hongjinghao authored Jun 05, 2023 and Simon McVittie committed Jun 06, 2023

Normally, it's enough to rely on a message being given a serial number
by the DBusConnection just before it is actually sent. However, in the
rare case where the policy blocks the driver from sending a message
(due to a deny rule or the outgoing message quota being full), we need
to get a valid serial number sooner, so that we can copy it into the
DBUS_HEADER_FIELD_REPLY_SERIAL field (which is mandatory) in the error
message sent to monitors. Otherwise, the dbus-daemon will crash with
an assertion failure if at least one Monitoring client is attached,
because zero is not a valid serial number to copy.

This fixes a denial-of-service vulnerability: if a privileged user is
monitoring the well-known system bus using a Monitoring client like
dbus-monitor or `busctl monitor`, then an unprivileged user can cause
denial-of-service by triggering this crash. A mitigation for this
vulnerability is to avoid attaching Monitoring clients to the system
bus when t...