In the Linux kernel, the following vulnerability has been resolved: scsi: target: Disable interrupts while holding delayed_cmd_lock target

In the Linux kernel, the following vulnerability has been resolved:

scsi: target: Disable interrupts while holding delayed_cmd_lock

target_do_delayed_work() takes dev->delayed_cmd_lock with a plain
spin_lock(). It runs from system_percpu_wq, so interrupts stay enabled
while the lock is held. Every other acquisition of that lock uses
spin_lock_irqsave(): target_handle_task_attr() and
transport_complete_ordered_sync() in target_core_transport.c, and
target_non_ordered_release() in target_core_device.c.

That was safe while every caller of transport_complete_task_attr() ran
in process context: target_complete_ok_work() from target_completion_wq,
transport_complete_qf() from dev->qf_work_queue, and
transport_generic_request_failure() from the submit and execute paths.

Since commit 06933066d88a ("scsi: target: Add support for completing
commands from backend context") that is no longer true. With
complete_type=1 on a fabric that sets direct_compl_supp,
target_complete() runs target_complete_ok_work() inline in the backend's
completion context, which target_complete_cmd_with_sense() documents as
"May be called from interrupt context". For iblock that is the bio
end_io.

target_complete_ok_work() starts with transport_complete_task_attr(),
which takes delayed_cmd_lock in two ways:

• Every ORDERED command is put on dev->delayed_cmd_list by
  target_handle_task_attr() and dispatched from there by
  target_do_delayed_work(), which marks it SCF_TASK_ORDERED_SYNC. Its
  completion therefore goes through transport_complete_ordered_sync().

• A SIMPLE command that completes while an ORDERED command is pending
  drops the last reference to the killed dev->non_ordered, and
  percpu_ref_put() then runs target_non_ordered_release() in the
  completing context.

Both take the lock and, if commands are waiting, call
schedule_work(&dev->delayed_cmd_work), which queues on system_percpu_wq,
i.e. on the CPU that took the completion. Further completions for the
same queue are typically delivered there too, so one arriving while the
work holds the lock spins on it forever.

Reaching this needs complete_type=1, which is not the default
(vhost-scsi, the only fabric with direct_compl_supp, defaults to
TARGET_QUEUE_COMPL), a backend whose I/O completes from interrupt
context, and ORDERED commands from the initiator. Linux virtio_scsi
guests send only VIRTIO_SCSI_S_SIMPLE and never take the ordered path;
any guest that uses the ORDERED task attribute, which vhost-scsi
accepts, does.

target_do_delayed_work() is a work item and always runs in process
context, so spin_lock_irq() is sufficient.

Found by inspection; no runtime report. A single ORDERED command goes
through both acquisitions, so with CONFIG_PROVE_LOCKING the first one
completed through direct completion should produce an inconsistent lock
state report on delayed_cmd_lock (hardirq or softirq, depending on where
the backend completes).