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mkbox: an experiment in nonprivileged sandboxing in Linux
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Requires kernel 3.12 or newer with CONFIG_USER_NS.

Tested on Ubuntu 14.04 LTS / Linux 3.13.7.

Huge thanks to Andy Lutomirski, who provided the roadmap in the form of
a patch to Sandstorm (sandstorm-io/sandstorm#12) as
well as kindly assisting with some early debugging as I fell into various
pits he had previously explored.

Disclaimer:  This is a toy built to learn about these APIs and is
certainly incomplete, likely buggy, etc.  Use at your own risk.

"make test" will build mkbox and create sandbox (which will be r/o /),
and databox (which will be r/w /data), and copy /bin/busybox into
sandbox/bin and create busybox's symlinks in there as well, then
invoke: mkbox sandbox `pwd`/databox 

computer$ id
uid=1000(somebody) gid=1000(somebody) groups=1000(somebody)

computer$ make test
cc -Wall -O1 -g -o mkbox mkbox.c
mkdir -p sandbox databox sandbox/bin
cp /bin/busybox sandbox/bin
chmod 755 sandbox/bin/busybox
( cd sandbox/bin && for x in $(busybox --list) ; do ln -fs busybox $x ; done )
./mkbox sandbox `pwd`/databox
mkbox: pid=14259, child=14260


BusyBox v1.21.1 (Ubuntu 1:1.21.0-1ubuntu1) built-in shell (ash)
Enter 'help' for a list of built-in commands.

/ $ id
uid=3333 gid=3333 groups=65534,3333

/ $ ls -l
drwxrwxr-x    2 3333     3333          4096 Apr 27 04:34 bin
drwxrwxr-x    2 3333     3333          4096 Apr 27 04:33 data
drwxr-xr-x    2 3333     3333            80 Apr 27 04:34 dev


/* in another shell */

computer$ cat /proc/14259/mounts
rootfs / rootfs rw 0 0
/dev/root / ext3 ro,nosuid,noatime,errors=remount-ro,barrier=0,data=writeback 0 0
/dev/root /data ext3 rw,noatime,errors=remount-ro,barrier=0,data=writeback 0 0
sandbox-dev /dev tmpfs ro,nosuid,noexec,noatime,size=64k,nr_inodes=16,mode=755,uid=1000,gid=1000 0 0
devtmpfs /dev/null devtmpfs rw,relatime,size=1019296k,nr_inodes=254824,mode=755 0 0
devtmpfs /dev/zero devtmpfs rw,relatime,size=1019296k,nr_inodes=254824,mode=755 0 0