Enable a secure compute environment in a jail that blocks certain syscalls.
Because of the dependence on seccomp, this project is Linux specific, and it will not build on other operating systems (not even other UNIX like systems).
First, ensure that you have the following dependencies installed:
Ubuntu/Debian
sudo apt install libseccomp-dev
Fedora/CentOS/Red hat
sudo dnf install libseccomp-devel
To compile capejail, type the following into a terminal:
make
After experimentation, I've found that existing jail programs such nsjail and firejail do not work inside of AWS nitro enclaves. This jail implementation was built specifically for safeguarding Cape from user code when running inside of an enclave.
The security is multilayer.
- User code is run in a
chrootso that it cannot access files outside of its designated jail. - User code is run as a different user with locked down permissions.
- User code is run with seccomp filters set to block harmful syscalls that can be used to escape the jail.
- User code is run in a separate PID namespace, so it is not able to see or interact with any other processes on the system, other than its own child processes.
- Optionally, the jail can unshare the networking namespace, which removes the jailed process's ability to perform any networking.
The chroot environment can be customized to the level of security desired.
For example, by restricting access to /proc and certain devices in /dev/
AND running the jail as a non-root user, you can keep the jailed user from
tampering with other processes and devices.
For a comprehensive list of why certain syscalls are blocked, please see SYSCALLS.md.
$ capejail -h
capejail: enable a secure compute environment in a jail that blocks certain syscalls
usage:
capejail [OPTION] -- PROGRAM [ARGS]
-e add an environment variable in the form of KEY=VALUE
example: capejail -e HOME=/home/jailuser -e FOO=bar -- ...
-h display this help message
-n disable networking for the jailed process
-d directory to start in within jail
-r path to chroot directory to use in jail
-u user to run as within the jail
-I insecure mode, launch with seccomp disabled
NOTE: should be run as root or with sudo to allow chroot
First, create a chroot directory. One way to do this is to run a docker container, and mount a volume to it where you will install the system to.
docker run --rm -it -v ~/chroot:/chroot debian
Next, within the docker container, copy the necessary directories into the chroot:
mkdir /chroot
cp -r /bin /chroot/
cp -r /sbin /chroot/
cp -r /usr /chroot/
cp -r /etc /chroot/
rm -f /chroot/etc/bash.bashrc
cp -r /lib /chroot/
cp -r /lib64 /chroot/
mkdir /chroot/dev
mknod -m 666 /chroot/dev/null c 1 3
mknod -m 666 /chroot/dev/zero c 1 5
mknod -m 666 /chroot/dev/random c 1 8
mknod -m 666 /chroot/dev/urandom c 1 9
After exiting the docker container, create a new user on the host system for running as within the jail.
adduser jailuser
And now you'll have a chroot environment setup at ~/chroot. At this point,
you're ready to run capejail.
(py310) [kyle@fedora ~]$ grep PRETTY_NAME /etc/os-release
PRETTY_NAME="Fedora Linux 36 (Workstation Edition)"
(py310) [kyle@fedora ~]$ sudo capejail -r ~/chroot -u jailuser -- bash
[jail]$ grep PRETTY_NAME /etc/os-release
PRETTY_NAME="Debian GNU/Linux 11 (bullseye)"
[jail]$ echo $USER
jailuser
[jail]$ pwd
/
[jail]$ ls -l
total 8
drwxr-xr-x 1 root root 730 Jun 2 20:15 bin
drwxr-xr-x 1 root root 14 Jun 2 18:52 dev
drwxr-xr-x 1 root root 1274 Jun 2 20:15 etc
drwxr-xr-x 1 root root 84 Jun 2 20:15 lib
drwxr-xr-x 1 root root 40 May 24 20:28 lib64
drwxr-xr-x 1 root root 1008 Jun 2 20:15 sbin
drwxr-xr-x 1 root root 0 Jun 2 19:58 tmp
drwxr-xr-x 1 root root 84 May 24 20:28 usr
drwxr-xr-x 1 root root 90 Jun 2 14:27 var
[jail]$ chroot /tmp # for security, this is supposed to fail
Bad system call (core dumped)
[jail]$ exit
exit
(py310) [kyle@fedora ~]$
(py310) [kyle@fedora ~]$ # Note, the '-I' flag is used here because ping will
(py310) [kyle@fedora ~]$ # be killed by an illegal syscall before it has a
(py310) [kyle@fedora ~]$ # chance to be blocked by unsharing the network namespace.
(py310) [kyle@fedora ~]$ # DON'T USE '-I' IN PRODUCTION!
(py310) [kyle@fedora ~]$ sudo capejail -u jailuser -I -n -r /home/kyle/chroot-ping/ -- bash
[jail]$ ping google.com # with the '-n' flag, network calls are not allowed
ping: socket: Operation not permittedFirst step for allowing a new syscall, is discovering which syscall you need.
Let's look at an example that uses ping. We see in this example that we get
the message Bad system call (core dumped) when using the ping program.
(py310) [kyle@fedora capejail]$ sudo capejail -- bash
[jail]# ping google.com
Bad system call (core dumped)
We can use strace to monitor which systemcalls are being used by this program.
The -f flag will follow forks, which is necessary because capejail forks the
user program into a new process. -c will show a summary of all of the syscalls
made.
(py310) [kyle@fedora capejail]$ strace -f -c ping -c 1 google.com
PING google.com (172.217.4.78) 56(84) bytes of data.
64 bytes from lga15s47-in-f78.1e100.net (172.217.4.78): icmp_seq=1 ttl=119 time=12.4 ms
--- google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 12.443/12.443/12.443/0.000 ms
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
16.08 0.000770 15 49 mmap
12.49 0.000598 598 1 execve
11.34 0.000543 19 28 7 openat
7.64 0.000366 12 29 newfstatat
6.10 0.000292 9 31 close
5.97 0.000286 17 16 recvmsg
5.68 0.000272 27 10 socket
4.28 0.000205 9 21 read
3.95 0.000189 12 15 mprotect
3.74 0.000179 29 6 sendto
2.53 0.000121 8 14 setsockopt
2.21 0.000106 17 6 write
2.09 0.000100 10 10 pread64
1.75 0.000084 16 5 1 connect
1.36 0.000065 10 6 ppoll
1.25 0.000060 7 8 4 prctl
1.11 0.000053 7 7 capget
1.00 0.000048 9 5 getsockopt
0.88 0.000042 8 5 getsockname
0.86 0.000041 10 4 brk
0.84 0.000040 4 9 rt_sigprocmask
0.75 0.000036 7 5 munmap
0.67 0.000032 10 3 bind
0.58 0.000028 9 3 rt_sigaction
0.52 0.000025 6 4 2 recvfrom
0.48 0.000023 11 2 lseek
0.46 0.000022 5 4 futex
0.42 0.000020 10 2 ioctl
0.42 0.000020 20 1 1 access
0.42 0.000020 10 2 getrandom
0.31 0.000015 7 2 1 arch_prctl
0.29 0.000014 7 2 getuid
0.27 0.000013 13 1 capset
0.27 0.000013 13 1 rseq
0.21 0.000010 10 1 setuid
0.19 0.000009 9 1 setitimer
0.17 0.000008 8 1 set_tid_address
0.15 0.000007 7 1 getpid
0.15 0.000007 7 1 set_robust_list
0.15 0.000007 7 1 prlimit64
0.00 0.000000 0 1 uname
------ ----------- ----------- --------- --------- ----------------
100.00 0.004789 14 324 16 total
I put together a small bash function that will search the list of syscalls and find which ones are missing from the allow list from capejail.
function search_syscalls()
{
for item in $(cat /tmp/syscalls);
do
grep $item seccomp.c > /dev/null;
if [[ $? != 0 ]]; then
echo $item
fi
done
}
To use this function, place each of the syscalls that strace detected into the
text file /tmp/syscalls like so. Please note that each line only contains the
exact name of the syscall as output by strace.
(py310) [kyle@fedora capejail]$ cat /tmp/syscalls
mmap
openat
execve
newfstatat
close
mprotect
read
socket
recvmsg
setsockopt
sendto
connect
munmap
write
pread64
rt_sigprocmask
prctl
ppoll
capget
getsockopt
access
brk
getsockname
rt_sigaction
bind
setitimer
recvfrom
ioctl
getrandom
lseek
futex
getuid
arch_prctl
capset
setuid
getpid
uname
prlimit64
rseq
set_tid_address
set_robust_list
Now we can call our search_syscalls function to search which syscalls we are
missing. We can see below that we are missing 3 syscalls.
(py310) [kyle@fedora capejail]$ search_syscalls
setsockopt
capset
setuid
If we want to allow these syscalls (which in actuality, we do not as these can
be used to exploit the system) then we can add them to the allow list found in
seccomp.c.
By using the SCMP_SYS macro, we can then add these syscalls to the allow list
as follows:
diff --git a/seccomp.c b/seccomp.c
index 6a62dc4..8051406 100644
--- a/seccomp.c
+++ b/seccomp.c
@@ -6,6 +6,9 @@
#include "seccomp.h"
static const int ALLOWED_SYSCALLS[] = {
+ SCMP_SYS(setsockopt),
+ SCMP_SYS(capset),
+ SCMP_SYS(setuid),
/*
* Check user's permissions for a file.
*/We will now recompile:
make
Finally, we will run capejail again, and see that ping is no longer being killed by a bad syscall:
(py310) [kyle@fedora capejail]$ sudo ./capejail -- ping -c 3 google.com
PING google.com (172.217.4.78) 56(84) bytes of data.
64 bytes from lga15s47-in-f78.1e100.net (172.217.4.78): icmp_seq=1 ttl=119 time=12.9 ms
64 bytes from ord37s18-in-f14.1e100.net (172.217.4.78): icmp_seq=2 ttl=119 time=11.8 ms
64 bytes from ord37s18-in-f14.1e100.net (172.217.4.78): icmp_seq=3 ttl=119 time=11.6 ms
--- google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2004ms
rtt min/avg/max/mdev = 11.600/12.080/12.854/0.552 ms
./capejail: 'ping' exited with status 0
./capejail: shutting down
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