net: dsa: mxl862xx: fix use-after-free of DSA ports in crc_err_work

net: dsa: mxl862xx: fix use-after-free of DSA ports in crc_err_work

Upon an MDIO CRC error mxl862xx_crc_err_work_fn() walks the DSA ports
and closes the CPU port conduits:

	dsa_switch_for_each_cpu_port(dp, priv->ds)
		dev_close(dp->conduit);

mxl862xx_remove() unregisters the switch before cancelling this work:

	set_bit(MXL862XX_FLAG_WORK_STOPPED, &priv->flags);
	cancel_delayed_work_sync(&priv->stats_work);
	dsa_unregister_switch(ds);
	mxl862xx_host_shutdown(priv);

dsa_unregister_switch() frees the dsa_port objects. If a CRC error
schedules the work during teardown it can run after the ports have been
freed and dereference freed memory.

Guard the port walk with MXL862XX_FLAG_WORK_STOPPED, which is already set
before dsa_unregister_switch(). DSA tears the ports down under
rtnl_lock(), so checking the flag under rtnl_lock() means the work either
runs before teardown and sees valid ports, or runs afterwards, observes
the flag and skips the walk. This mirrors the host_flood_work handler,
which skips torn-down ports under rtnl_lock().

Link: https://sashiko.dev/#/patchset/cover.1780968180.git.daniel%40makrotopia.org?part=2
Fixes: a319d0c8c8ce ("net: dsa: mxl862xx: add CRC for MDIO communication")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/5e55169926c02f2b914e5ada529d7453b943cda4.1781702256.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>