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ssh_alert.c
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ssh_alert.c
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/*******************************************************************
* Project: SSH-Alert
* SSH-Alert detects new SSH connection
* attempts in real time and alerts the user.
*
* Author: Kfiros (Kfir Shtober)
* Year: 2015
*
* File: ssh_alert.c
* Description: This file contains the main logic for SSH connections
* monitoring.
*******************************************************************/
/*******************************************************************
* Includes
*******************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/user.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <errno.h>
#include <limits.h>
#include <signal.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include "includes/general.h"
#include "includes/ssh_alert.h"
#include "includes/logger.h"
pid_t g_sshd_pid = -1;
/*******************************************************************
* Name: get_sockaddr_rounded_size
* Description: This function calculates the size of the sockaddr
* struct in bytes, rounded upward (by int size).
*******************************************************************/
static int get_sockaddr_rounded_size() {
return sizeof(struct sockaddr) + \
(sizeof(int) - sizeof(struct sockaddr) % (sizeof(int)));
}
/*******************************************************************
* Name: read_proc_mem
* Description: This function reads remote process' memory, using
PTRACE_PEEKDATA. The read block size is sizeof(int).
*******************************************************************/
static int read_proc_mem(pid_t sshd_pid,
int * remote_addr,
int * read_mem,
size_t num_memb) {
int ret = SUCCESS;
unsigned int i = 0;
if (NULL == remote_addr) {
ret = ERR_SSH_REMOTE_ADDR_NULL_POINTER;
goto cleanup;
}
errno = 0;
/* Read sshd process' sockkadr struct from its memory */
for(i = 0; num_memb > i; ++i) {
read_mem[i] = ptrace(PTRACE_PEEKDATA,
sshd_pid, (int *) remote_addr + i,
NULL);
if (0 != errno) {
ret = ERR_SSH_PTRACE_PEEK_TRACEE_DATA;
goto cleanup;
}
}
cleanup:
return ret;
}
/*******************************************************************
* Name: hook_accept
* Description: This function examines the 'accept' syscall. Before
* the 'accept' syscall returns, it acquires its
* arguments.
*******************************************************************/
static int hook_accept(pid_t sshd_pid, struct user_regs_struct * uregs) {
int ret = SUCCESS;
int call_rv;
struct ssh_conn_entry conn = {0};
size_t read_mem_size;
int * read_mem = NULL;
/* Calculate the size of memory needed to be read from the sshd process */
read_mem_size = get_sockaddr_rounded_size();
read_mem = calloc(1, read_mem_size);
if (NULL == read_mem) {
ret = ERR_SSH_CALLOC_USER_MEM;
goto cleanup;
}
/* Syscall parameters convention (x86-64) - %rdi, %rsi, %rdx, %r10, %r8, %r9 */
/* accept(int sockfd, struct sockaddr * addr, socklen_t * addrlen) */
call_rv = read_proc_mem(sshd_pid,
(int * ) uregs->rsi,
read_mem,
read_mem_size / sizeof(int));
if (SUCCESS != call_rv) {
ret = call_rv;
goto cleanup;
}
/* Assign the required data in the ssh_conn_entry struct */
conn.client_ip = ((struct sockaddr_in *) read_mem)->sin_addr.s_addr;
conn.conn_time = time(NULL);
/* Log and notify about new ssh connection attempt */
LOGGER_log_and_notify(&conn);
cleanup:
if (NULL != read_mem) {
free(read_mem);
}
return ret;
}
/*******************************************************************
* Name: hook_connections
* Description: This function polls on syscalls and wait for the
* 'accept' syscall. When 'accept' syscall is revealed,
* it handles it accordingly.
*******************************************************************/
static int hook_connections(pid_t sshd_pid) {
int ret = SUCCESS;
int call_rv;
int status;
int sig = 0;
struct user_regs_struct uregs;
bool enter = true;
while (true) {
/* Intercept the next syscall */
call_rv = ptrace(PTRACE_SYSCALL, sshd_pid, 0, sig);
if (-1 == call_rv) {
ret = ERR_SSH_PTRACE_SYSCALL;
goto cleanup;
}
/* Wait for signals */
call_rv = waitpid(-1, &status, __WALL);
if (-1 == call_rv) {
ret = ERR_SSH_PTRACE_SYSCALL_WAIT;
goto cleanup;
}
/* In case child exited */
if (WIFEXITED(status)) {
ret = ERR_SSH_PTRACE_TRACEE_EXITED;
goto cleanup;
}
if (WIFSTOPPED(status)){
sig = WSTOPSIG(status);
/* Check for other signals except SIGTRAP */
if (SIGTRAP != sig) {
/* In this case - we need to signal the child process
and restart the syscall... */
continue;
} else {
sig = 0;
}
}
/* Get user registers */
call_rv = ptrace(PTRACE_GETREGS, sshd_pid, 0, &uregs);
if (-1 == call_rv) {
ret = ERR_SSH_PTRACE_SYSCALL_GETREGS;
goto cleanup;
}
/* Checking for an accept syscall */
if(SYS_accept == uregs.orig_rax) {
/* Intercept the function only when it returns */
if (true == enter) {
enter = false;
} else {
enter = true;
call_rv = hook_accept(sshd_pid, &uregs);
if (SUCCESS != call_rv) {
ret = call_rv;
goto cleanup;
}
}
}
}
cleanup:
return ret;
}
/*******************************************************************
* Name: obtain_sshd_pid
* Description: This function obtains the sshd PID from its pid file,
* usually located at /var/run/sshd.pid. Obviously there
* are other ways to accomplish this task, however, I find
* my way very hermetic.
*******************************************************************/
static int obtain_sshd_pid(pid_t * sshd_pid) {
int ret = SUCCESS;
long int converted_pid;
FILE * sshd_pid_fd = NULL;
char read_buff[SSHD_PID_FILE_MAX_SIZE];
char * endptr;
/* Open the sshd.pid file for reading */
sshd_pid_fd = fopen(SSHD_PID_FILE, "rb");
if (NULL == sshd_pid_fd) {
LOGGER_log("ERROR: Cannot open %s file \n", SSHD_PID_FILE);
ret = ERR_SSH_OPEN_PID_FILE;
goto cleanup;
}
/* Read from the requested file */
if (NULL == fgets(read_buff,
SSHD_PID_FILE_MAX_SIZE,
sshd_pid_fd)) {
LOGGER_log("ERROR: Cannot read from sshd.pid file \n");
ret = ERR_SSH_READ_PID_FILE;
goto cleanup;
}
/* Set errno to zero before converting the pid string */
errno = 0;
converted_pid = strtol(read_buff, &endptr, 10);
if ((ERANGE == errno && (LONG_MAX == converted_pid || LONG_MIN == converted_pid)) ||
(0 != errno && 0 == converted_pid) || (read_buff == endptr)) {
ret = ERR_SSH_PID_CONVERTION;
goto cleanup;
}
*sshd_pid = (pid_t) converted_pid;
cleanup:
if (NULL != sshd_pid_fd) {
fclose(sshd_pid_fd);
}
return ret;
}
/*******************************************************************
* Name: attach_remote_process
* Description: This function attaches to the remote sshd process.
* Actually, in this version of the program, there is
* no distinction between real sshd processes and other
* processes.
*******************************************************************/
static int attach_remote_process(pid_t sshd_pid) {
int ret = SUCCESS;
int call_rv;
int status;
LOGGER_log("[*] Attaching to process: %d\n", sshd_pid);
call_rv = ptrace(PTRACE_ATTACH, sshd_pid, 0, 0);
if (-1 == call_rv) {
ret = ERR_SSH_PTRACE_ATTACH;
goto cleanup;
}
/* Wait for signal status from the attached process */
call_rv = waitpid(-1, &status, __WALL);
if (-1 == call_rv) {
ret = ERR_SSH_PTRACE_ATTACH_WAIT;
goto cleanup;
}
cleanup:
return ret;
}
/*******************************************************************
* Name: SSH_ALERT_start
* Description: Main entry point of the ssh_alert module. This function
* attaches to the sshd process and starts monitoring it.
*******************************************************************/
int SSH_ALERT_start(pid_t arg_pid) {
int ret = SUCCESS;
int call_rv;
pid_t sshd_pid;
/* Obtain the pid of the running sshd deamon */
if (-1 != arg_pid) { /* As argument */
sshd_pid = arg_pid;
} else {
call_rv = obtain_sshd_pid(&sshd_pid); /* From pid file */
if (SUCCESS != call_rv) {
ret = call_rv;
goto cleanup;
}
}
g_sshd_pid = sshd_pid;
/* Attach to the remote process */
call_rv = attach_remote_process(sshd_pid);
if (SUCCESS != call_rv) {
LOGGER_log("ERROR: Couldn't attach to remote process \n");
ret = call_rv;
goto cleanup;
}
LOGGER_log("[*] Attached to process successfully \n");
/* Poll on syscalls and hook the `accept` syscall */
call_rv = hook_connections(sshd_pid);
if (SUCCESS != call_rv) {
LOGGER_log("ERROR: Critical error while tracing the SSH process \n");
ret = call_rv;
goto cleanup;
}
cleanup:
return ret;
}
/*******************************************************************
* Name: SSH_ALERT_stop
* Description: When SIGINT is raised, this function is called.
* It detaches from the SSHD process, in order to
* restore it to its original state.
*******************************************************************/
void SSH_ALERT_stop() {
int ret = SUCCESS;
int call_rv;
int status;
/* Check whether we already attached to the SSHD process */
if (-1 == g_sshd_pid) {
/* If not, exit */
goto cleanup;
}
LOGGER_log("\n[*] Detaching from remote sshd process \n");
/* First - we need to stop the remote SSHD process. Only afterward
* we are able to detach from the sshd process. */
call_rv = kill(g_sshd_pid, SIGSTOP);
if (SUCCESS != call_rv) {
ret = ERR_SSH_STOP_SSHD_PROCESS_SIG;
goto cleanup;
}
call_rv = waitpid(g_sshd_pid, &status, 0);
if (-1 == call_rv) {
ret = ERR_SSH_DETACH_WAIT;
goto cleanup;
}
if (!(WIFSTOPPED(status))) {
ret = ERR_SSH_WAIT_SSHD_STOP_SIG;
goto cleanup;
}
/* Detach */
call_rv = ptrace(PTRACE_DETACH, g_sshd_pid, 0, 0);
if (-1 == call_rv) {
ret = ERR_SSH_PTRACE_DETACH;
goto cleanup;
}
/* Finally, we continue the SSHD process with a SIGCONT signal */
call_rv = kill(g_sshd_pid, SIGCONT);
if (SUCCESS != call_rv) {
ret = ERR_SSH_DETACH_CONTINUE;
goto cleanup;
}
cleanup:
if (SUCCESS != ret) {
LOGGER_log("ERROR: Failed detaching from remote SSHD process.\n\
The SSHD daemon may left in an unstable state. \n");
} else {
LOGGER_log("[*] Detached successfully \n");
}
/* In any case, exit */
LOGGER_log("[*] Exiting \n");
exit(ret);
}