西湖论剑2023 Berkeley writeup ——bpf字节码逆向-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_41866334/article/details/128953487

分析

非预期

题目出的不好，有非预期，strings一下binary 直接能找到源码根本不需要逆向。

int BPF_KPROBE(LBB0_1, char *arg1)
        u64 id = bpf_get_current_pid_tgid();
        pid_t pid = (pid_t) id;
        if (bpf_map_update_elem(&execs, &pid, &empty_event, BPF_NOEXIST)) {
        event = bpf_map_lookup_elem(&execs, &pid);
        if (!event) {
        bpf_probe_read_user_str(event->args, ARGSIZE, arg1);
    for(int i=0; i<256; i++)
            unsigned char uc1 = flag[i/8];
            unsigned char uc2 = ~(flag[i/8] + arr[i%8]);
            output[i] = key[uc1 ^ uc2];
LBB0_2
uretprobe
        id = bpf_get_current_pid_tgid();
        pid = (pid_t) id;
        if (!event)
    unsigned char *output = event->output;
            output[i] = key[output[i] ^ key[i]];
        for(int i=0; i<256; i++)
            if(output[i] != cipher[i])
        bpf_map_delete_elem(&execs, &pid);
int BPF_KRETPROBE(LBB0_2)
cipher
LICENSE
empty_event
____LBB0_2.____fmt
____LBB0_2.____fmt.1

正经逆向

我傻了xdm，看了官方wp发现ghidra直接有ebpf插件能反编译，那还要啥自行车。https://github.com/Nalen98/eBPF-for-Ghidra/tree/v0.2 里照着install下的第三个item做就能安装，成功后重启，然后拖进去一把梭。

当然如果想正经逆向字节码，首先按照bpf,ebpf一些原理以及逆向基于libbpf-bootstrap编写的bpf文件，把字节码dump出来进行分析。然后还有两个小知识要知道：
一是bpf 字节码中的relocation问题，实际上和elf的relocation没有本质区别，也可以通过llvm-objdump -dr bpf_dump.mem 直接获得
在这里插入图片描述
另一个就是call 指令的分析，可以查看BPF-Opcodes链接。
call (disp32|imm32)
Jump and link to the offset disp32, or to the kernel helper function identified by imm32.
所以本题中的call后跟着的都是kernel helper function 的编号。
这个编号可以在源码中调用序号和调用函数之间对应的map表中查看。

题解

from z3 import *

key = [
    0xC1, 0xD1, 0x02, 0x61, 0xD6, 0xF7, 0x13, 0xA2, 0x9B, 0x20, 
    0xD0, 0x4A, 0x8F, 0x7F, 0xEE, 0xB9, 0x00, 0x63, 0x34, 0xB0, 
    0x33, 0xB7, 0x8A, 0x8B, 0x94, 0x60, 0x2E, 0x8E, 0x21, 0xFF, 
    0x90, 0x82, 0xD5, 0x87, 0x96, 0x78, 0x22, 0xB6, 0x48, 0x6C, 
    0x45, 0xC7, 0x5A, 0x16, 0x80, 0xFD, 0xE4, 0x8C, 0xBF, 0x01, 
    0x1F, 0x4B, 0x79, 0x24, 0xA0, 0xB4, 0x23, 0x4D, 0x3B, 0xC5, 
    0x5D, 0x6F, 0x0D, 0xC9, 0xD4, 0xCA, 0x55, 0xE0, 0x39, 0xAD, 
    0x2B, 0xCD, 0x2C, 0xEC, 0xC2, 0x6B, 0x30, 0xE6, 0x0C, 0xA8, 
    0x9A, 0x2F, 0xF6, 0xE8, 0xBB, 0x32, 0x57, 0xFB, 0x0B, 0x9D, 
    0xF2, 0x3F, 0xB5, 0xF9, 0x59, 0xE5, 0x10, 0xCF, 0x51, 0x41, 
    0xE9, 0x50, 0xDF, 0x26, 0x74, 0x58, 0xCB, 0x64, 0x54, 0x73, 
    0xAB, 0xF4, 0xB2, 0x9F, 0x18, 0xF8, 0x4E, 0xFE, 0x08, 0x1D, 
    0x4F, 0x49, 0xD3, 0xAC, 0x38, 0x12, 0x77, 0x11, 0x69, 0x07, 
    0x1C, 0x99, 0xB3, 0xE7, 0x3D, 0x05, 0xD8, 0xFC, 0x70, 0x46, 
    0x93, 0x09, 0x65, 0x89, 0xB1, 0xC6, 0x52, 0xFA, 0xD2, 0x0E, 
    0xA9, 0x17, 0xE3, 0x91, 0xA1, 0x68, 0x5B, 0x2A, 0xF0, 0xC3, 
    0x42, 0xCC, 0x29, 0xDE, 0xDC, 0x85, 0x98, 0x31, 0x5C, 0xBC, 
    0x2D, 0xEF, 0x5E, 0x7E, 0xAF, 0x67, 0x62, 0xA7, 0x56, 0x88, 
    0xA4, 0x43, 0x40, 0xE1, 0x37, 0x9E, 0x36, 0x76, 0x71, 0x84, 
    0xBD, 0x06, 0x8D, 0x47, 0x7D, 0x53, 0xD7, 0xC8, 0xCE, 0x15, 
    0x92, 0x95, 0x4C, 0x28, 0x6D, 0x75, 0xEB, 0x7C, 0xF3, 0xBE, 
    0xAA, 0xB8, 0xED, 0x03, 0x3C, 0x27, 0x3E, 0x19, 0xDD, 0xA6, 
    0x66, 0x25, 0x1E, 0xC4, 0x6E, 0xC0, 0xE2, 0xDB, 0x3A, 0xD9, 
    0x81, 0xA5, 0x1B, 0xF5, 0x04, 0xAE, 0xBA, 0xEA, 0x97, 0x83, 
    0x35, 0x44, 0xA3, 0x7A, 0x1A, 0xF1, 0x86, 0xDA, 0x7B, 0x14, 
    0x72, 0x9C, 0x6A, 0x0F, 0x5F, 0x0A ]

cipher = [
    0xF3, 0x27, 0x47, 0x1B, 0x8F, 0x09, 0xFB, 0x17, 0x70, 0x48, 
    0xB0, 0x53, 0x32, 0xDB, 0xC0, 0xB8, 0x63, 0x2D, 0x40, 0x4B, 
    0xF5, 0x16, 0xF0, 0x35, 0xE7, 0xDF, 0xEA, 0xA2, 0x9C, 0x41, 
    0xB3, 0x25, 0xD7, 0x0C, 0x33, 0x9C, 0x7B, 0x5A, 0xCD, 0x13, 
    0xBB, 0xEE, 0x3E, 0x0E, 0xF2, 0xCF, 0x35, 0xDA, 0xAF, 0xA2, 
    0x66, 0x7D, 0x38, 0x37, 0x67, 0x1E, 0x1F, 0x6B, 0x7B, 0x30, 
    0x0B, 0x7A, 0x02, 0xA9, 0xC8, 0x61, 0x27, 0x41, 0xDB, 0x01, 
    0x22, 0x31, 0x6F, 0xB6, 0xD4, 0x1B, 0x04, 0xD3, 0x94, 0xB8, 
    0x46, 0xC7, 0x24, 0xCF, 0xBD, 0xAF, 0x0B, 0xDC, 0x2E, 0xBB, 
    0xB2, 0x71, 0xF4, 0x99, 0x57, 0x36, 0xD1, 0x95, 0x52, 0x92, 
    0xBA, 0x6D, 0xF3, 0x30, 0x50, 0x59, 0x9B, 0xEA, 0x2F, 0x83, 
    0xDC, 0xF0, 0xDE, 0x57, 0xA1, 0xAC, 0xD2, 0x51, 0xA2, 0x1D, 
    0x59, 0xA8, 0x00, 0xB6, 0xE2, 0x65, 0x41, 0x0C, 0x4F, 0xEB, 
    0xF0, 0x2E, 0x58, 0x2A, 0x1F, 0xF4, 0x95, 0x72, 0x88, 0x7C, 
    0xA9, 0x0E, 0xCB, 0x3C, 0x42, 0xB9, 0xF3, 0x49, 0x9B, 0x52, 
    0x98, 0x12, 0xA3, 0x17, 0x51, 0xC0, 0x59, 0x40, 0x0A, 0xBC, 
    0xE8, 0x4C, 0x04, 0xFB, 0x13, 0x0A, 0x17, 0x3F, 0xE6, 0x36, 
    0x97, 0xDF, 0xB3, 0xE2, 0x42, 0x7F, 0xF8, 0xCC, 0x0E, 0xD1, 
    0x77, 0xC4, 0xA8, 0x46, 0x48, 0xE3, 0xF1, 0x0A, 0xEF, 0x94, 
    0x56, 0x54, 0x5B, 0xCA, 0xBD, 0xDD, 0x7F, 0x56, 0x47, 0xC2, 
    0x99, 0xFA, 0x89, 0xCC, 0xE1, 0xB9, 0x3A, 0x78, 0xE2, 0x37, 
    0x58, 0x01, 0x1B, 0xC3, 0x4B, 0xE6, 0x8C, 0xF3, 0xE5, 0xB6, 
    0x71, 0x9E, 0x63, 0xAF, 0x11, 0xCE, 0x87, 0xF6, 0x6E, 0xDE, 
    0xC8, 0xB1, 0xD0, 0x7A, 0x15, 0x6C, 0x10, 0x08, 0x99, 0x7B, 
    0x22, 0x55, 0x10, 0x7A, 0x82, 0x73, 0xFC, 0x62, 0xCB, 0x34, 
    0xA7, 0xB7, 0x62, 0xFA, 0x6B, 0x9F ]

arr = [0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01]

s = Solver()
flag = [BitVec('flag%i' % i, 8) for i in range(32)]

uc_xor = [0 for _ in range(256)]
for i in range(256):
    temp = key.index(cipher[i]) ^ key[i]
    uc_xor[i] = key.index(temp)

for i in range(32):
    s.add(flag[i] > 32)
    s.add(flag[i] < 127)

for i in range(256):
    uc1 = flag[i // 8]
    uc2 = ~(flag[i // 8] + arr[i % 8])
    s.add(uc1 ^ uc2 == uc_xor[i])

if s.check() == sat:
    ans = s.model()
    print(''.join([chr(ans[flag[i]].as_long()) for i in range(32)]))

引用

bpf,ebpf一些原理以及逆向基于libbpf-bootstrap编写的bpf文件

BPF LLVM Relocations

BPF-Opcodes
Calling kernel functions from BPF
源码中调用序号和调用函数之间对应的map表

附录

字节码部分

$ llvm-objdump -d Berkeley.bpf.o 

Berkeley.bpf.o:	file format elf64-bpf

Disassembly of section uprobe:

0000000000000000 <LBB0_1>:
       0:	79 18 70 00 00 00 00 00	r8 = *(u64 *)(r1 + 112)  ；args
       1:	85 00 00 00 0e 00 00 00	call 14
       2:	63 0a fc ff 00 00 00 00	*(u32 *)(r10 - 4) = r0
       3:	bf a2 00 00 00 00 00 00	r2 = r10
       4:	07 02 00 00 fc ff ff ff	r2 += -4
       5:	18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r1 = 0 ll
       7:	18 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r3 = 0 ll
       9:	b7 04 00 00 01 00 00 00	r4 = 1
      10:	85 00 00 00 02 00 00 00	call 2
      11:	55 00 2a 00 00 00 00 00	if r0 != 0 goto +42 <LBB0_4>
      12:	bf a2 00 00 00 00 00 00	r2 = r10
      13:	07 02 00 00 fc ff ff ff	r2 += -4
      14:	18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r1 = 0 ll
      16:	85 00 00 00 01 00 00 00	call 1
      17:	bf 06 00 00 00 00 00 00	r6 = r0
      18:	15 06 23 00 00 00 00 00	if r6 == 0 goto +35 <LBB0_4>
      19:	bf 67 00 00 00 00 00 00	r7 = r6   ; event
      20:	07 07 00 00 20 00 00 00	r7 += 32  ; event->args
      21:	bf 71 00 00 00 00 00 00	r1 = r7
      22:	b7 02 00 00 80 00 00 00	r2 = 128
      23:	bf 83 00 00 00 00 00 00	r3 = r8
      24:	85 00 00 00 72 00 00 00	call 114
      25:	b7 01 00 00 00 00 00 00	r1 = 0
      26:	07 06 00 00 20 01 00 00	r6 += 288  ; event->output

00000000000000d8 <LBB0_3>:
      27:	bf 12 00 00 00 00 00 00	r2 = r1  ; r1 = i
      28:	77 02 00 00 03 00 00 00	r2 >>= 3
      29:	57 02 00 00 ff ff ff 1f	r2 &= 536870911
      30:	bf 73 00 00 00 00 00 00	r3 = r7  ； r7 ptr to flag  event->args
      31:	0f 23 00 00 00 00 00 00	r3 += r2  ; r3 = &flag[i/8]
      32:	bf 12 00 00 00 00 00 00	r2 = r1
      33:	57 02 00 00 07 00 00 00	r2 &= 7
      34:	67 02 00 00 02 00 00 00	r2 <<= 2  ;u32, offset is 4 
      35:	18 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r4 = 0 ll  ; ptr to arr
      37:	0f 24 00 00 00 00 00 00	r4 += r2
      38:	61 42 00 00 00 00 00 00	r2 = *(u32 *)(r4 + 0) ; arr[i%8]
      39:	71 33 00 00 00 00 00 00	r3 = *(u8 *)(r3 + 0)
      40:	bf 34 00 00 00 00 00 00	r4 = r3
      41:	0f 24 00 00 00 00 00 00	r4 += r2
      42:	af 43 00 00 00 00 00 00	r3 ^= r4
      43:	a7 03 00 00 ff ff ff ff	r3 ^= -1  ; not 
      44:	57 03 00 00 ff 00 00 00	r3 &= 255
      45:	18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r2 = 0 ll  ; ptr to key 
      47:	0f 32 00 00 00 00 00 00	r2 += r3  ; &key[uc1^uc2]
      48:	bf 63 00 00 00 00 00 00	r3 = r6   ; output 
      49:	0f 13 00 00 00 00 00 00	r3 += r1
      50:	71 22 00 00 00 00 00 00	r2 = *(u8 *)(r2 + 0)
      51:	73 23 00 00 00 00 00 00	*(u8 *)(r3 + 0) = r2
      52:	07 01 00 00 01 00 00 00	r1 += 1
      53:	55 01 e5 ff 00 01 00 00	if r1 != 256 goto -27 <LBB0_3>

00000000000001b0 <LBB0_4>:
      54:	b7 00 00 00 00 00 00 00	r0 = 0
      55:	95 00 00 00 00 00 00 00	exit

Disassembly of section uretprobe:

0000000000000000 <LBB0_2>:
       0:	85 00 00 00 0e 00 00 00	call 14
       1:	63 0a fc ff 00 00 00 00	*(u32 *)(r10 - 4) = r0
       2:	bf a2 00 00 00 00 00 00	r2 = r10
       3:	07 02 00 00 fc ff ff ff	r2 += -4
       4:	18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r1 = 0 ll
       6:	85 00 00 00 01 00 00 00	call 1
       7:	15 00 29 00 00 00 00 00	if r0 == 0 goto +41 <LBB1_7>
       8:	b7 01 00 00 00 00 00 00	r1 = 0
       9:	07 00 00 00 20 01 00 00	r0 += 288

0000000000000050 <LBB1_2>:
      10:	18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r2 = 0 ll ; ptr to key
      12:	0f 12 00 00 00 00 00 00	r2 += r1
      13:	71 22 00 00 00 00 00 00	r2 = *(u8 *)(r2 + 0)
      14:	bf 03 00 00 00 00 00 00	r3 = r0  ; ptr to output 
      15:	0f 13 00 00 00 00 00 00	r3 += r1
      16:	71 34 00 00 00 00 00 00	r4 = *(u8 *)(r3 + 0)
      17:	af 42 00 00 00 00 00 00	r2 ^= r4
      18:	57 02 00 00 ff 00 00 00	r2 &= 255
      19:	18 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r4 = 0 ll ; ptr to cipher 
      21:	0f 24 00 00 00 00 00 00	r4 += r2
      22:	71 42 00 00 00 00 00 00	r2 = *(u8 *)(r4 + 0)
      23:	73 23 00 00 00 00 00 00	*(u8 *)(r3 + 0) = r2
      24:	07 01 00 00 01 00 00 00	r1 += 1
      25:	55 01 f0 ff 00 01 00 00	if r1 != 256 goto -16 <LBB1_2>
      26:	b7 02 00 00 00 00 00 00	r2 = 0
      27:	05 00 04 00 00 00 00 00	goto +4 <LBB1_5>

00000000000000e0 <LBB1_4>:
      28:	18 01 00 00 2b 02 00 00 00 00 00 00 00 00 00 00	r1 = 555 ll
      30:	07 02 00 00 01 00 00 00	r2 += 1
      31:	15 02 0a 00 00 01 00 00	if r2 == 256 goto +10 <LBB1_6>

0000000000000100 <LBB1_5>:
      32:	bf 03 00 00 00 00 00 00	r3 = r0
      33:	0f 23 00 00 00 00 00 00	r3 += r2
      34:	18 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r4 = 0 ll
      36:	0f 24 00 00 00 00 00 00	r4 += r2
      37:	18 01 00 00 20 02 00 00 00 00 00 00 00 00 00 00	r1 = 544 ll
      39:	71 44 00 00 00 00 00 00	r4 = *(u8 *)(r4 + 0)
      40:	71 33 00 00 00 00 00 00	r3 = *(u8 *)(r3 + 0)
      41:	1d 43 f2 ff 00 00 00 00	if r3 == r4 goto -14 <LBB1_4>

0000000000000150 <LBB1_6>:
      42:	b7 02 00 00 0b 00 00 00	r2 = 11
      43:	85 00 00 00 06 00 00 00	call 6
      44:	bf a2 00 00 00 00 00 00	r2 = r10
      45:	07 02 00 00 fc ff ff ff	r2 += -4
      46:	18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00	r1 = 0 ll
      48:	85 00 00 00 03 00 00 00	call 3

0000000000000188 <LBB1_7>:
      49:	b7 00 00 00 00 00 00 00	r0 = 0
      50:	95 00 00 00 00 00 00 00	exit