[逆向][writeup]0ctf2015 r0ops
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这道逆向题相对其分值来说，实在是太难了，想要逆向这个程序，需要足够的耐心，而且还需要有一定的数学知识。
这是一个64位的ELF程序。首先用IDA进行分析，开始部分的逻辑是比较简单的：
1 void *__usercall [email protected]<rax>(__int64 [email protected]<rbp>) 2 { 3 *(_DWORD *)(a1 - 4) = accept(3, 0LL, 0LL); 4 recv(*(_DWORD *)(a1 - 4), &qword_E0B10C0, 0x1000uLL, 0); 5 close(*(_DWORD *)(a1 - 4)); 6 qmemcpy(&unk_E0AF0A0, &unk_E0B00A0, 0x1000uLL); 7 return &unk_E0AF8A0; 8 }
程序打开了一个套接字（描述符为3），然后监听这个套接字，在接收到消息后，会进行拷贝操作，然后返回。
最关键的就是返回操作。动态调试发现函数返回时，ret指令会取出栈上地址并跳转到该地址处执行一系列指令，结束后会再次返回到栈上，重复下一次过程。大概如下：
.text:000000000DEAD1F4 jmp short loc_DEAD1F8 .text:000000000DEAD1F8 pop rcx .text:000000000DEAD1F9 retn .text:000000000DEAD271 jmp short loc_DEAD275 .text:000000000DEAD275 pop r9 .text:000000000DEAD277 retn
换句话说，整个stack就是由一系列的ROP gadgets构成的ROPChain。在进行动态调试时，发现gadgets中含有大量的垃圾指令，比如无效的赋值，重复的拷贝等等，从而增大了我们理解程序的难度。
栈上的内容我们是可以获得的，想到可以写一个脚本，结合代码段与栈上的内容来还原这个ROPChain，但是后来发现gadgets中有不少rsp操作和条件跳转操作，这些操作的结果取决于动态执行的内容，用脚本来进行静态还原相当于写一个解释器了，难度可想而知，最终放弃。
耐着性子一步步跟踪gadget的执行，去掉无效的干扰指令，记录下有效指令，及相关内存、寄存器的值，如下：
1 .text:000000000DEAD1F8 pop rcx ;0x0000000000000008 2 .text:000000000DEAD275 pop r9 ;0x1337DEADBEEF0095 3 .text:000000000DEAD127 mov rax, [rdi] ;rax = 8bytes user input 4 .text:000000000DEAD208 mov [rsi], rax ;[rsi] = 8bytes user input 5 .text:000000000DEAD26B mov r8, [rsi] ;ri = 8bytes user input 6 .text:000000000DEAD107 add rdi, 8 ;rdi points to next 8 bytes user input 7 .text:000000000DEAD27E mov [rsi], r9 ;[rsi] = 0x1337DEADBEEF0095 8 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x1337DEADBEEF0095 9 .text:000000000DEAD1F0 pop rbx ;rbx = 0x000000000000CAFE 10 .text:000000000DEAD145 imul rax, rbx ;rax = 0x1337DEADBEEF0095 * 0x000000000000CAFE 11 .text:000000000DEAD208 mov [rsi], rax 12 .text:000000000DEAD288 mov r9, [rsi] ;r9 = 0x1337DEADBEEF0095 * 0x000000000000CAFE 13 .text:000000000DEAD1F0 pop rbx ;rbx = 0x000000000000BEEF 14 .text:000000000DEAD131 add rax, rbx ;rax = 0x1337DEADBEEF0095 * 0x000000000000CAFE + 0x000000000000BEEF 15 .text:000000000DEAD208 mov [rsi], rax 16 .text:000000000DEAD288 mov r9, [rsi] ;r9 = 0x1337DEADBEEF0095 * 0x000000000000CAFE + 0x000000000000BEEF 17 .text:000000000DEAD2CC pop r12 ;r12 = 0x0000000000000001 18 .text:000000000DEAD292 pop r10 ;r10 = 0x0000000000003419 = 13337 19 .text:000000000DEAD1E8 pop rax ;rax = 0x0000000000000000 20 .text:000000000DEAD1F0 pop rbx ;rbx = 0x0000000000000000 21 .text:000000000DEAD200 pop rdx ;rdx = 0x00000000000001D8 22 .text:000000000DEAD19F cmp rax, rbx ; 23 .text:000000000DEAD1A2 jnz short locret_DEAD1A7 ;not jmp 24 .text:000000000DEAD1A4 add rsp, rdx ;rsp = 0x000000000E0AFBD0 25 .text:000000000DEAD29B mov [rsi], r10 26 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000003419 27 .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 28 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 29 .text:000000000DEAD1AE cmp rax, rbx ; 30 .text:000000000DEAD1B1 jz short locret_DEAD1B6 31 .text:000000000DEAD1B3 add rsp, rdx ;rsp = 0x000000000E0AF9F8 32 .text:000000000DEAD212 mov rax, [rsi] ;rax = r10 = 0x0000000000003419 33 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 34 .text:000000000DEAD177 and rax, rbx ;rax = rax & rbx = 0x0000000000000001 35 .text:000000000DEAD208 mov [rsi], rax ;[rsi] = 0x0000000000000001 36 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000001 37 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 38 .text:000000000DEAD200 pop rdx ;0x0000000000000068 39 .text:000000000DEAD1AE cmp rax, rbx ; 40 .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;jmped 41 .text:000000000DEAD1B6 retn 42 .text:000000000DEAD2D5 mov [rsi], r12 ;[rsi] = 0x0000000000000001 43 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000001 44 .text:000000000DEAD261 mov [rsi], r8 ;[rsi] = input8 45 text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8 46 .text:000000000DEAD145 imul rax, rbx ;rax = 0x0000000000000001 * input8 47 .text:000000000DEAD208 mov [rsi], rax ;rax = input8 48 .text:000000000DEAD2DF mov r12, [rsi] ;r12 = input8 r12用于保存幂的值 49 .text:000000000DEAD261 mov [rsi], r8 ;[rsi] = input8 50 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8 51 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8 52 .text:000000000DEAD145 imul rax, rbx ;rax = input8*input* 53 .text:000000000DEAD208 mov [rsi], rax 54 .text:000000000DEAD26B mov r8, [rsi] ;r8 = input8*input8 55 .text:000000000DEAD29B mov [rsi], r10 ;*rsi = 0x0000000000003419 56 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000003419 57 .text:000000000DEAD195 shr rax, 1 ;rax = 0x0000000000001A0C 58 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000001A0C 59 .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0x0000000000001A0C 60 .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 61 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 62 .text:000000000DEAD1AE cmp rax, rbx 63 .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;if r10/2 > 0 64 .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AF9F8 65 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 66 .text:000000000DEAD177 and rax, rbx ;rax = 0x0000000000000000 67 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000000 68 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 69 .text:000000000DEAD200 pop rdx ;0x0000000000000068 70 .text:000000000DEAD1AE cmp rax, rbx 71 .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;not jmp 72 .text:000000000DEAD1B3 add rsp, rdx ;rsp = 0x000000000E0AFB20 73 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input8*input8 74 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8*input8 75 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8*input8 76 .text:000000000DEAD145 imul rax, rbx ;rax = input8*input8*input8*input8 77 .text:000000000DEAD208 mov [rsi], rax ;*rsi = input8*input8*input8*input8 78 .text:000000000DEAD26B mov r8, [rsi] ;r8 = input8*input8*input8*input8 79 .text:000000000DEAD29B mov [rsi], r10 ;*rsi = 0x0000000000001A0C 80 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000001A0C 81 .text:000000000DEAD195 shr rax, 1 ;0x0000000000000D06 82 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000D06 83 .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0x0000000000000D06 84 .text:000000000DEAD1F0 pop rbx ;rbx = 0x0000000000000000 85 .text:000000000DEAD200 pop rdx ;rdx = 0x0000000000000068 86 .text:000000000DEAD1AE cmp rax, rbx 87 .text:000000000DEAD1B1 jz short locret_DEAD1B6;not jmp 88 .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AF9F8 89 .text:000000000DEAD212 mov rax, [rsi] ; 90 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 91 .text:000000000DEAD177 and rax, rbx ;0x0000000000000000 92 .text:000000000DEAD208 mov [rsi], rax ;*rsi=0x0000000000000000 93 .text:000000000DEAD212 mov rax, [rsi] ;rax = r11 = 0x0000000000000000 94 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 95 .text:000000000DEAD200 pop rdx ;0x0000000000000068 96 .text:000000000DEAD1AE cmp rax, rbx 97 .text:000000000DEAD1B1 jz short locret_DEAD1B6 ; not jmp 98 .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AFB20 99 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input8*input8*input8*input8 100 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8*input8*input8*input8 101 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8*input8*input8*input8 102 .text:000000000DEAD145 imul rax, rbx ;rax = input8*input8*input8*input8*input8*input8*input8*input8 103 .text:000000000DEAD208 mov [rsi], rax 104 .text:000000000DEAD26B mov r8, [rsi] ;r8 = input8*input8*input8*input8*input8*input8*input8*input8 105 .text:000000000DEAD29B mov [rsi], r10 ;0x0000000000000D06 106 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000D06 107 .text:000000000DEAD195 shr rax, 1 ;rax = 0x0000000000000683 108 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000683 109 .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0x0000000000000683 110 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000683 111 .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 112 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 113 .text:000000000DEAD1AE cmp rax, rbx 114 .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;if r10/2 > 0 115 .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AF9F8 116 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000683 117 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 118 .text:000000000DEAD177 and rax, rbx ;rax = 0x0000000000000001 119 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000001 120 .text:000000000DEAD212 mov rax, [rsi] 121 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 122 .text:000000000DEAD200 pop rdx ;0x0000000000000068 123 .text:000000000DEAD1AE cmp rax, rbx 124 .text:000000000DEAD1B1 jz short locret_DEAD1B6;jmped 125 .text:000000000DEAD1B6 retn 126 .text:000000000DEAD2D5 mov [rsi], r12 ;*rsi = input8 127 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8 128 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input8*input8*input8*input8*input8*input8*input8*input8 129 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8*input8*input8*input8*input8*input8*input8*input8 130 .text:000000000DEAD145 imul rax, rbx ;rax = input(9) 131 .text:000000000DEAD208 mov [rsi], rax 132 .text:000000000DEAD2DF mov r12, [rsi] ;r12 = r12 * r8 = input(9) 133 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input(8) 134 .text:000000000DEAD212 mov rax, [rsi] 135 .text:000000000DEAD226 mov rbx, [rsi] 136 .text:000000000DEAD145 imul rax, rbx 137 .text:000000000DEAD208 mov [rsi], rax 138 .text:000000000DEAD26B mov r8, [rsi] 139 .text:000000000DEAD29B mov [rsi], r10 140 .text:000000000DEAD212 mov rax, [rsi] 141 .text:000000000DEAD195 shr rax, 1 142 .text:000000000DEAD208 mov [rsi], rax 143 .text:000000000DEAD2A5 mov r10, [rsi] 144 .text:000000000DEAD29B mov [rsi], r10 145 .text:000000000DEAD212 mov rax, [rsi] 146 .text:000000000DEAD1F0 pop rbx ；0x0000000000000000 147 ……
程序似乎没有结束的尽头，而且我们能够明显的感觉程序执行进入了一个循环。通过这些已经执行的代码，我们来大概分析一下程序的执行逻辑，并用伪码进行表示。如下：
；rcx = 8，应该是一个8次的循环 .text:000000000DEAD1F8 pop rcx ;0x0000000000000008 ;r8 = input[rcx], r9 = 0x1337DEADBEEF0095 .text:000000000DEAD275 pop r9 ;0x1337DEADBEEF0095 .text:000000000DEAD127 mov rax, [rdi] ;rax = 8bytes user input .text:000000000DEAD208 mov [rsi], rax ;[rsi] = 8bytes user input .text:000000000DEAD26B mov r8, [rsi] ;ri = 8bytes user input .text:000000000DEAD107 add rdi, 8 ;rdi points to next 8 bytes user input ;r9 = r9 * 0x000000000000CAFE + 0x000000000000BEEF .text:000000000DEAD27E mov [rsi], r9 ;[rsi] = 0x1337DEADBEEF0095 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x1337DEADBEEF0095 .text:000000000DEAD1F0 pop rbx ;rbx = 0x000000000000CAFE .text:000000000DEAD145 imul rax, rbx ;rax = 0x1337DEADBEEF0095 * 0x000000000000CAFE .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD288 mov r9, [rsi] ;r9 = 0x1337DEADBEEF0095 * 0x000000000000CAFE .text:000000000DEAD1F0 pop rbx ;rbx = 0x000000000000BEEF .text:000000000DEAD131 add rax, rbx ;rax = 0x1337DEADBEEF0095 * 0x000000000000CAFE + 0x000000000000BEEF .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD288 mov r9, [rsi] ;r9 = 0x1337DEADBEEF0095 * 0x000000000000CAFE + 0x000000000000BEEF ;r12 = 1, r10 = 13337 .text:000000000DEAD2CC pop r12 ;r12 = 0x0000000000000001 .text:000000000DEAD292 pop r10 ;r10 = 0x0000000000003419 = 13337 .text:000000000DEAD1E8 pop rax ;rax = 0x0000000000000000 .text:000000000DEAD1F0 pop rbx ;rbx = 0x0000000000000000 .text:000000000DEAD200 pop rdx ;rdx = 0x00000000000001D8 .text:000000000DEAD19F cmp rax, rbx ; .text:000000000DEAD1A2 jnz short locret_DEAD1A7 ;not jmp .text:000000000DEAD1A4 add rsp, rdx ;rsp = 0x000000000E0AFBD0 .text:000000000DEAD29B mov [rsi], r10 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000003419 .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 .text:000000000DEAD1AE cmp rax, rbx ; .text:000000000DEAD1B1 jz short locret_DEAD1B6 .text:000000000DEAD1B3 add rsp, rdx ;rsp = 0x000000000E0AF9F8 if (r10 & 0x01 == 0x01) { r12 = r12 * r8; r8 = r8 * r8; } .text:000000000DEAD212 mov rax, [rsi] ;rax = r10 = 0x0000000000003419 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD177 and rax, rbx ;rax = rax & rbx = 0x0000000000000001 .text:000000000DEAD208 mov [rsi], rax ;[rsi] = 0x0000000000000001 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000001 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD200 pop rdx ;0x0000000000000068 .text:000000000DEAD1AE cmp rax, rbx ; .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;jmped .text:000000000DEAD1B6 retn .text:000000000DEAD2D5 mov [rsi], r12 ;[rsi] = 0x0000000000000001 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000001 .text:000000000DEAD261 mov [rsi], r8 ;[rsi] = input8 text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8 .text:000000000DEAD145 imul rax, rbx ;rax = 0x0000000000000001 * input8 .text:000000000DEAD208 mov [rsi], rax ;rax = input8 .text:000000000DEAD2DF mov r12, [rsi] ;r12 = input8 r12用于保存幂的值 .text:000000000DEAD261 mov [rsi], r8 ;[rsi] = input8 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8 .text:000000000DEAD145 imul rax, rbx ;rax = input8*input* .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD26B mov r8, [rsi] ;r8 = input8*input8 r10 = r10 / 2; .text:000000000DEAD29B mov [rsi], r10 ;*rsi = 0x0000000000003419 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000003419 .text:000000000DEAD195 shr rax, 1 ;rax = 0x0000000000001A0C .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000001A0C .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0x0000000000001A0C if (r10 != 0) .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;if r10/2 > 0 .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AF9F8 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 if (r10 & 0x01 == 0) { r8 = r8 * r8; } .text:000000000DEAD177 and rax, rbx ;rax = 0x0000000000000000 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000000 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD200 pop rdx ;0x0000000000000068 .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;not jmp .text:000000000DEAD1B3 add rsp, rdx ;rsp = 0x000000000E0AFB20 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input8*input8 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8*input8 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8*input8 .text:000000000DEAD145 imul rax, rbx ;rax = input8*input8*input8*input8 .text:000000000DEAD208 mov [rsi], rax ;*rsi = input8*input8*input8*input8 .text:000000000DEAD26B mov r8, [rsi] ;r8 = input8*input8*input8*input8 r10 = r10 /2; .text:000000000DEAD29B mov [rsi], r10 ;*rsi = 0x0000000000001A0C .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000001A0C .text:000000000DEAD195 shr rax, 1 ;0x0000000000000D06 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000D06 .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0x0000000000000D06 if (r10 > 0) .text:000000000DEAD1F0 pop rbx ;rbx = 0x0000000000000000 .text:000000000DEAD200 pop rdx ;rdx = 0x0000000000000068 .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6;not jmp .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AF9F8 if (r10 & 0x01 == 0) { r8 = r8*r8; } .text:000000000DEAD212 mov rax, [rsi] ; .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD177 and rax, rbx ;0x0000000000000000 .text:000000000DEAD208 mov [rsi], rax ;*rsi=0x0000000000000000 .text:000000000DEAD212 mov rax, [rsi] ;rax = r11 = 0x0000000000000000 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD200 pop rdx ;0x0000000000000068 .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6 ; not jmp .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AFB20 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input8*input8*input8*input8 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8*input8*input8*input8 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8*input8*input8*input8 .text:000000000DEAD145 imul rax, rbx ;rax = input8*input8*input8*input8*input8*input8*input8*input8 .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD26B mov r8, [rsi] ;r8 = input8*input8*input8*input8*input8*input8*input8*input8 r10 = r10 / 2 .text:000000000DEAD29B mov [rsi], r10 ;0x0000000000000D06 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000D06 .text:000000000DEAD195 shr rax, 1 ;rax = 0x0000000000000683 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000683 .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0x0000000000000683 if (r10 > 0) .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000683 .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6 ;if r10/2 > 0 if (r10 & 0x01 == 1) { r12 = r12 * r8; r8 = r8 * r8; } .text:000000000DEAD1B3 add rsp, rdx ;0x000000000E0AF9F8 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0x0000000000000683 .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD177 and rax, rbx ;rax = 0x0000000000000001 .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0x0000000000000001 .text:000000000DEAD212 mov rax, [rsi] .text:000000000DEAD1F0 pop rbx ;0x0000000000000001 .text:000000000DEAD200 pop rdx ;0x0000000000000068 .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6;jmped .text:000000000DEAD1B6 retn .text:000000000DEAD2D5 mov [rsi], r12 ;*rsi = input8 .text:000000000DEAD212 mov rax, [rsi] ;rax = input8 .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input8*input8*input8*input8*input8*input8*input8*input8 .text:000000000DEAD226 mov rbx, [rsi] ;rbx = input8*input8*input8*input8*input8*input8*input8*input8 .text:000000000DEAD145 imul rax, rbx ;rax = input(9) .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD2DF mov r12, [rsi] ;r12 = r12 * r8 = input(9) .text:000000000DEAD261 mov [rsi], r8 ;*rsi = input(8) .text:000000000DEAD212 mov rax, [rsi] .text:000000000DEAD226 mov rbx, [rsi] .text:000000000DEAD145 imul rax, rbx .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD26B mov r8, [rsi] r10 = r10 / 2; .text:000000000DEAD29B mov [rsi], r10 .text:000000000DEAD212 mov rax, [rsi] .text:000000000DEAD195 shr rax, 1 .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD2A5 mov r10, [rsi] .text:000000000DEAD29B mov [rsi], r10 .text:000000000DEAD212 mov rax, [rsi] .text:000000000DEAD1F0 pop rbx ；0x0000000000000000
将我们写的伪码汇总一下，大概如下：
r9 = 0x1337DEADBEEF0095; r9 = 0x000000000000CAFE + 0x000000000000BEEF r12 = 1, r10 = 13337 if (r10 & 0x01 == 0x01) { r12 = r12 * r8; r8 = r8 * r8; } r10 = r10 / 2; if (r10 != 0) if (r10 & 0x01 == 0) { r8 = r8 * r8; } r10 = r10 /2; if (r10 != 0) if (r10 & 0x01 == 0) { r8 = r8*r8; } r10 = r10 / 2 if (r10 ! 0) if (r10 & 0x01 == 1) { r12 = r12 * r8; r8 = r8 * r8; } r10 = r10 / 2;
从上述伪码大概可以看，开始的时候，程序进行了初始化的赋值操作，然后会对r10 & 0x01进行判断，如果该值不为0，会执行“r12 = r12 * r8; r8 = r8 * r8;”，如果该值为0，执行“r8 = r8 * r8;”，然后r10右移一位，重复这个过程，直到r10为0为止。
那么用C语言表示这段代码的逻辑，如下：
r9 = 0x1337DEADBEEF0095; r12 = 1 r9 = r9 * 0x000000000000CAFE + 0x000000000000BEEF; for (r10 = 13337; r10 != 0; r10 = r10/2) { if (r10 & 0x01) { r12 = r12 * r8 } r8 = r8 * r8; }
上述处理都是r10不为0的时候。那么当r10为0的时候，处理逻辑是怎样的呢？由于r10的初始值为13337，经过13次右移1位后，r10将为0，我们在移位处设置断点：
重新运行程序进行调试，前13次断在该位置时，我们直接按”F9”，在第14次断在该位置时，我们按”F8”进行单步跟踪，此时有效执行的gadgets及分析如下：
if (r10 == 0 ) .text:000000000DEAD208 mov [rsi], rax ;*rsi = 0 .text:000000000DEAD2A5 mov r10, [rsi] ;r10 = 0 .text:000000000DEAD212 mov rax, [rsi] ;rax = 0 .text:000000000DEAD1F0 pop rbx ;0x0000000000000000 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFDE0 .text:000000000DEAD1AE cmp rax, rbx ; .text:000000000DEAD1B1 jz short locret_DEAD1B6 if (r12 != r9) { handlemsg () } .text:000000000DEAD2D5 mov [rsi], r12 .text:000000000DEAD212 mov rax, [rsi] .text:000000000DEAD27E mov [rsi], r9 .text:000000000DEAD226 mov rbx, [rsi] .text:000000000DEAD200 pop rdx .text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6 .text:000000000DEAD200 pop rdx ;0xFFFFFFFFFFFFFC38 .text:000000000DEAD1DF loop loc_DEAD1DB
这段用伪码来表示的执行逻辑为：
if (r10 == 0) { if (r12 != r9) { handlemsg () } }
即在上述for循环结束时，判断r12是否等于r9，如果不相等，则继续监听socket，重复整个过程。
至此，用c语言还原的完整代码为：
r9 = 0x1337DEADBEEF0095; r12 = 1 r9 = r9 * 0x000000000000CAFE + 0x000000000000BEEF; for (r10 = 13337; r10 != 0; r10 = r10/2) { if (r10 & 0x01) { r12 = r12 * r8 } r8 = r8 * r8; } if (r9 != r12) { handlemsg() }
那么还有一个分支我们没有走到，即r9如果等于r12，程序会执行什么操作？还记得之前POP RCX后，RCX==8吗？我们猜想此时会进入外层的这个循环。动态调试一下来验证我们的想法。重复上述过程，在第14次断在.text:000000000DEAD195 shr rax, 1时，采用单步调试，在执行到下面的判断时，我们可以修改寄存器的值来满足这个判断，进而执行r9==r12后的逻辑：
.text:000000000DEAD1AE cmp rax, rbx .text:000000000DEAD1B1 jz short locret_DEAD1B6
在执行到这个位置后，寄存器内容如下：
此时rax为0，rbx为0x2724090C0798E4C5，我们修改rax寄存器的值为0x2724090C0798E4C5，这样就满足了r9==r12，修改后，我们跟踪gadgets的执行，有效指令如下：
text:000000000DEAD1FC jmp short loc_DEAD200 .text:000000000DEAD200 pop rdx .text:000000000DEAD1DF loop loc_DEAD1DB .text:000000000DEAD1DB add rsp, rdx .text:000000000DEAD127 mov rax, [rdi] .text:000000000DEAD208 mov [rsi], rax .text:000000000DEAD26B mov r8, [rsi]
此时，rcx变为7，rdi指向用户输入的下一个8字节，开始了一个新的循环过程。
在8个外循环执行后，程序将打印出flag。
综合上述分析，我们可以用C语言表示该程序的执行逻辑了：
#include <stdio.h> #include <stdlib.h> long long r8[8] = {???, ???, ???, ???, ???, ???, ???, ???}; int main () { r9 = 0x1337DEADBEEF0095; for (rcx = 0; rcx < 8; rcx ++) { r9 = r9 * 0x000000000000CAFE + 0x000000000000BEEF; r12 = 1; for (r10 = 13337; r10 > 0; r10 = r10 / 2) { if (r10 & 0x01) r12 = r12 * r8[rcx]; r8[rcx] = r8[rcx] * r8[rcx]; } if (r12 != r9) { handlemsg(); } } printflag (); }
R9的初始值为0x1337DEADBEEF0095，每次循环后会变化，我们可以计算出每次循环后R9的值分别为：
long long r9[8] = { 0x2724090c0798e4c5, 0x44e477ee2e372c65, 0xa150eec963c67d25, 0xeab7d48b9db01ba5, 0xf01b0cf36a8c5ea5, 0x930eeb9679f4d8a5, 0xaeb27b8833e1e4a5, 0x2a900a13b88bcca5 };
从我们分析的结果来看，R12为用户输入数据的N次幂，N是多少呢？仿照这个程序我们写一段C代码来计算一下：
int getpower () { long long magic = 0x0000000000003419; int twopower = 1; int r8power = 0; for (; magic > 0; magic = magic >> 1) { if (magic & 0x01) { r8power += twopower; } twopower = twopower*2; } return r8power; }
可以知道幂的次数是13337，也就是程序监听的端口。于是问题转化为，给定Y，求X，使得X满足如下条件：
X^13337 = Y
实际上，由于X是一个64位整数，在计算其幂时，很可能会溢出，所以实际问题应该是给定Y，求X，使得X满足如下条件：
X^13337 %2^64 = Y
这就是个数学问题了，本人数学功底太差，不知道怎么去求解。但是看网上有人用欧拉定理来求解，有人用广义费马小定理来求解，膜拜这些数学大牛们。
网上还看到有大牛们用python的pack函数来求解，得到了flag，没想明白是怎么做到的，代码如下：
import sys,struct def f(x): return struct.pack(‘Q‘, x) p = ‘‘ p += f(0xd5b028b6c97155a5) p += f(0x51a2c3e8e288fa45) p += f(0x561720a3f926b105) p += f(0xa325ec548e4e0385) p += f(0x5369761ad6ccde85) p += f(0x9475802813002885) p += f(0xcadd6a0bdc679485) p += f(0x7d67b37124bcbc85) sys.stdout.write(p)
将上述脚本的输出传递给r0ops即可打印flag:
[email protected]:/home/ctf/0ctf2015/r0ops# python roops.py | nc localhost 13337 [email protected]:/home/ctf/0ctf2015/r0ops# ./r0ops YOU WIN! FLAG IS: 0ctf{c97155a5e288fa45f926b1058e4e0385d6ccde8513002885dc67948524bcbc85}
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