Re: [Discussion] systemd-run0 policy issue

Stephen Smalley <[email protected]> writes:

> On Thu, Aug 13, 2026 at 9:11 AM Jan Janasek <[email protected]> wrote:
>>
>> Hi everyone,
>>
>> (I originally posted this to the fedora selinux list, but was advised
>> that this is the proper ML for such architectural discussions.)
>>
>> We’ve been analyzing the integration of systemd-run0 with SELinux and
>> hit a fundamental architectural roadblock, related to bug
>> https://bugzilla.redhat.com/show_bug.cgi?id=2359828. Unlike sudo,
>> where the parent process is the user's shell (allowing PAM to easily
>> calculate and transition the context), run0 delegates execution via
>> D-Bus to PID 1 (init_t). This breaks standard SELinux PAM transitions
>> and creates a severe design challenge for maintaining user isolation.
>> (Currently, wrapping commands via run0 --via-shell acts as a temporary
>> workaround, but we are looking for a robust architectural fix.)
>>
>> When pam_selinux.so attempts to transition the user directly into
>> their target domain over a specific binary, it immediately triggers
>> strict policy conflicts. To demonstrate the exact mechanics of what
>> happens when PAM forces this transition, I wrote a Python reproducer
>> (labeled as bin_t and running in the unconfined_t domain) that uses
>> setexeccon to simulate a transition (e.g., to staff_t) and then calls
>> os.execl("/usr/bin/dnf5", ...). It triggers these distinct SELinux
>> denials:
>>
>> # 1. The Transition Denial: The kernel blocks the domain transition
>> type=AVC msg=audit(07/23/2026 09:23:00.584:636) : avc:  denied  {
>> transition } for  pid=4945 comm=python3 path=/usr/bin/dnf5
>> dev="nvme0n1p2" ino=146474
>> scontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
>> tcontext=staff_u:staff_r:staff_t:s0-s0:c0.c1023 tclass=process
>> permissive=1
>>
>> # 2. The Entrypoint Denial: The target domain lacks entrypoint
>> permissions on the binary
>> type=AVC msg=audit(07/23/2026 09:23:00.584:637) : avc:  denied  {
>> entrypoint } for  pid=4945 comm=python3 path=/usr/bin/dnf5
>> dev="nvme0n1p2" ino=146474
>> scontext=staff_u:staff_r:staff_t:s0-s0:c0.c1023
>> tcontext=system_u:object_r:rpm_exec_t:s0 tclass=file permissive=1
>>
>> Fixing this purely in the Base Policy would require writing transition
>> and entrypoint rules for every single interactive tool a user might
>> run. This is unmaintainable, massively expands the attack surface, and
>> breaks the principle of least privilege.
>>
>> We explored a few potential ways to handle this and would like the
>> community's input:
>>
>> 1. SELinux-aware run0 (Upstream fix). Modify run0/systemd to be
>> natively SELinux-aware. Instead of relying blindly on pam_selinux.so
>> (which only sees init_t as the caller), run0 could dynamically
>> calculate the correct target context based on the originating user's
>> session and explicitly set it for the execution (e.g., via the
>> SELinuxContext= property of the transient unit).
>> 2. Leave pam_selinux.so commented out in /lib/pam.d/systemd-run0. If
>> we leave pam_selinux.so disabled in the run0 PAM stack, processes
>> simply inherit init_t. This means we would have to write static
>> type_transition rules into the base policy for every single
>> interactive binary users might run. (For example, run0 dnf only works
>> right now because an init_t -> rpm_exec_t transition historically
>> exists).
>> 3. Enable PAM + Explicit Default Contexts (A Flawed PoC). We tried
>> keeping pam_selinux.so enabled and explicitly defining the transition
>> path in /etc/selinux/targeted/contexts/users/unconfined_u:
>> system_r:init_t:s0 unconfined_r:systemd_run_t:s0
>> unconfined_r:unconfined_t:s0 While this correctly routes run0 payloads
>> into an isolated domain (systemd_run_t) without polluting init_t, it
>> causes unacceptable collateral damage. The SELinux get_default_context
>> API only sees the caller (init_t) and the user (unconfined_u). Because
>> run0 sessions and systemd --user sessions share these exact same
>> inputs, this global mapping breaks systemd --user instances by
>> throwing them into systemd_run_t instead of unconfined_t.
>>
>> Given that we cannot safely rely on global policy rules (which expands
>> PID 1's attack surface) or PAM default contexts (which breaks user
>> sessions), we'd love to hear your thoughts. Are there other approaches
>> or configuration options we might have missed? Or is pushing for run0
>> to become natively SELinux-aware the most viable path forward?
>>
>> Any thoughts or guidance would be greatly appreciated.
>
> Option 1 seems best to me, but this issue might also be related:
> https://github.com/SELinuxProject/selinux/issues/485

Disagree because this functionality is not that much more than some
abstraction/wrapper around systemd's PAMName= functionality. In my view it would
be more robust to make any solution to this issue also work for the
underlying PAMName= technology it is built on.

>

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Dominick Grift
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