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logind: make ReleaseSession "unprivileged" and allow closing of own session

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Fixes https://github.com/systemd/systemd/issues/28514.

Quoting https://github.com/systemd/systemd/issues/28514#issuecomment-1831781486:
> Whenever PAM is enabled for a service, we set up the PAM session and then
> fork off a process whose only job is to eventually close the PAM session when
> the service dies. That services we run with service privileges, both to
> minimize attack surface and because we want to use PR_SET_DEATHSIG to be get
> a notification via signal whenever the main process dies. But that only works
> if we have the same credentials as that main process.
>
> Now, if pam_systemd runs inside the PAM stack (which it normally does) it's
> session close hook will ask logind to synchronously end the session via a bus
> call. Currently that call is not accessible to unprivileged clients. And
> that's the part we need to relax: allow users to end their own sessions.

The check is implemented in a way that allows the kill if the sender is in
the target session.

I found 'sudo systemctl --user -M "zbyszek@" is-system-running' to
be a convenient reproducer.

Before:
May 16 16:25:26 x1c systemd[1]: run-u24754.service: Deactivated successfully.
May 16 16:25:26 x1c dbus-broker[1489]: A security policy denied :1.24757 to send method call /org/freedesktop/login1:org.freedesktop.login1.Manager.ReleaseSession to org.freedesktop.login1.
May 16 16:25:26 x1c (sd-pam)[3036470]: pam_systemd(login:session): Failed to release session: Access denied
May 16 16:25:26 x1c systemd[1]: Stopping session-114.scope...
May 16 16:25:26 x1c systemd[1]: session-114.scope: Deactivated successfully.
May 16 16:25:26 x1c systemd[1]: Stopped session-114.scope.
May 16 16:25:26 x1c systemd[1]: session-c151.scope: Deactivated successfully.
May 16 16:25:26 x1c systemd-logind[1513]: Session c151 logged out. Waiting for processes to exit.
May 16 16:25:26 x1c systemd-logind[1513]: Removed session c151.
After:
May 16 17:02:15 x1c systemd[1]: run-u24770.service: Deactivated successfully.
May 16 17:02:15 x1c systemd[1]: Stopping session-115.scope...
May 16 17:02:15 x1c systemd[1]: session-c153.scope: Deactivated successfully.
May 16 17:02:15 x1c systemd[1]: session-115.scope: Deactivated successfully.
May 16 17:02:15 x1c systemd[1]: Stopped session-115.scope.
May 16 17:02:15 x1c systemd-logind[1513]: Session c153 logged out. Waiting for processes to exit.
May 16 17:02:15 x1c systemd-logind[1513]: Removed session c153.

Edit: this seems to also fix https://github.com/systemd/systemd/issues/8598.
It seems that with the call to ReleaseSession, we wait for the pam session
close hooks to finish. I inserted a 'sleep(10)' after the call to ReleaseSession
in pam_systemd, and things block on that, nothing is killed prematurely.

(cherry picked from commit fc0bb7ccc7)

Resolves https://bugzilla.redhat.com/show_bug.cgi?id=2221337.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2024-05-16 17:06:24 +02:00 committed by Luca Boccassi
parent 092f6b60ce
commit 97ad9a336a
3 changed files with 13 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
man/org.freedesktop.login1.xml
View File

@ -103,7 +103,6 @@ node /org/freedesktop/login1 {
out s seat_id,
out u vtnr,
out b existing);
@org.freedesktop.systemd1.Privileged("true")
ReleaseSession(in s session_id);
ActivateSession(in s session_id);
ActivateSessionOnSeat(in s session_id,

11
src/login/logind-dbus.c
View File

@ -1116,7 +1116,7 @@ static int method_create_session_pidfd(sd_bus_message *message, void *userdata,
static int method_release_session(sd_bus_message *message, void *userdata, sd_bus_error *error) {
Manager *m = ASSERT_PTR(userdata);
Session *session;
Session *session, *sender_session;
const char *name;
int r;
@ -1130,6 +1130,13 @@ static int method_release_session(sd_bus_message *message, void *userdata, sd_bu
if (r < 0)
return r;
r = get_sender_session(m, message, /* consult_display= */ false, error, &sender_session);
if (r < 0)
return r;
if (session != sender_session)
return sd_bus_error_set(error, BUS_ERROR_NOT_IN_CONTROL, "You are not in control of this session");
r = session_release(session);
if (r < 0)
return r;
@ -3666,7 +3673,7 @@ static const sd_bus_vtable manager_vtable[] = {
SD_BUS_ARGS("s", session_id),
SD_BUS_NO_RESULT,
method_release_session,
0),
SD_BUS_VTABLE_UNPRIVILEGED),
SD_BUS_METHOD_WITH_ARGS("ActivateSession",
SD_BUS_ARGS("s", session_id),
SD_BUS_NO_RESULT,

4
src/login/org.freedesktop.login1.conf
View File

@ -262,6 +262,10 @@
send_interface="org.freedesktop.login1.Manager"
send_member="FlushDevices"/>
<allow send_destination="org.freedesktop.login1"
send_interface="org.freedesktop.login1.Manager"
send_member="ReleaseSession"/>
<allow send_destination="org.freedesktop.login1"
send_interface="org.freedesktop.login1.Seat"
send_member="Terminate"/>