i know this is kinda retarded but I just can't figure it out. I'm debugging this:
xor eax,eax
mov ah,[var1]
mov al,[var2]
call addition
stop: jmp stop
var1: db 5
var2: db 6
addition:
add ah,al
ret
the numbers that I find on addresses var1 and var2 are 0x0E and 0x07. I know it's not segmented, but that ain't reason for it to do such escapades, because the addition call works just fine. Could you please explain to me where is my mistake?
I see the problem, dunno how to fix it yet though. The thing is, for some reason the instruction pointer starts at 0x100 and all the segment registers at 0x1628. To address the instruction the used combination is i guess [cs:ip] (one of the segment registers and the instruction pointer for sure). The offset to var1 is 0x10 (probably because from the begining of the code it's the 0x10th byte in order), i tried to examine the memory and what i got was:
1628:100 8 bytes
1628:108 8 bytes
1628:110 <- wtf? (assume another 8 bytes)
1628:118 ...
whatever tricks are there in the memory [cs:var1] points somewhere else than in my code, which is probably where the label .data would usually address ds.... probably.. i don't know what is supposed to be at 1628:10
ok, i found out what caused the assness and wasted me whole fuckin day. the behaviour described above is just correct, the code is fully functional. what i didn't know is that grdb debugger for some reason sets the begining address to 0x100... the sollution is to insert the directive ORG 0x100 on the first line and that's the whole thing. the code was working because instruction pointer has the right address to first instruction and goes one by one, but your assembler doesn't know what effective address will be your program stored at so it pretty much remains relative to first line of the code which means all the variables (if not using label for data section) will remain pointing as if it started at 0x0. which of course wouldn't work with DOS. and grdb apparently emulates some DOS features... sry for the language, thx everyone for effort, hope this will spare someone's time if having the same problem...
heheh.. at least now i know the reason why to use .data section :))))
Assuming that is x86 assembly, var1 and var2 must reside in the .data section.
Explanation: I'm not going to explain exactly how the executable file is structured (not to mention this is platform-specific), but here's a general idea as to why what you're doing is not working.
Assembly code must be divided into data sections due to the fact that each data section corresponds directly (or almost directly) to a specific part of the binary/executable file. All global variables must be defined in the .data sections since they have a corresponding location in the binary file which is where all global data resides.
Defining a global variable (or a globally accessed part of the memory) inside the code section will lead to undefined behavior. Some x86 assemblers might even throw an error on this.
Related
I'm writing some ARM64 assembly code for macOS, and it needs to access a global variable.
I tried to use the solution in this SO answer, and it works fine if I just call the function as is. However, my application needs to patch some instructions of this function, and the way I'm doing it, the function gets moved somewhere else in memory in the process. Note the adrp/ldr pair is untouched during patching.
However, if I try to run the function after moving it elsewhere in memory, it no longer returns correct results. This happens even if I just memcpy() the code as is, without patching. After tracing with a debugger, I isolated the issue to the address of the global valuable being incorrectly loaded by the adrp/ldr pair (and weirdly, the ldr is assembled as an add, as seen with objdump straight after compiling the binary -- not sure if it's somehow related to the issue here.)
What would be the correct way to load a global variable, so that it survives the function being copied somewhere else and run from there?
Note the adrp/ldr pair is untouched during patching.
There's the issue. If you rip code out of the binary it's in, then you effectively need to re-link it.
There's two ways of dealing with this:
If you have complete control over the segment layout, then you could have one executable segment with all of your assembly in it, and right next to it one segment with all addresses that code needs, and make sure the assembly ONLY has references to things on that page. Then wherever you copy your assembly, you'd also copy the data page next to it. This would enable you to make use of static addresses that get rebased by the dynamic linker at the time your binary is loaded. This might look something like:
.section __ASM,__asm,regular
.globl _asm_stub
.p2align 2
_asm_stub:
adrp x0, _some_ref#PAGE
ldr x0, [x0, _some_ref#PAGEOFF]
ret
.section __REF,__ref
.globl _some_ref
.p2align 3
_some_ref:
.8byte _main
Compile that with -Wl,-segprot,__ASM,rx,rx and you'll get an executable __ASM and a writeable __REF segment. Those two would have to maintain their relative position to each other when they get copied around.
(Note that on arm64 macOS you cannot put symbol references into executable segments for the dynamic linker to rebase, because it will fault and crash while trying to do so, and even if it were able to do that, it would invalidate the code signature.)
You act as a linker, scanning for PC-relative instructions and re-linking them as you go. The list of PC-relative instructions in arm64 is quite short, so it should be a feasible amount of work:
adr and adrp
b and bl
b.cond (and bc.cond with FEAT_HBC)
cbz and cbnz
tbz and tbnz
ldr and ldrsw (literal)
ldr (SIMD & FP literal)
prfm (literal)
(You can look for the string PC[] in the ARMv8 Reference Manual to find all uses.)
For each of those you'd have to check whether their target address lies within the range that's being copied or not. If it does, then you'd leave the instruction alone (unless you copy the code to a different offset within the 4K page than it was before, in which case you have to fix up adrp instructions). If it isn't then you'll have to recalculate the offset and emit a new instruction. Some of the instructions have a really low maximum offset (tbz/tbnz ±32KiB). But usually the only instructions that reference addresses across function boundaries are adr, adrp, b, bl and ldr. If all code on the page is written by you then you can do adrp+add instead of adr and adrp+ldr instead of just ldr, and if you have compiler-generated code on there, then all adr's and ldr's will have a nop before or after, which you can use to turn them into an adrp combo. That should get your maximum reference range up to ±128MiB.
So recently I've been wanting to call some win32 calls from assembly, and I've been using NASM as my external assembler. I was calling SendMessage in my code in the following way:
call __imp__SendMessageW#16
This was assembled into a relative jump (0xE8 opcode) and the result was an access violation. In the debugger, the computed jump offset seemed to be the correct one (in that __imp__SendMessageW#16 really did seem to reside there) but nonetheless it did not work. Examining the assembly produced by Visual Studio when I called the function from C++, I noticed that it wasn't a relative immediate jump it was using, but instead (in the language of MASM) a call dword ptr [__imp__SendMessageW#16], corresponding to an 0xFF15 opcode. After some futzing around I figured out that NASM syntax encodes this as call dword near [dword __imp__SendMessageW#16], and making the change my code suddenly worked.
My question is, why does one work and not the other? Is there some relocation of code going on that causes the relative immediate call to jump somewhere unfriendly? I've never been much of an assembly programmer but my impression was always that the two calls should do the same thing and the main difference is that one is position independent and the other is not (assuming that they move the IP to the same place). The relocation of code theory makes sense given that, but then how do you explain the debugger showing the right address?
Also: what's the logic behind the [] syntax in this call? The offset is still an immediate (just little endian encoded immediately after 0xFF15), there's no memory access going on here beyond the instruction fetch (I tend to think of [] as a dereference outside the context of lea).
call dword[__imp__SendMessageW#16]
_imp_SendMessageW#16 is an address to your imports section that contains the address of the API function. You use the square brackets to deference (call the address STORED by this address)
I need to find null-free replacements for the following instructions so I can put the following code in shellcode.
The first instruction I need to convert to null-free is:
mov ebx, str ; the string containing /dev/zero
The string str is defined in my .data section.
The second is:
mov eax,0x5a
Thanks!
Assuming what you want to learn is how assembly code is made up, what type of instruction choices ends up in assembly code with specific properties, then (on x86/x64) do the following:
Pick up Intel's instruction set reference manuals (four volumes as of this writing, I think). They contain opcode tables (instruction binary formats), and detailed lists of all allowed opcodes for a specific assembly mnemonic (instruction name).
Familiarize yourself with those and mentally divide them into two groups - those that match your expected properties (like, not containing the 'x' character ... or any other specific one), and those that don't. The 2nd category you need to eliminate from your code if they're present.
Compile your code telling the compiler not to discard compile intermediates:gcc -save-temps -c csource.c
Disassemble the object file:objdump -d csource.o
The disassembly output from objdump will contain the binary instructions (opcodes) as well as the instruction names (mnemonics), i.e. you'll see exactly which opcode format was chosen. You can now check whether any opcodes in there are from the 2nd set as per 1. above.
The creative bit of the work comes in now. When you've found an instruction in the disassembly output that doesn't match the expectations/requirements you have, look up / create a substitute (or, more often, a substitute sequence of several instructions) that gives the same end result but is only made up from instructions that do match what you need.
Go back to the compile intermediates from above, find the csource.s assembly, make changes, reassemble/relink, test.
If you want to make your assembly code standalone (i.e. not using system runtime libraries / making system calls directly), consult documentation on your operating system internals (how to make syscalls), and/or disassemble the runtime libraries that ordinarily do so on your behalf, to learn how it's done.
Since 5. is definitely homework, of the same sort like create a C for() loop equivalent to a given while() loop, don't expect too much help there. The instruction set reference manuals and experiments with the (dis)assembler are what you need here.
Additionally, if you're studying, attend lessons on how compilers work / how to write compilers - they do cover how assembly instruction selection is done by compilers, and I can well imagine it to be an interesting / challenging term project to e.g. write a compiler whose output is guaranteed to contain the character '?' (0x3f) but never '!' (0x21). You get the idea.
You mention the constant load via xor to clear plus inc and shl to get any set of bits you want.
The least fragile way I can think of to load an unknown constant (your unknown str) is to load the constant xor with some value like 0xAAAAAAAA and then xor that back out in a subsequent instruction. For example to load 0x1234:
0: 89 1d 9e b8 aa aa mov %ebx,0xaaaab89e
6: 31 1d aa aa aa aa xor %ebx,0xaaaaaaaa
You could even choose the 0xAAAAAAAA to be some interesting ascii!
I would like to test a buffer-overflow by writing "Hello World" to console (using Windows XP 32-Bit). The shellcode needs to be null-free in order to be passed by "scanf" into the program I want to overflow. I've found plenty of assembly-tutorials for Linux, however none for Windows. Could someone please step me through this using NASM? Thxxx!
Assembly opcodes are the same, so the regular tricks to produce null-free shellcodes still apply, but the way to make system calls is different.
In Linux you make system calls with the "int 0x80" instruction, while on Windows you must use DLL libraries and do normal usermode calls to their exported functions.
For that reason, on Windows your shellcode must either:
Hardcode the Win32 API function addresses (most likely will only work on your machine)
Use a Win32 API resolver shellcode (works on every Windows version)
If you're just learning, for now it's probably easier to just hardcode the addresses you see in the debugger. To make the calls position independent you can load the addresses in registers. For example, a call to a function with 4 arguments:
PUSH 4 ; argument #4 to the function
PUSH 3 ; argument #3 to the function
PUSH 2 ; argument #2 to the function
PUSH 1 ; argument #1 to the function
MOV EAX, 0xDEADBEEF ; put the address of the function to call
CALL EAX
Note that the argument are pushed in reverse order. After the CALL instruction EAX contains the return value, and the stack will be just like it was before (i.e. the function pops its own arguments). The ECX and EDX registers may contain garbage, so don't rely on them keeping their values after the call.
A direct CALL instruction won't work, because those are position dependent.
To avoid zeros in the address itself try any of the null-free tricks for x86 shellcode, there are many out there but my favorite (albeit lengthy) is encoding the values using XOR instructions:
MOV EAX, 0xDEADBEEF ^ 0xFFFFFFFF ; your value xor'ed against an arbitrary mask
XOR EAX, 0xFFFFFFFF ; the arbitrary mask
You can also try NEG EAX or NOT EAX (sign inversion and bit flipping) to see if they work, it's much cheaper (two bytes each).
You can get help on the different API functions you can call here: http://msdn.microsoft.com
The most important ones you'll need are probably the following:
WinExec(): http://msdn.microsoft.com/en-us/library/ms687393(VS.85).aspx
LoadLibrary(): http://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx
GetProcAddress(): http://msdn.microsoft.com/en-us/library/ms683212%28v=VS.85%29.aspx
The first launches a command, the next two are for loading DLL files and getting the addresses of its functions.
Here's a complete tutorial on writing Windows shellcodes: http://www.codeproject.com/Articles/325776/The-Art-of-Win32-Shellcoding
Assembly language is defined by your processor, and assembly syntax is defined by the assembler (hence, at&t, and intel syntax) The main difference (at least i think it used to be...) is that windows is real-mode (call the actual interrupts to do stuff, and you can use all the memory accessible to your computer, instead of just your program) and linux is protected mode (You only have access to memory in your program's little cubby of memory, and you have to call int 0x80 and make calls to the kernel, instead of making calls to the hardware and bios) Anyway, hello world type stuff would more-or-less be the same between linux and windows, as long as they are compatible processors.
To get the shellcode from your program you've made, just load it into your target system's
debugger (gdb for linux, and debug for windows) and in debug, type d (or was it u? Anyway, it should say if you type h (help)) and between instructions and memory will be the opcodes.
Just copy them all over to your text editor into one string, and maybe make a program that translates them all into their ascii values. Not sure how to do this in gdb tho...
Anyway, to make it into a bof exploit, enter aaaaa... and keep adding a's until it crashes
from a buffer overflow error. But find exactly how many a's it takes to crash it. Then, it should tell you what memory adress that was. Usually it should tell you in the error message. If it says '9797[rest of original return adress]' then you got it. Now u gotta use ur debugger to find out where this was. disassemble the program with your debugger and look for where scanf was called. Set a breakpoint there, run and examine the stack. Look for all those 97's (which i forgot to mention is the ascii number for 'a'.) and see where they end. Then remove breakpoint and type the amount of a's you found out it took (exactly the amount. If the error message was "buffer overflow at '97[rest of original return adress]" then remove that last a, put the adress you found examining the stack, and insert your shellcode. If all goes well, you should see your shellcode execute.
Happy hacking...
I'm writing a program for a 8051 microcontroller. In the first part of the program I do some calculations and based on the result, I either light the LED or not (using CLR P1.7, where P1.7 is the port the LED is attached to in the microcontroller).
In the next part of the program I want to retrieve the bit, perhaps store it somewhere, and use it in a if-jump instruction like JB. How can I do that?
Also, I've seen the instruction MOV C, P1.7 in a code sample. What's the C here?
The C here is the 8051's carry flag - called that because it can be used to hold the "carry" when doing addition operations on multiple bytes.
It can also be used as a single-bit register - so (as here) where you want to move bits around, you can load it with a port value (such as P1.7) then store it somewhere else, for example:
MOV C, P1.7
MOV <bit-address>, C
Then later you can branch on it using:
JB <bit-address>, <label>
Some of the special function registers are also bit addressable. I believe its all the ones ending in 0 or 8. Don't have a reference in front of me but you can do something like setb r0.1. That way if you need the carry for something you dont have to worry about pushing it and using up space on your stack.