# 6.1.7 pwn 0CTF2015 freenote - [题目简介](#题目解析) - [题目逆向](#题目逆向) - [Exploit](#exploit) - [参考资料](#参考资料) [下载文件](../src/writeup/6.1.7_pwn_0ctf2015_freenote) ## 题目解析 ``` $ file freenote freenote: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=dd259bb085b3a4aeb393ec5ef4f09e312555a64d, stripped $ checksec -f freenote RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE Partial RELRO Canary found NX enabled No PIE No RPATH No RUNPATH Yes 0 2 freenote ``` 因为没有 PIE，即使本机开启 ASLR 也没有关系。 玩一下，它有 List、New、Edit、Delete 四个功能： ``` $ ./freenote == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 2 Length of new note: 5 Enter your note: AAAA Done. == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 1 0. AAAA == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 3 Note number: 0 Length of note: 10 Enter your note: BBBBBBBBB Done. == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 1 0. BBBBBBBBB == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 4 Note number: 0 Done. == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 1 You need to create some new notes first. == 0ops Free Note == 1. List Note 2. New Note 3. Edit Note 4. Delete Note 5. Exit ==================== Your choice: 5 Bye ``` 然后漏洞似乎也很明显，如果我们两次 Delete 同一个笔记，则触发 double free： ``` *** Error in `./freenote': double free or corruption (!prev): 0x0000000000672830 *** Aborted ``` 在 Ubuntu 14.04 上把程序跑起来： ``` $ socat tcp4-listen:10001,reuseaddr,fork exec:"env LD_PRELOAD=./libc.so_1 ./freenote" & ``` ## 题目逆向 我们先来看一下 main 函数： ``` [0x00400770]> pdf @ main / (fcn) main 60 | main (); | ; var int local_4h @ rbp-0x4 | ; DATA XREF from 0x0040078d (entry0) | 0x00401087 55 push rbp | 0x00401088 4889e5 mov rbp, rsp | 0x0040108b 4883ec10 sub rsp, 0x10 | 0x0040108f b800000000 mov eax, 0 | 0x00401094 e864f9ffff call sub.setvbuf_9fd ; int setvbuf(FILE*stream, char*buf, int mode, size_t size) | 0x00401099 b800000000 mov eax, 0 | 0x0040109e e8a6f9ffff call sub.malloc_a49 ; void *malloc(size_t size) | ; JMP XREF from 0x0040110f (main + 136) | 0x004010a3 b800000000 mov eax, 0 | 0x004010a8 e8ebf8ffff call sub.0ops_Free_Note_998 | 0x004010ad 8945fc mov dword [local_4h], eax | 0x004010b0 837dfc05 cmp dword [local_4h], 5 ; [0x5:4]=-1 ; 5 | ,=< 0x004010b4 774e ja 0x401104 | | 0x004010b6 8b45fc mov eax, dword [local_4h] | | 0x004010b9 488b04c5f812. mov rax, qword [rax*8 + 0x4012f8] ; [0x4012f8:8]=0x401104 \ | 0x004010c1 ffe0 jmp rax ``` main 函数首先调用 `sub.malloc_a49()` 分配一块内存并进行初始化，然后读取用户输入，根据偏移跳转到相应的函数上去（典型的switch实现）： ``` [0x00400770]> px 48 @ 0x4012f8 - offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF 0x004012f8 0411 4000 0000 0000 c310 4000 0000 0000 ..@.......@..... 0x00401308 cf10 4000 0000 0000 db10 4000 0000 0000 ..@.......@..... 0x00401318 e710 4000 0000 0000 f310 4000 0000 0000 ..@.......@..... ``` ``` [0x00400770]> pd 22 @ 0x4010c3 : 0x004010c3 b800000000 mov eax, 0 : 0x004010c8 e847faffff call sub.You_need_to_create_some_new_notes_first._b14 ,==< 0x004010cd eb40 jmp 0x40110f |: 0x004010cf b800000000 mov eax, 0 |: 0x004010d4 e8e9faffff call sub.Length_of_new_note:_bc2 ,===< 0x004010d9 eb34 jmp 0x40110f ||: 0x004010db b800000000 mov eax, 0 ||: 0x004010e0 e8a2fcffff call sub.Note_number:_d87 ,====< 0x004010e5 eb28 jmp 0x40110f |||: 0x004010e7 b800000000 mov eax, 0 |||: 0x004010ec e88cfeffff call sub.No_notes_yet._f7d ,=====< 0x004010f1 eb1c jmp 0x40110f ||||: 0x004010f3 bfe6124000 mov edi, 0x4012e6 ||||: 0x004010f8 e8c3f5ffff call sym.imp.puts ; int puts(const char *s) ||||: 0x004010fd b800000000 mov eax, 0 ,======< 0x00401102 eb0d jmp 0x401111 |||||: ; JMP XREF from 0x004010b4 (main) |||||: 0x00401104 bfea124000 mov edi, str.Invalid ; 0x4012ea ; "Invalid!" |||||: 0x00401109 e8b2f5ffff call sym.imp.puts ; int puts(const char *s) |||||: 0x0040110e 90 nop |||||| ; JMP XREF from 0x004010cd (main + 70) |||||| ; JMP XREF from 0x004010d9 (main + 82) |||||| ; JMP XREF from 0x004010e5 (main + 94) |||||| ; JMP XREF from 0x004010f1 (main + 106) |`````=< 0x0040110f eb92 jmp 0x4010a3 ; main+0x1c | ; JMP XREF from 0x00401102 (main + 123) `------> 0x00401111 c9 leave 0x00401112 c3 ret ``` 所以四个功能对应的函数如下： - List：`sub.You_need_to_create_some_new_notes_first._b14` - New：`sub.Length_of_new_note:_bc2` - Edit：`call sub.Note_number:_d87` - Delete：`call sub.No_notes_yet._f7d` 函数 `sub.malloc_a49` 如下： ``` [0x00400770]> pdf @ sub.malloc_a49 / (fcn) sub.malloc_a49 203 | sub.malloc_a49 (); | ; var int local_4h @ rbp-0x4 | ; CALL XREF from 0x0040109e (main) | 0x00400a49 55 push rbp | 0x00400a4a 4889e5 mov rbp, rsp | 0x00400a4d 4883ec10 sub rsp, 0x10 | 0x00400a51 bf10180000 mov edi, 0x1810 | 0x00400a56 e8d5fcffff call sym.imp.malloc ; void *malloc(size_t size) | 0x00400a5b 488905461620. mov qword [0x006020a8], rax ; [0x6020a8:8]=0 | 0x00400a62 488b053f1620. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | 0x00400a69 48c700000100. mov qword [rax], 0x100 ; [0x100:8]=-1 ; 256 ; Notes 结构体成员 max | 0x00400a70 488b05311620. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | 0x00400a77 48c740080000. mov qword [rax + 8], 0 ; Notes 结构体成员 length | 0x00400a7f c745fc000000. mov dword [local_4h], 0 ; [local_4h] 是 Note 的序号 | ,=< 0x00400a86 eb7d jmp 0x400b05 | | ; JMP XREF from 0x00400b0c (sub.malloc_a49) | .--> 0x00400a88 488b0d191620. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 ; Notes 结构体的地址 | :| 0x00400a8f 8b45fc mov eax, dword [local_4h] | :| 0x00400a92 4863d0 movsxd rdx, eax | :| 0x00400a95 4889d0 mov rax, rdx | :| 0x00400a98 4801c0 add rax, rax ; '#' | :| 0x00400a9b 4801d0 add rax, rdx ; '(' | :| 0x00400a9e 48c1e003 shl rax, 3 ; 序号 *24 | :| 0x00400aa2 4801c8 add rax, rcx ; '&' | :| 0x00400aa5 4883c010 add rax, 0x10 ; 序号对应的 Note 地址 | :| 0x00400aa9 48c700000000. mov qword [rax], 0 ; Note 结构体成员 isValid 初始化为 0 | :| 0x00400ab0 488b0df11520. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | :| 0x00400ab7 8b45fc mov eax, dword [local_4h] | :| 0x00400aba 4863d0 movsxd rdx, eax | :| 0x00400abd 4889d0 mov rax, rdx | :| 0x00400ac0 4801c0 add rax, rax ; '#' | :| 0x00400ac3 4801d0 add rax, rdx ; '(' | :| 0x00400ac6 48c1e003 shl rax, 3 | :| 0x00400aca 4801c8 add rax, rcx ; '&' | :| 0x00400acd 4883c010 add rax, 0x10 | :| 0x00400ad1 48c740080000. mov qword [rax + 8], 0 ; Note 结构体成员 length 初始化为 0 | :| 0x00400ad9 488b0dc81520. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | :| 0x00400ae0 8b45fc mov eax, dword [local_4h] | :| 0x00400ae3 4863d0 movsxd rdx, eax | :| 0x00400ae6 4889d0 mov rax, rdx | :| 0x00400ae9 4801c0 add rax, rax ; '#' | :| 0x00400aec 4801d0 add rax, rdx ; '(' | :| 0x00400aef 48c1e003 shl rax, 3 | :| 0x00400af3 4801c8 add rax, rcx ; '&' | :| 0x00400af6 4883c020 add rax, 0x20 | :| 0x00400afa 48c700000000. mov qword [rax], 0 ; Note 结构体成员 content 初始化为 0 | :| 0x00400b01 8345fc01 add dword [local_4h], 1 ; 序号 +1 | :| ; JMP XREF from 0x00400a86 (sub.malloc_a49) | :`-> 0x00400b05 817dfcff0000. cmp dword [local_4h], 0xff ; [0xff:4]=-1 ; 255 ; 循环初始化 Note | `==< 0x00400b0c 0f8e76ffffff jle 0x400a88 | 0x00400b12 c9 leave \ 0x00400b13 c3 ret ``` 该函数调用 malloc 在堆上分配 0x1810 字节的内存用来存放数据。我们可以猜测这里还存在两个结构体，Note 和 Notes： ```c struct Note { int isValid; // 1 表示笔记存在，0 表示笔记不存在 int length; // 笔记长度 char *content; // 指向笔记内容的指针 } Note; struct Notes { int max; // 笔记总数上限 0x100 int length; // 当前笔记数量 Note notes[256]; // 笔记 } Notes; ``` 接下来依次分析程序四个功能的实现，先从 List 开始： ``` [0x00400770]> pdf @ sub.You_need_to_create_some_new_notes_first._b14 / (fcn) sub.You_need_to_create_some_new_notes_first._b14 174 | sub.You_need_to_create_some_new_notes_first._b14 (); | ; var int local_4h @ rbp-0x4 | ; CALL XREF from 0x004010c8 (main + 65) | 0x00400b14 55 push rbp | 0x00400b15 4889e5 mov rbp, rsp | 0x00400b18 4883ec10 sub rsp, 0x10 | 0x00400b1c 488b05851520. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | 0x00400b23 488b4008 mov rax, qword [rax + 8] ; [0x8:8]=-1 ; 8 ; 取出 Notes 结构体成员 length | 0x00400b27 4885c0 test rax, rax ; 判断 length 是否为 0 | ,=< 0x00400b2a 0f8e86000000 jle 0x400bb6 ; 如果为 0，即没有笔记时跳转，该函数结束 | | 0x00400b30 c745fc000000. mov dword [local_4h], 0 ; [local_4h] 初始化为 0 | ,==< 0x00400b37 eb66 jmp 0x400b9f | || ; JMP XREF from 0x00400bb2 (sub.You_need_to_create_some_new_notes_first._b14) | .---> 0x00400b39 488b0d681520. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | :|| 0x00400b40 8b45fc mov eax, dword [local_4h] | :|| 0x00400b43 4863d0 movsxd rdx, eax | :|| 0x00400b46 4889d0 mov rax, rdx | :|| 0x00400b49 4801c0 add rax, rax ; '#' | :|| 0x00400b4c 4801d0 add rax, rdx ; '(' | :|| 0x00400b4f 48c1e003 shl rax, 3 | :|| 0x00400b53 4801c8 add rax, rcx ; '&' | :|| 0x00400b56 4883c010 add rax, 0x10 ; rax 为 local_4h 序号处的 Note | :|| 0x00400b5a 488b00 mov rax, qword [rax] ; 取出 Note 结构体成员 isValid | :|| 0x00400b5d 4883f801 cmp rax, 1 ; 1 | ,====< 0x00400b61 7538 jne 0x400b9b ; 如果 isValid != 1，跳过打印 | |:|| 0x00400b63 488b0d3e1520. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | |:|| 0x00400b6a 8b45fc mov eax, dword [local_4h] | |:|| 0x00400b6d 4863d0 movsxd rdx, eax | |:|| 0x00400b70 4889d0 mov rax, rdx | |:|| 0x00400b73 4801c0 add rax, rax ; '#' | |:|| 0x00400b76 4801d0 add rax, rdx ; '(' | |:|| 0x00400b79 48c1e003 shl rax, 3 | |:|| 0x00400b7d 4801c8 add rax, rcx ; '&' | |:|| 0x00400b80 4883c020 add rax, 0x20 | |:|| 0x00400b84 488b10 mov rdx, qword [rax] | |:|| 0x00400b87 8b45fc mov eax, dword [local_4h] | |:|| 0x00400b8a 89c6 mov esi, eax | |:|| 0x00400b8c bf1d124000 mov edi, str.d.__s ; 0x40121d ; "%d. %s\n" | |:|| 0x00400b91 b800000000 mov eax, 0 | |:|| 0x00400b96 e845fbffff call sym.imp.printf ; int printf(const char *format) ; 打印出序号和内容 | |:|| ; JMP XREF from 0x00400b61 (sub.You_need_to_create_some_new_notes_first._b14) | `----> 0x00400b9b 8345fc01 add dword [local_4h], 1 | :|| ; JMP XREF from 0x00400b37 (sub.You_need_to_create_some_new_notes_first._b14) | :`--> 0x00400b9f 8b45fc mov eax, dword [local_4h] ; eax = [local_4h] | : | 0x00400ba2 4863d0 movsxd rdx, eax ; rdx = eax == [local_4h] | : | 0x00400ba5 488b05fc1420. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 ; 取出 Notes 地址 | : | 0x00400bac 488b00 mov rax, qword [rax] ; 取出结构体成员 max == 0x100 | : | 0x00400baf 4839c2 cmp rdx, rax ; 比较当前笔记序号 rdx 与 max | `===< 0x00400bb2 7c85 jl 0x400b39 ; 遍历所有笔记 | ,==< 0x00400bb4 eb0a jmp 0x400bc0 | || ; JMP XREF from 0x00400b2a (sub.You_need_to_create_some_new_notes_first._b14) | |`-> 0x00400bb6 bf28124000 mov edi, str.You_need_to_create_some_new_notes_first. ; 0x401228 ; "You need to create some new notes first." | | 0x00400bbb e800fbffff call sym.imp.puts ; int puts(const char *s) | | ; JMP XREF from 0x00400bb4 (sub.You_need_to_create_some_new_notes_first._b14) | `--> 0x00400bc0 c9 leave \ 0x00400bc1 c3 ret ``` 该函数会打印出所有 isValid 成员为 1 的笔记。 New 的实现如下： ``` [0x00400770]> pdf @ sub.Length_of_new_note:_bc2 / (fcn) sub.Length_of_new_note:_bc2 453 | sub.Length_of_new_note:_bc2 (int arg_1000h); | ; var int local_14h @ rbp-0x14 | ; var int local_10h @ rbp-0x10 | ; var int local_ch @ rbp-0xc | ; var int local_8h @ rbp-0x8 | ; var int local_0h @ rbp-0x0 | ; arg int arg_1000h @ rbp+0x1000 | ; CALL XREF from 0x004010d4 (main + 77) | 0x00400bc2 55 push rbp | 0x00400bc3 4889e5 mov rbp, rsp | 0x00400bc6 4883ec20 sub rsp, 0x20 | 0x00400bca 488b05d71420. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | 0x00400bd1 488b5008 mov rdx, qword [rax + 8] ; [0x8:8]=-1 ; 8 ; 取出 Notes 成员 length | 0x00400bd5 488b05cc1420. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | 0x00400bdc 488b00 mov rax, qword [rax] ; 取出 Notes 成员 max | 0x00400bdf 4839c2 cmp rdx, rax | ,=< 0x00400be2 7c0f jl 0x400bf3 ; 当 length < max 时继续，否则表示笔记本已满 | | 0x00400be4 bf51124000 mov edi, str.Unable_to_create_new_note. ; 0x401251 ; "Unable to create new note." | | 0x00400be9 e8d2faffff call sym.imp.puts ; int puts(const char *s) | ,==< 0x00400bee e992010000 jmp 0x400d85 | || ; JMP XREF from 0x00400be2 (sub.Length_of_new_note:_bc2) | |`-> 0x00400bf3 c745ec000000. mov dword [local_14h], 0 ; [local_14h] 初始化为 0 | |,=< 0x00400bfa e96d010000 jmp 0x400d6c | || ; JMP XREF from 0x00400d7f (sub.Length_of_new_note:_bc2) | .---> 0x00400bff 488b0da21420. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | :|| 0x00400c06 8b45ec mov eax, dword [local_14h] | :|| 0x00400c09 4863d0 movsxd rdx, eax | :|| 0x00400c0c 4889d0 mov rax, rdx | :|| 0x00400c0f 4801c0 add rax, rax ; '#' | :|| 0x00400c12 4801d0 add rax, rdx ; '(' | :|| 0x00400c15 48c1e003 shl rax, 3 | :|| 0x00400c19 4801c8 add rax, rcx ; '&' | :|| 0x00400c1c 4883c010 add rax, 0x10 | :|| 0x00400c20 488b00 mov rax, qword [rax] | :|| 0x00400c23 4885c0 test rax, rax ; rax 表示成员 isValid | ,====< 0x00400c26 0f853c010000 jne 0x400d68 ; 当 isValid 为 1 时，表示当前序号处笔记已经存在，跳过 | |:|| 0x00400c2c bf6c124000 mov edi, str.Length_of_new_note: ; 0x40126c ; "Length of new note: " | |:|| 0x00400c31 b800000000 mov eax, 0 | |:|| 0x00400c36 e8a5faffff call sym.imp.printf ; int printf(const char *format) | |:|| 0x00400c3b b800000000 mov eax, 0 | |:|| 0x00400c40 e809fdffff call sub.atoi_94e ; int atoi(const char *str) | |:|| 0x00400c45 8945f0 mov dword [local_10h], eax ; [local_10h] 是笔记长度 | |:|| 0x00400c48 837df000 cmp dword [local_10h], 0 | ,=====< 0x00400c4c 7f0f jg 0x400c5d ; 大于 0 时 | ||:|| 0x00400c4e bf81124000 mov edi, str.Invalid_length ; 0x401281 ; "Invalid length!" | ||:|| 0x00400c53 e868faffff call sym.imp.puts ; int puts(const char *s) | ,======< 0x00400c58 e928010000 jmp 0x400d85 | |||:|| ; JMP XREF from 0x00400c4c (sub.Length_of_new_note:_bc2) | |`-----> 0x00400c5d 817df0001000. cmp dword [local_10h], 0x1000 ; [0x1000:4]=-1 | |,=====< 0x00400c64 7e07 jle 0x400c6d ; 小于等于 0x1000 时 | |||:|| 0x00400c66 c745f0001000. mov dword [local_10h], 0x1000 ; 否则 [local_10h] = 0x1000 | |||:|| ; JMP XREF from 0x00400c64 (sub.Length_of_new_note:_bc2) | |`-----> 0x00400c6d 8b45f0 mov eax, dword [local_10h] | | |:|| 0x00400c70 99 cdq | | |:|| 0x00400c71 c1ea19 shr edx, 0x19 | | |:|| 0x00400c74 01d0 add eax, edx | | |:|| 0x00400c76 83e07f and eax, 0x7f | | |:|| 0x00400c79 29d0 sub eax, edx | | |:|| 0x00400c7b ba80000000 mov edx, 0x80 ; 128 | | |:|| 0x00400c80 29c2 sub edx, eax | | |:|| 0x00400c82 89d0 mov eax, edx | | |:|| 0x00400c84 c1f81f sar eax, 0x1f | | |:|| 0x00400c87 c1e819 shr eax, 0x19 | | |:|| 0x00400c8a 01c2 add edx, eax | | |:|| 0x00400c8c 83e27f and edx, 0x7f | | |:|| 0x00400c8f 29c2 sub edx, eax | | |:|| 0x00400c91 89d0 mov eax, edx | | |:|| 0x00400c93 89c2 mov edx, eax | | |:|| 0x00400c95 8b45f0 mov eax, dword [local_10h] | | |:|| 0x00400c98 01d0 add eax, edx | | |:|| 0x00400c9a 8945f4 mov dword [local_ch], eax | | |:|| 0x00400c9d 8b45f4 mov eax, dword [local_ch] | | |:|| 0x00400ca0 4898 cdqe | | |:|| 0x00400ca2 4889c7 mov rdi, rax ; rdi 最终为 ((128 - [local_10h] % 128) % 128 + [local_10h]) | | |:|| 0x00400ca5 e886faffff call sym.imp.malloc ; void *malloc(size_t size) | | |:|| 0x00400caa 488945f8 mov qword [local_8h], rax ; [local_8h] 为 Note 内容的地址 | | |:|| 0x00400cae bf91124000 mov edi, str.Enter_your_note: ; 0x401291 ; "Enter your note: " | | |:|| 0x00400cb3 b800000000 mov eax, 0 | | |:|| 0x00400cb8 e823faffff call sym.imp.printf ; int printf(const char *format) | | |:|| 0x00400cbd 8b55f0 mov edx, dword [local_10h] | | |:|| 0x00400cc0 488b45f8 mov rax, qword [local_8h] | | |:|| 0x00400cc4 89d6 mov esi, edx | | |:|| 0x00400cc6 4889c7 mov rdi, rax | | |:|| 0x00400cc9 e88ffbffff call sub.read_85d ; ssize_t read(int fildes, void *buf, size_t nbyte) ; 读入笔记内容 | | |:|| 0x00400cce 488b0dd31320. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | | |:|| 0x00400cd5 8b45ec mov eax, dword [local_14h] | | |:|| 0x00400cd8 4863d0 movsxd rdx, eax | | |:|| 0x00400cdb 4889d0 mov rax, rdx | | |:|| 0x00400cde 4801c0 add rax, rax ; '#' | | |:|| 0x00400ce1 4801d0 add rax, rdx ; '(' | | |:|| 0x00400ce4 48c1e003 shl rax, 3 | | |:|| 0x00400ce8 4801c8 add rax, rcx ; '&' | | |:|| 0x00400ceb 4883c010 add rax, 0x10 | | |:|| 0x00400cef 48c700010000. mov qword [rax], 1 ; 设置 Note 结构体成员 isValid 为 1 | | |:|| 0x00400cf6 488b35ab1320. mov rsi, qword [0x006020a8] ; [0x6020a8:8]=0 | | |:|| 0x00400cfd 8b45f0 mov eax, dword [local_10h] | | |:|| 0x00400d00 4863c8 movsxd rcx, eax | | |:|| 0x00400d03 8b45ec mov eax, dword [local_14h] | | |:|| 0x00400d06 4863d0 movsxd rdx, eax | | |:|| 0x00400d09 4889d0 mov rax, rdx | | |:|| 0x00400d0c 4801c0 add rax, rax ; '#' | | |:|| 0x00400d0f 4801d0 add rax, rdx ; '(' | | |:|| 0x00400d12 48c1e003 shl rax, 3 | | |:|| 0x00400d16 4801f0 add rax, rsi ; '+' | | |:|| 0x00400d19 4883c010 add rax, 0x10 | | |:|| 0x00400d1d 48894808 mov qword [rax + 8], rcx ; 设置 Note 结构体成员 length 为 [local_10h] | | |:|| 0x00400d21 488b0d801320. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | | |:|| 0x00400d28 8b45ec mov eax, dword [local_14h] | | |:|| 0x00400d2b 4863d0 movsxd rdx, eax | | |:|| 0x00400d2e 4889d0 mov rax, rdx | | |:|| 0x00400d31 4801c0 add rax, rax ; '#' | | |:|| 0x00400d34 4801d0 add rax, rdx ; '(' | | |:|| 0x00400d37 48c1e003 shl rax, 3 | | |:|| 0x00400d3b 4801c8 add rax, rcx ; '&' | | |:|| 0x00400d3e 488d5020 lea rdx, [rax + 0x20] ; 32 | | |:|| 0x00400d42 488b45f8 mov rax, qword [local_8h] | | |:|| 0x00400d46 488902 mov qword [rdx], rax ; 设置 Note 结构体成员 content 为 [local_8h] | | |:|| 0x00400d49 488b05581320. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | | |:|| 0x00400d50 488b5008 mov rdx, qword [rax + 8] ; [0x8:8]=-1 ; 8 ; 取出 Notes 成员 length | | |:|| 0x00400d54 4883c201 add rdx, 1 ; length +1 | | |:|| 0x00400d58 48895008 mov qword [rax + 8], rdx ; 写回 length | | |:|| 0x00400d5c bfa3124000 mov edi, str.Done. ; 0x4012a3 ; "Done." | | |:|| 0x00400d61 e85af9ffff call sym.imp.puts ; int puts(const char *s) | |,=====< 0x00400d66 eb1d jmp 0x400d85 | |||:|| ; JMP XREF from 0x00400c26 (sub.Length_of_new_note:_bc2) | ||`----> 0x00400d68 8345ec01 add dword [local_14h], 1 | || :|| ; JMP XREF from 0x00400bfa (sub.Length_of_new_note:_bc2) | || :|`-> 0x00400d6c 8b45ec mov eax, dword [local_14h] ; eax = [local_14h] | || :| 0x00400d6f 4863d0 movsxd rdx, eax ; rdx 表示序号 | || :| 0x00400d72 488b052f1320. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | || :| 0x00400d79 488b00 mov rax, qword [rax] ; 取出 Notes 成员 max | || :| 0x00400d7c 4839c2 cmp rdx, rax ; 比较序号与 max | || `===< 0x00400d7f 0f8c7afeffff jl 0x400bff ; 遍历 | || | ; JMP XREF from 0x00400bee (sub.Length_of_new_note:_bc2) | || | ; JMP XREF from 0x00400c58 (sub.Length_of_new_note:_bc2) | || | ; JMP XREF from 0x00400d66 (sub.Length_of_new_note:_bc2) | ``--`--> 0x00400d85 c9 leave \ 0x00400d86 c3 ret ``` 该函数首先对你输入的大小进行判断，如果小于 128 字节，则默认分配 128 字节的空间，如果大于 128 字节且小于 4096 字节时，则分配比输入稍大的 128 字节的整数倍的空间，如果大于 4096 字节，则默认分配 4096 字节。 Edit 的实现如下： ``` [0x00400770]> pdf @ sub.Note_number:_d87 / (fcn) sub.Note_number:_d87 502 | sub.Note_number:_d87 (int arg_1000h); | ; var int local_1ch @ rbp-0x1c | ; var int local_18h @ rbp-0x18 | ; var int local_14h @ rbp-0x14 | ; var int local_0h @ rbp-0x0 | ; arg int arg_1000h @ rbp+0x1000 | ; CALL XREF from 0x004010e0 (main + 89) | 0x00400d87 55 push rbp | 0x00400d88 4889e5 mov rbp, rsp | 0x00400d8b 53 push rbx | 0x00400d8c 4883ec18 sub rsp, 0x18 | 0x00400d90 bfa9124000 mov edi, str.Note_number: ; 0x4012a9 ; "Note number: " | 0x00400d95 b800000000 mov eax, 0 | 0x00400d9a e841f9ffff call sym.imp.printf ; int printf(const char *format) | 0x00400d9f b800000000 mov eax, 0 | 0x00400da4 e8a5fbffff call sub.atoi_94e ; int atoi(const char *str) | 0x00400da9 8945e8 mov dword [local_18h], eax ; [local_18h] 为要修改的笔记序号 | 0x00400dac 837de800 cmp dword [local_18h], 0 ; 进行检查，确保序号是有效的 | ,=< 0x00400db0 783f js 0x400df1 | | 0x00400db2 8b45e8 mov eax, dword [local_18h] | | 0x00400db5 4863d0 movsxd rdx, eax | | 0x00400db8 488b05e91220. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | | 0x00400dbf 488b00 mov rax, qword [rax] | | 0x00400dc2 4839c2 cmp rdx, rax | ,==< 0x00400dc5 7d2a jge 0x400df1 | || 0x00400dc7 488b0dda1220. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | || 0x00400dce 8b45e8 mov eax, dword [local_18h] | || 0x00400dd1 4863d0 movsxd rdx, eax | || 0x00400dd4 4889d0 mov rax, rdx | || 0x00400dd7 4801c0 add rax, rax ; '#' | || 0x00400dda 4801d0 add rax, rdx ; '(' | || 0x00400ddd 48c1e003 shl rax, 3 | || 0x00400de1 4801c8 add rax, rcx ; '&' | || 0x00400de4 4883c010 add rax, 0x10 | || 0x00400de8 488b00 mov rax, qword [rax] | || 0x00400deb 4883f801 cmp rax, 1 ; 1 | ,===< 0x00400def 740f je 0x400e00 ; 修改笔记的 isValid 必须为 1 | ||| ; JMP XREF from 0x00400db0 (sub.Note_number:_d87) | ||| ; JMP XREF from 0x00400dc5 (sub.Note_number:_d87) | |``-> 0x00400df1 bfb7124000 mov edi, str.Invalid_number ; 0x4012b7 ; "Invalid number!" | | 0x00400df6 e8c5f8ffff call sym.imp.puts ; int puts(const char *s) | | ,=< 0x00400dfb e976010000 jmp 0x400f76 | | | ; JMP XREF from 0x00400def (sub.Note_number:_d87) | `---> 0x00400e00 bfc7124000 mov edi, str.Length_of_note: ; 0x4012c7 ; "Length of note: " | | 0x00400e05 b800000000 mov eax, 0 | | 0x00400e0a e8d1f8ffff call sym.imp.printf ; int printf(const char *format) | | 0x00400e0f b800000000 mov eax, 0 | | 0x00400e14 e835fbffff call sub.atoi_94e ; int atoi(const char *str) | | 0x00400e19 8945e4 mov dword [local_1ch], eax ; [local_1ch] 为新大小 | | 0x00400e1c 837de400 cmp dword [local_1ch], 0 ; 进行检查，确保新的大小是有效的 | ,==< 0x00400e20 7f0f jg 0x400e31 | || 0x00400e22 bf81124000 mov edi, str.Invalid_length ; 0x401281 ; "Invalid length!" | || 0x00400e27 e894f8ffff call sym.imp.puts ; int puts(const char *s) | ,===< 0x00400e2c e945010000 jmp 0x400f76 | ||| ; JMP XREF from 0x00400e20 (sub.Note_number:_d87) | |`--> 0x00400e31 817de4001000. cmp dword [local_1ch], 0x1000 ; [0x1000:4]=-1 | |,==< 0x00400e38 7e07 jle 0x400e41 | ||| 0x00400e3a c745e4001000. mov dword [local_1ch], 0x1000 | ||| ; JMP XREF from 0x00400e38 (sub.Note_number:_d87) | |`--> 0x00400e41 8b45e4 mov eax, dword [local_1ch] | | | 0x00400e44 4863c8 movsxd rcx, eax | | | 0x00400e47 488b355a1220. mov rsi, qword [0x006020a8] ; [0x6020a8:8]=0 | | | 0x00400e4e 8b45e8 mov eax, dword [local_18h] | | | 0x00400e51 4863d0 movsxd rdx, eax | | | 0x00400e54 4889d0 mov rax, rdx | | | 0x00400e57 4801c0 add rax, rax ; '#' | | | 0x00400e5a 4801d0 add rax, rdx ; '(' | | | 0x00400e5d 48c1e003 shl rax, 3 | | | 0x00400e61 4801f0 add rax, rsi ; '+' | | | 0x00400e64 4883c010 add rax, 0x10 | | | 0x00400e68 488b4008 mov rax, qword [rax + 8] ; [0x8:8]=-1 ; 8 | | | 0x00400e6c 4839c1 cmp rcx, rax ; 比较新大小与原大小是否相同 | |,==< 0x00400e6f 0f84b7000000 je 0x400f2c ; 如果相同，跳过重新分配空间的过程，直接修改笔记 | ||| 0x00400e75 8b45e4 mov eax, dword [local_1ch] | ||| 0x00400e78 99 cdq | ||| 0x00400e79 c1ea19 shr edx, 0x19 | ||| 0x00400e7c 01d0 add eax, edx | ||| 0x00400e7e 83e07f and eax, 0x7f | ||| 0x00400e81 29d0 sub eax, edx | ||| 0x00400e83 ba80000000 mov edx, 0x80 ; 128 | ||| 0x00400e88 29c2 sub edx, eax | ||| 0x00400e8a 89d0 mov eax, edx | ||| 0x00400e8c c1f81f sar eax, 0x1f | ||| 0x00400e8f c1e819 shr eax, 0x19 | ||| 0x00400e92 01c2 add edx, eax | ||| 0x00400e94 83e27f and edx, 0x7f | ||| 0x00400e97 29c2 sub edx, eax | ||| 0x00400e99 89d0 mov eax, edx | ||| 0x00400e9b 89c2 mov edx, eax | ||| 0x00400e9d 8b45e4 mov eax, dword [local_1ch] | ||| 0x00400ea0 01d0 add eax, edx | ||| 0x00400ea2 8945ec mov dword [local_14h], eax | ||| 0x00400ea5 488b1dfc1120. mov rbx, qword [0x006020a8] ; [0x6020a8:8]=0 | ||| 0x00400eac 8b45ec mov eax, dword [local_14h] | ||| 0x00400eaf 4863c8 movsxd rcx, eax | ||| 0x00400eb2 488b35ef1120. mov rsi, qword [0x006020a8] ; [0x6020a8:8]=0 | ||| 0x00400eb9 8b45e8 mov eax, dword [local_18h] | ||| 0x00400ebc 4863d0 movsxd rdx, eax | ||| 0x00400ebf 4889d0 mov rax, rdx | ||| 0x00400ec2 4801c0 add rax, rax ; '#' | ||| 0x00400ec5 4801d0 add rax, rdx ; '(' | ||| 0x00400ec8 48c1e003 shl rax, 3 | ||| 0x00400ecc 4801f0 add rax, rsi ; '+' | ||| 0x00400ecf 4883c020 add rax, 0x20 | ||| 0x00400ed3 488b00 mov rax, qword [rax] | ||| 0x00400ed6 4889ce mov rsi, rcx ; rsi 为分配的大小，算法和 New 过程中的一样 | ||| 0x00400ed9 4889c7 mov rdi, rax ; rdi 为 Note 成员 content | ||| 0x00400edc e85ff8ffff call sym.imp.realloc ; void *realloc(void *ptr, size_t size) | ||| 0x00400ee1 4889c1 mov rcx, rax | ||| 0x00400ee4 8b45e8 mov eax, dword [local_18h] | ||| 0x00400ee7 4863d0 movsxd rdx, eax | ||| 0x00400eea 4889d0 mov rax, rdx | ||| 0x00400eed 4801c0 add rax, rax ; '#' | ||| 0x00400ef0 4801d0 add rax, rdx ; '(' | ||| 0x00400ef3 48c1e003 shl rax, 3 | ||| 0x00400ef7 4801d8 add rax, rbx ; '%' | ||| 0x00400efa 4883c020 add rax, 0x20 | ||| 0x00400efe 488908 mov qword [rax], rcx | ||| 0x00400f01 488b35a01120. mov rsi, qword [0x006020a8] ; [0x6020a8:8]=0 | ||| 0x00400f08 8b45e4 mov eax, dword [local_1ch] | ||| 0x00400f0b 4863c8 movsxd rcx, eax | ||| 0x00400f0e 8b45e8 mov eax, dword [local_18h] | ||| 0x00400f11 4863d0 movsxd rdx, eax | ||| 0x00400f14 4889d0 mov rax, rdx | ||| 0x00400f17 4801c0 add rax, rax ; '#' | ||| 0x00400f1a 4801d0 add rax, rdx ; '(' | ||| 0x00400f1d 48c1e003 shl rax, 3 | ||| 0x00400f21 4801f0 add rax, rsi ; '+' | ||| 0x00400f24 4883c010 add rax, 0x10 | ||| 0x00400f28 48894808 mov qword [rax + 8], rcx ; 将新的大小写回 Note 的 length | ||| ; JMP XREF from 0x00400e6f (sub.Note_number:_d87) | |`--> 0x00400f2c bf91124000 mov edi, str.Enter_your_note: ; 0x401291 ; "Enter your note: " | | | 0x00400f31 b800000000 mov eax, 0 | | | 0x00400f36 e8a5f7ffff call sym.imp.printf ; int printf(const char *format) | | | 0x00400f3b 488b0d661120. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | | | 0x00400f42 8b45e8 mov eax, dword [local_18h] | | | 0x00400f45 4863d0 movsxd rdx, eax | | | 0x00400f48 4889d0 mov rax, rdx | | | 0x00400f4b 4801c0 add rax, rax ; '#' | | | 0x00400f4e 4801d0 add rax, rdx ; '(' | | | 0x00400f51 48c1e003 shl rax, 3 | | | 0x00400f55 4801c8 add rax, rcx ; '&' | | | 0x00400f58 4883c020 add rax, 0x20 | | | 0x00400f5c 488b00 mov rax, qword [rax] | | | 0x00400f5f 8b55e4 mov edx, dword [local_1ch] | | | 0x00400f62 89d6 mov esi, edx | | | 0x00400f64 4889c7 mov rdi, rax | | | 0x00400f67 e8f1f8ffff call sub.read_85d ; ssize_t read(int fildes, void *buf, size_t nbyte) ; 读入新的笔记内容 | | | 0x00400f6c bfa3124000 mov edi, str.Done. ; 0x4012a3 ; "Done." | | | 0x00400f71 e84af7ffff call sym.imp.puts ; int puts(const char *s) | | | ; JMP XREF from 0x00400dfb (sub.Note_number:_d87) | | | ; JMP XREF from 0x00400e2c (sub.Note_number:_d87) | `-`-> 0x00400f76 4883c418 add rsp, 0x18 | 0x00400f7a 5b pop rbx | 0x00400f7b 5d pop rbp \ 0x00400f7c c3 ret ``` 该函数在输入了笔记序号和大小之后，会先判断新的大小与现在的大小是否相同，如果相同，则不重新分配空间，直接编辑其内容，否则调用 `realloc()` 重新分配一块空间（地址可能与原地址相同，也可能不相同）。 Delete 的实现如下： ``` [0x00400770]> pdf @ sub.No_notes_yet._f7d / (fcn) sub.No_notes_yet._f7d 266 | sub.No_notes_yet._f7d (); | ; var int local_4h @ rbp-0x4 | ; var int local_0h @ rbp-0x0 | ; CALL XREF from 0x004010ec (main + 101) | 0x00400f7d 55 push rbp | 0x00400f7e 4889e5 mov rbp, rsp | 0x00400f81 4883ec10 sub rsp, 0x10 | 0x00400f85 488b051c1120. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | 0x00400f8c 488b4008 mov rax, qword [rax + 8] ; [0x8:8]=-1 ; 8 | 0x00400f90 4885c0 test rax, rax ; 检查 Notes 成员 length 是否为零，即是否有笔记 | ,=< 0x00400f93 0f8ee2000000 jle 0x40107b | | 0x00400f99 bfa9124000 mov edi, str.Note_number: ; 0x4012a9 ; "Note number: " | | 0x00400f9e b800000000 mov eax, 0 | | 0x00400fa3 e838f7ffff call sym.imp.printf ; int printf(const char *format) | | 0x00400fa8 b800000000 mov eax, 0 | | 0x00400fad e89cf9ffff call sub.atoi_94e ; int atoi(const char *str) | | 0x00400fb2 8945fc mov dword [local_4h], eax ; [local_4h] 为要删除笔记的序号 | | 0x00400fb5 837dfc00 cmp dword [local_4h], 0 ; 检查笔记序号是否有效 | ,==< 0x00400fb9 7815 js 0x400fd0 | || 0x00400fbb 8b45fc mov eax, dword [local_4h] | || 0x00400fbe 4863d0 movsxd rdx, eax | || 0x00400fc1 488b05e01020. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | || 0x00400fc8 488b00 mov rax, qword [rax] | || 0x00400fcb 4839c2 cmp rdx, rax | ,===< 0x00400fce 7c0f jl 0x400fdf | ||| ; JMP XREF from 0x00400fb9 (sub.No_notes_yet._f7d) | |`--> 0x00400fd0 bfb7124000 mov edi, str.Invalid_number ; 0x4012b7 ; "Invalid number!" | | | 0x00400fd5 e8e6f6ffff call sym.imp.puts ; int puts(const char *s) | |,==< 0x00400fda e9a6000000 jmp 0x401085 | ||| ; JMP XREF from 0x00400fce (sub.No_notes_yet._f7d) | `---> 0x00400fdf 488b05c21020. mov rax, qword [0x006020a8] ; [0x6020a8:8]=0 | || 0x00400fe6 488b5008 mov rdx, qword [rax + 8] ; [0x8:8]=-1 ; 8 ; 取出 Notes 成员 length | || 0x00400fea 4883ea01 sub rdx, 1 ; 将 length -1 | || 0x00400fee 48895008 mov qword [rax + 8], rdx ; 将新的 length 写回去 | || 0x00400ff2 488b0daf1020. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | || 0x00400ff9 8b45fc mov eax, dword [local_4h] | || 0x00400ffc 4863d0 movsxd rdx, eax | || 0x00400fff 4889d0 mov rax, rdx | || 0x00401002 4801c0 add rax, rax ; '#' | || 0x00401005 4801d0 add rax, rdx ; '(' | || 0x00401008 48c1e003 shl rax, 3 | || 0x0040100c 4801c8 add rax, rcx ; '&' | || 0x0040100f 4883c010 add rax, 0x10 | || 0x00401013 48c700000000. mov qword [rax], 0 ; 修改 Note 成员 isValid 为 0 | || 0x0040101a 488b0d871020. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | || 0x00401021 8b45fc mov eax, dword [local_4h] | || 0x00401024 4863d0 movsxd rdx, eax | || 0x00401027 4889d0 mov rax, rdx | || 0x0040102a 4801c0 add rax, rax ; '#' | || 0x0040102d 4801d0 add rax, rdx ; '(' | || 0x00401030 48c1e003 shl rax, 3 | || 0x00401034 4801c8 add rax, rcx ; '&' | || 0x00401037 4883c010 add rax, 0x10 | || 0x0040103b 48c740080000. mov qword [rax + 8], 0 ; 修改 Note 成员 length 为 0 | || 0x00401043 488b0d5e1020. mov rcx, qword [0x006020a8] ; [0x6020a8:8]=0 | || 0x0040104a 8b45fc mov eax, dword [local_4h] | || 0x0040104d 4863d0 movsxd rdx, eax | || 0x00401050 4889d0 mov rax, rdx | || 0x00401053 4801c0 add rax, rax ; '#' | || 0x00401056 4801d0 add rax, rdx ; '(' | || 0x00401059 48c1e003 shl rax, 3 | || 0x0040105d 4801c8 add rax, rcx ; '&' | || 0x00401060 4883c020 add rax, 0x20 | || 0x00401064 488b00 mov rax, qword [rax] | || 0x00401067 4889c7 mov rdi, rax ; rdi 为 Note 成员 content 指向的地址 | || 0x0040106a e841f6ffff call sym.imp.free ; void free(void *ptr) | || 0x0040106f bfa3124000 mov edi, str.Done. ; 0x4012a3 ; "Done." | || 0x00401074 e847f6ffff call sym.imp.puts ; int puts(const char *s) | ,===< 0x00401079 eb0a jmp 0x401085 | ||| ; JMP XREF from 0x00400f93 (sub.No_notes_yet._f7d) | ||`-> 0x0040107b bfd8124000 mov edi, str.No_notes_yet. ; 0x4012d8 ; "No notes yet." | || 0x00401080 e83bf6ffff call sym.imp.puts ; int puts(const char *s) | || ; JMP XREF from 0x00400fda (sub.No_notes_yet._f7d) | || ; JMP XREF from 0x00401079 (sub.No_notes_yet._f7d) | ``--> 0x00401085 c9 leave \ 0x00401086 c3 ret ``` 该函数在读入要删除的笔记序号后，首先将 Notes 结构体成员 `length -1`，然后将对应的 Note 结构体的 `isValid` 和 `length` 修改为 `0`，然后 free 掉笔记的内容（`*content`）。 ## Exploit 在上面逆向的过程中我们发现，程序存在 double free 漏洞。在 Delete 的时候，只是设置了 `isValid =0` 作为标记，而没有将该笔记从 Notes 中移除，也没有将 `content` 设置为 NULL，然后就调用了 free 函数。整个过程没有对 `isValid` 是否已经为 `0` 做任何检查。于是我们可以对同一个笔记 Delete 两次，造成 double free，修改 GOT 表，改变程序的执行流。 ## 参考资料 [0CTF 2015 Quals CTF: freenote](https://github.com/ctfs/write-ups-2015/tree/master/0ctf-2015/exploit/freenote)