Re: [PATCH] host/roots: Sandbox xdg-desktop-portal-spectrum-host

Demi Marie Obenour
2025-12-13T21:22:04Z

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On 12/13/25 14:12, Alyssa Ross wrote:
> Demi Marie Obenour <demiobenour@gmail.com> writes:
>
>> It is quite possible that these Landlock rules are unnecessarily
>> permissive, but all of the paths to which read and execute access is
>> granted are part of the root filesystem and therefore assumed to be
>> public knowledge. Removing access from any of them would only increase
>> the risk of accidental breakage in the future, and would not provide any
>> security improvements. seccomp *could* provide some improvements, but
>> the effort needed is too high for now.
>>
>> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
>> ---
>> .../template/data/service/xdg-desktop-portal-spectrum-host/run | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>
> Are you sure this is working as intended? There's no rule allowing
> access to Cloud Hypervisor's VSOCK socket, and yet it still seems to be
> able to access that. Don't you need to set a rule that *restricts*
> filesystem access and then add holes? Did you ever see this deny
> anything?

'man 1 setpriv' states that '--landlock-access fs' blocks all
filesystem access unless a subsequent --landlock-rule permits it.
I tried running with no --landlock-rule flags and the execve of
xdg-desktop-portal-spectrum-host failed as expected.

The socket is passed over stdin, and I'm pretty sure Landlock
doesn't restrict using an already-open file descriptor.
xdg-desktop-portal-spectrum-host does need to find the path to the
socket, but I don't think it ever accesses that path.

> And wouldn't it make more sense to implement this inside the program
> itself, since it's code we control that will only ever run on Spectrum?
> That way it could even drop privileges after they're no longer needed,
> perhaps. There's a nice landlock Rust crate that's already used by
> Cloud Hypervisor.

It definitely does make far more sense to use Landlock from within
the process itself. That should allow dropping all filesystem and
network access.

>> diff --git a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run
>> index d2bf78cefc3837b5d5369dbab819606e71bf1fc5..c3d67b6520d490c71bdce0f1056b2960115108b3 100755
>> --- a/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run
>> +++ b/host/rootfs/image/etc/s6-linux-init/run-image/service/vm-services/template/data/service/xdg-desktop-portal-spectrum-host/run
>> @@ -12,4 +12,12 @@ s6-ipcserver-socketbinder -a 0700 /run/vm/by-id/${VM}/vsock_219
>> if { fdmove 1 3 echo }
>> fdclose 3
>>
>> +unshare -inu --
>> +setpriv
>> + --landlock-access fs
>> + --landlock-rule path-beneath:read-file,execute:/nix/store
>> + --landlock-rule path-beneath:read-file,execute:/usr/bin
>> + --landlock-rule path-beneath:read-file,execute:/usr/lib
>> + --landlock-rule path-beneath:read-file:/run/vm/by-id/${VM}/portal-bus
>> + --
>> xdg-desktop-portal-spectrum-host
>>
>> ---
>> base-commit: 59cda41acc455513cf9936e99b8d97647955ac07
>> change-id: 20251212-sandbox-dbus-portal-4f98ba29c23a
>>
>> --
>> Sincerely,
>> Demi Marie Obenour (she/her/hers)

--
Sincerely,
Demi Marie Obenour (she/her/hers)

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