opcodes is desired, a push of an encrypted rva to these opcodes is done. Sadly, the rva is only 32bit's so you cannot easily run your own sequence of vm handlers. I suppose you could but you would need to be near the vmp'ed module in memory.
```
[vm handler index][immediate value up to 8 bytes]
```
*note that the vm handler index and immediate values are all encrypted*
#### opcode encryption
each opcode of a virtual instruction is encrypted by itself.
##### first opcode - vm handler index
```nasm
; note trans is short for transformation...
; this includes: xor, neg, not, nor, add, sub, etc
mov al, [rsi] ; RSI is VIP...
; BL is the byte address of the encrypted opcode...
; (only for the first decryption of the first opcode....)
trans al, bl ; AL is the encrypted opcode...
; note that there is ALWAYS three transformations...
trans al
trans al
trans al
trans bl, al ; at this point AL contains the decrypted opcode
; (which is the vm handler index)...
; BL now can decrypt the next opcode...
; also note that the transformation done to BL is also done to AL...
```
*first opcode decryption of the first virtual instruction*
```nasm
mov al, [rsi] ; RSI is VIP...
trans al, bl ; AL is the encrypted opcode...
; note that there is ALWAYS three transformations...
trans al
trans al
trans al
trans bl, al ; BL contains the decrypt key from the last virtual instruction...
; read the assembly above at this instruction for BL...
```
*first opcode decryption of the second virtual instruction*
##### second opcode - immediate values
Each virtual instruction can have a second opcode. This second opcode is decrypted by the "rolling" decryption key stored in `BL`. An example of a second opcode would be the first virtual instruction always executed.
```nasm
movzx eax, byte ptr [rsi]
; transformations done by vm handlers on immediate values are unique...
; unlike the `calc_jmp` block of code which vm handlers use to get to the next virtual instruction...
sub al, bl
; again, three unique transformations done on AL...
ror al, 2
not al
inc al
; the same transformation done al AL is then done on BL with the decrypted opcode...
sub bl, al
; this is some sort of VPOP and store instruction where a value comes off the virtual stack and into a place in RDI
; RDI seems to be scratch space, heap, or a per-binary virtual register mapping, not sure yet.
