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

Demi Marie Obenour
2025-12-14T00:23:13Z

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On 12/13/25 16:42, Alyssa Ross wrote:
> Demi Marie Obenour <demiobenour@gmail.com> writes:
>
>> 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.
>
> I've been looking into this a bit myself, and from what I can tell
> Landlock just doesn't restrict connecting to sockets at all, even if
> they're inside directories that would otherwise be inaccessible. It's
> able to connect to both Cloud Hypervisor's VSOCK socket and the D-Bus
> socket even with a maximally restrictive landlock rule. So you were
> right after all, sorry!

That's not good at all! It's a trivial sandbox escape in so many cases.
For instance, with access to D-Bus I can just call `systemd-run`.

I'm CCing the Landlock and LSM mailing lists because if you are
correct, then this is a bad security hole.

> I will still go ahead with doing this in the program though, since I
> already got that far.

Would it make sense to connect to the sockets and then block connect()
and friends using seccomp?
--
Sincerely,
Demi Marie Obenour (she/her/hers)

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