bubblewrap: Add --not-a-security-boundary flag to enable fail-open behavior

[ao2]

Some callers of bwrap (e.g. xdg-dbus-proxy, Steam Runtime) use it purely to adjust filesystem layout, without any expectation of a security boundary between the sandbox and the host.

For these callers, hard failures during sandbox setup (such as an automount timeout on a bind source) are unnecessarily fatal.

So add a new --not-a-security-boundary option that can be used to relax the bubblewrap behavior in these specific cases, and allow it to "fail-open".

[ao2]

cc @smcv even though it's still a draft

[smcv]

I'd like some smoke-test coverage for this in the test suite. Setting up a failing automounter (so that there will genuinely be an error to ignore) is probably too difficult to do in bubblewrap's test suite, but we can at least exercise the happy path where the new option makes no practical difference.

[smcv]

Why does only --bind fail open? If bubblewrap is not a security boundary, shouldn't --dev-bind and --ro-bind be equally willing to fail open?

Separately, I think we're getting enough flags that the repeated ?: operators make it harder to read, and it would be better more like:

bind_option_t bind_flags = 0;

if (opt_not_a_security_boundary)
  bind_flags |= BIND_FAIL_OPEN;

if (op->type == SETUP_RO_BIND_MOUNT)
  bind_flags |= BIND_READONLY;

if (op->type == SETUP_DEV_BIND_MOUNT)
  bind_flags |= BIND_DEVICES;

/* etc. */

setup_op_bind_mount (bind_flags, source, dest);

[smcv]

The commit message should have a Resolves: https://github.com/containers/bubblewrap/issues/653, and optionally also:

• Helps: https://github.com/flatpak/flatpak/issues/5112
• Helps: https://github.com/ValveSoftware/steam-for-linux/issues/10571
• Helps: https://github.com/ValveSoftware/steam-runtime/issues/766

Optionally also a mention of steamrt/tasks#535.

[smcv]

For manual testing, I see you've described how to reproduce the problem in ValveSoftware/steam-for-linux#10571 (comment) (it might be useful to summarize that in the commit message). It would be useful if you could create other mount points - perhaps /mnt/bad for the broken automount, and /mnt/good for a loopback mount that is working correctly - and then inspect /proc/self/mountinfo inside the container to see what happens after the error is ignored.

What I expect will happen is that /mnt/good will be remounted nodev, but the broken automount at /mnt/bad might lack the nodev option.

Similarly, if you request mounting them read-only, something like --ro-bind /mnt /mnt, I expect that /mnt/good will be remounted ro, but /mnt/bad will still be rw, which is why we need to fail closed for the default "yes, security boundary" use-case.

But it would be good to confirm this.

[smcv]

Perhaps worthwhile to say something like

Suggested change

	rather than causing <command>bwrap</command> to exit with an error.
	rather than causing <command>bwrap</command> to exit with an error.
	The effect of this option might be extended to make other sandbox
	setup operations non-fatal in future releases of bubblewrap.

to give us space to make more operations fail open if we find that it's desirable.

[ao2]

Some steps to manually test the change.

0. Prepare a loop device:

   $ dd if=/dev/zero of=good.bin bs=1 count=1M
   $ sudo mkfs.ext4 good.bin 
   $ sudo modprobe loop 
   $ sudo losetup loop0 good.bin 
   $ export LOOP_UUID="$(lsblk -n -o uuid /dev/loop0)"
   

1. Add an available and an unavailable automount:

   $ sudo -E sh -c 'echo "UUID=${LOOP_UUID} /mnt/good  ext4 defaults,noatime,nofail,inode_readahead_blks=64,nobarrier,x-systemd.automount    0  1" >> /etc/fstab'
   $ sudo sh -c 'echo "UUID=00000000-0000-0000-0000-000000000000 /mnt/bad  ext4 defaults,noatime,nofail,inode_readahead_blks=64,nobarrier,x-systemd.automount    0  1" >> /etc/fstab'
   $ sudo systemctl daemon-reload
   $ sudo systemctl restart mnt-bad.automount 
   $ sudo systemctl restart mnt-good.automount 
   

2. Check mount options on the host:

   $ mount | grep /mnt
   systemd-1 on /mnt/bad type autofs (rw,relatime,fd=76,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=15854)
   systemd-1 on /mnt/good type autofs (rw,relatime,fd=77,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=15857)
   

3. Check that mount options inside the container are not applied to unavailable automounts when passing --not-a-security-boundary, while automounts are being accessed:

   $ ls /mnt/good &>/dev/null &
   $ ls /mnt/bad &>/dev/null &
   $ ./_build/bwrap --not-a-security-boundary --bind / / grep /mnt /proc/self/mountinfo
   bwrap: Can't remount /newroot/mnt/bad submount (No such device), ignoring error
   860 769 0:41 / /mnt/bad rw,relatime master:45 - autofs systemd-1 rw,fd=107,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=144522
   861 769 0:66 / /mnt/good rw,relatime master:576 - autofs systemd-1 rw,fd=109,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=145727
   863 861 7:0 / /mnt/good rw,nosuid,nodev,noatime master:599 - ext4 /dev/loop0 rw,nobarrier,inode_readahead_blks=64
   

   We can see that nosuid,nodev are applied to the /mnt/good actually autmounted from /dev/loop0 but not to the inactive automounts.

4. Check that ro option is not applied when using --ro-bind on an unavailable automout that is accessed:

   In order to test the behavior with --ro-bind in a representative way we have
   to prevent /mnt/bad to be remounted twice, otherwise we may loose the
   reproducibility window related to the unavailable mount point being accessed at the same time when bubblewrap tries to remount it.

   Remember that if the unavailable automount is inactive (i.e. is not being accesses when bubblerwap runs) then systemd will just accept the new flags even without --not-a-security-boundary.

   We can achieve the desired behavior by using an alternative rootfs for /, which does not include /mnt.

   $ sudo debootstrap trixie test-rootfs
   $ ls /mnt/good &>/dev/null &
   $ ls /mnt/bad &>/dev/null &
   $ ./_build/bwrap --not-a-security-boundary --bind ./test-rootfs / --bind /proc /proc  --dev-bind /dev /dev --ro-bind /mnt /mnt grep /mnt /proc/self/mountinfo
   bwrap: Can't remount /newroot/mnt/bad submount (No such device), ignoring error
   742 725 254:1 /mnt /mnt ro,nosuid,nodev,relatime master:1 - ext4 /dev/mapper/Z13--vg-root rw,errors=remount-ro
   743 742 0:40 / /mnt/bad rw,relatime master:43 - autofs systemd-1 rw,fd=76,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=15854
   744 742 0:41 / /mnt/good rw,relatime master:45 - autofs systemd-1 rw,fd=77,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=15857
   745 744 7:0 / /mnt/good ro,nosuid,nodev,noatime master:568 - ext4 /dev/loop0 rw,nobarrier,inode_readahead_blks=64
   

   Where we can see that the ro option is not applied to /mnt/bad but is applied to the active /mnt/good.

[ao2]

I'll try to condense the comment above about the manual tests into something suitable for the commit message.