203 lines
8.9 KiB
Markdown
203 lines
8.9 KiB
Markdown
---
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title: "using qemu-user emulation to reverse engineer binaries"
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date: "2021-05-05"
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---
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QEMU is primarily known as the software which provides full system emulation under Linux's KVM. Also, it can be used without KVM to do full emulation of machines from the hardware level up. Finally, there is `qemu-user`, which allows for emulation of individual programs. That's what this blog post is about.
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The main use case for `qemu-user` is actually _not_ reverse-engineering, but simply running programs for one CPU architecture on another. For example, Alpine developers leverage `qemu-user` when they use `dabuild(1)` to cross-compile Alpine packages for other architectures: `qemu-user` is used to run the configure scripts, test suites and so on. For those purposes, `qemu-user` works quite well: we are even considering using it to build the entire `riscv64` architecture in the 3.15 release.
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However, most people don't realize that you can run a `qemu-user` emulator which targets the same architecture as the host. After all, that would be a little weird, right? Most also don't know that you can control the emulator using `gdb`, which is possible and allows you to debug binaries which detect if they are being debugged.
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You don't need `gdb` for this to be a powerful reverse engineering tool, however. The emulator itself includes many powerful tracing features. Lets look into them by writing and compiling a sample program, that does some recursion by [calculating whether a number is even or odd inefficiently](https://ariadne.space/2021/04/27/the-various-ways-to-check-if-an-integer-is-even/):
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```c
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#include <stdbool.h>
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#include <stdio.h>
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bool isOdd(int x);
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bool isEven(int x);
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bool isOdd(int x) {
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return x != 0 && isEven(x - 1);
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}
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bool isEven(int x) {
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return x == 0 || isOdd(x - 1);
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}
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int main(void) {
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printf("isEven(%d): %d\\n", 1025, isEven(1025));
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return 0;
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}
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```
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Compile this program with `gcc`, by doing `gcc -ggdb3 -Os example.c -o example`.
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The next step is to install the `qemu-user` emulator for your architecture, in this case we want the `qemu-x86_64` package:
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```
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$ doas apk add qemu-x86_64
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(1/1) Installing qemu-x86_64 (6.0.0-r1)
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$
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```
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Normally, you would also want to install the `qemu-openrc` package and start the `qemu-binfmt` service to allow for the emulator to handle any program that couldn't be run natively, but that doesn't matter here as we will be running the emulator directly.
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The first thing we will do is check to make sure the emulator can run our sample program at all:
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```
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$ qemu-x86_64 ./example
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isEven(1025): 0
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```
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Alright, all seems to be well. Before we jump into using `gdb` with the emulator, lets play around a bit with the tracing features. Normally when reverse engineering a program, it is common to use tracing programs like `strace`. These tracing programs are quite useful, but they suffer from a design flaw: they use `ptrace(2)` to accomplish the tracing, which can be detected by the program being traced. However, we can use qemu-user to do the tracing in a way that is transparent to the program being analyzed:
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```
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$ qemu-x86_64 -d strace ./example
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22525 arch_prctl(4098,274903714632,136818691500777464,274903714112,274903132960,465) = 0
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22525 set_tid_address(274903715728,274903714632,136818691500777464,274903714112,0,465) = 22525
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22525 brk(NULL) = 0x0000004000005000
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22525 brk(0x0000004000007000) = 0x0000004000007000
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22525 mmap(0x0000004000005000,4096,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0) = 0x0000004000005000
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22525 mprotect(0x0000004001899000,4096,PROT_READ) = 0
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22525 mprotect(0x0000004000003000,4096,PROT_READ) = 0
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22525 ioctl(1,TIOCGWINSZ,0x00000040018052b8) = 0 ({55,236,0,0})
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isEven(1025): 0
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22525 writev(1,0x4001805250,0x2) = 16
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22525 exit_group(0)
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```
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But we can do even more. For example, we can learn how a CPU would hypothetically break a program down into translation buffers full of micro-ops (these are TCG micro-ops but real CPUs are similar enough to gain a general understanding of the concept):
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```
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$ qemu-x86_64 -d op ./example
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OP:
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ld_i32 tmp11,env,$0xfffffffffffffff0
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brcond_i32 tmp11,$0x0,lt,$L0
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---- 000000400185eafb 0000000000000000
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discard cc_dst
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discard cc_src
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discard cc_src2
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discard cc_op
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mov_i64 tmp0,$0x0
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mov_i64 rbp,tmp0
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---- 000000400185eafe 0000000000000031
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mov_i64 tmp0,rsp
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mov_i64 rdi,tmp0
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---- 000000400185eb01 0000000000000031
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mov_i64 tmp2,$0x4001899dc0
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mov_i64 rsi,tmp2
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---- 000000400185eb08 0000000000000031
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mov_i64 tmp1,$0xfffffffffffffff0
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mov_i64 tmp0,rsp
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and_i64 tmp0,tmp0,tmp1
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mov_i64 rsp,tmp0
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mov_i64 cc_dst,tmp0
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---- 000000400185eb0c 0000000000000019
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mov_i64 tmp0,$0x400185eb11
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sub_i64 tmp2,rsp,$0x8
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qemu_st_i64 tmp0,tmp2,leq,0
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mov_i64 rsp,tmp2
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mov_i32 cc_op,$0x19
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goto_tb $0x0
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mov_i64 tmp3,$0x400185eb11
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st_i64 tmp3,env,$0x80
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exit_tb $0x7f72ebafc040
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set_label $L0
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exit_tb $0x7f72ebafc043
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[...]
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```
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If you want to trace the actual CPU registers for every instruction executed, that's possible too:
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```
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$ qemu-x86_64 -d cpu ./example
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RAX=0000000000000000 RBX=0000000000000000 RCX=0000000000000000 RDX=0000000000000000
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RSI=0000000000000000 RDI=0000000000000000 RBP=0000000000000000 RSP=0000004001805690
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R8 =0000000000000000 R9 =0000000000000000 R10=0000000000000000 R11=0000000000000000
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R12=0000000000000000 R13=0000000000000000 R14=0000000000000000 R15=0000000000000000
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RIP=000000400185eafb RFL=00000202 \[-------\] CPL=3 II=0 A20=1 SMM=0 HLT=0
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ES =0000 0000000000000000 00000000 00000000
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CS =0033 0000000000000000 ffffffff 00effb00 DPL=3 CS64 \[-RA\]
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SS =002b 0000000000000000 ffffffff 00cff300 DPL=3 DS \[-WA\]
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DS =0000 0000000000000000 00000000 00000000
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FS =0000 0000000000000000 00000000 00000000
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GS =0000 0000000000000000 00000000 00000000
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LDT=0000 0000000000000000 0000ffff 00008200 DPL=0 LDT
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TR =0000 0000000000000000 0000ffff 00008b00 DPL=0 TSS64-busy
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GDT= 000000400189f000 0000007f
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IDT= 000000400189e000 000001ff
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CR0=80010001 CR2=0000000000000000 CR3=0000000000000000 CR4=00000220
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DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000
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DR6=00000000ffff0ff0 DR7=0000000000000400
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CCS=0000000000000000 CCD=0000000000000000 CCO=EFLAGS
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EFER=0000000000000500
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[...]
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```
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You can also trace with disassembly for each translation buffer generated:
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```
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$ qemu-x86_64 -d in_asm ./example
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----------------
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IN:
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0x000000400185eafb: xor %rbp,%rbp
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0x000000400185eafe: mov %rsp,%rdi
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0x000000400185eb01: lea 0x3b2b8(%rip),%rsi # 0x4001899dc0
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0x000000400185eb08: and $0xfffffffffffffff0,%rsp
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0x000000400185eb0c: callq 0x400185eb11
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----------------
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IN:
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0x000000400185eb11: sub $0x190,%rsp
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0x000000400185eb18: mov (%rdi),%eax
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0x000000400185eb1a: mov %rdi,%r8
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0x000000400185eb1d: inc %eax
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0x000000400185eb1f: cltq
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0x000000400185eb21: mov 0x8(%r8,%rax,8),%rcx
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0x000000400185eb26: mov %rax,%rdx
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0x000000400185eb29: inc %rax
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0x000000400185eb2c: test %rcx,%rcx
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0x000000400185eb2f: jne 0x400185eb21
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[...]
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```
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All of these options, and more, can also be stacked. For more ideas, look at `qemu-x86_64 -d help`. Now, lets talk about using this with `gdb` using qemu-user's gdbserver functionality, which allows for `gdb` to control a remote machine.
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To start a program under gdbserver mode, we use the `-g` argument with a port number. For example, `qemu-x86_64 -g 1234 ./example` will start our example program with a gdbserver listening on port 1234. We can then connect to that gdbserver with `gdb`:
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```
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$ gdb ./example
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[...]
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Reading symbols from ./example...
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(gdb) target remote localhost:1234
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Remote debugging using localhost:1234
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0x000000400185eafb in ?? ()
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(gdb) br isEven
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Breakpoint 1 at 0x4000001233: file example.c, line 12.
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(gdb) c
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Continuing.
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Breakpoint 1, isEven (x=1025) at example.c:12
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12 return x == 0 || isOdd(x - 1);
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(gdb) bt full
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#0 isEven (x=1025) at example.c:12
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No locals.
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#1 0x0000004000001269 in main () at example.c:16
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No locals.
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```
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All of this is happening without any knowledge or cooperation of the program. As far as its concerned, its running as normal, there is no ptrace or any other weirdness.
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However, this is not 100% perfect: a program could be clever and run the `cpuid` instruction and check for `GenuineIntel` or `AuthenticAMD` and crash out if it doesn't see that it is running on a legitimate CPU. Thankfully, qemu-user has the ability to spoof CPUs with the `-cpu` option.
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If you find yourself needing to spoof the CPU, you'll probably have the best results with a simple CPU type like `-cpu Opteron_G1-v1` or similar. That CPU type spoofs an Opteron 240 processor, which was one of the first x86_64 CPUs on the market. You can get a full list of CPUs supported by your copy of the qemu-user emulator by doing `qemu-x86_64 -cpu help`.
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There's a lot more qemu-user emulation can do to help with reverse engineering, for some ideas, look at `qemu-x86_64 -h` or similar.
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