cxl/port: Hold port reference until decoder release

[ Upstream commit 74be987 ]

KASAN + DEBUG_KOBJECT_RELEASE reports a potential use-after-free in
cxl_decoder_release() where it goes to reference its parent, a cxl_port,
to free its id back to port->decoder_ida.

 BUG: KASAN: use-after-free in to_cxl_port+0x18/0x90 [cxl_core]
 Read of size 8 at addr ffff888119270908 by task kworker/35:2/379

 CPU: 35 PID: 379 Comm: kworker/35:2 Tainted: G           OE     5.17.0-rc2+ torvalds#198
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
 Workqueue: events kobject_delayed_cleanup
 Call Trace:
  <TASK>
  dump_stack_lvl+0x59/0x73
  print_address_description.constprop.0+0x1f/0x150
  ? to_cxl_port+0x18/0x90 [cxl_core]
  kasan_report.cold+0x83/0xdf
  ? to_cxl_port+0x18/0x90 [cxl_core]
  to_cxl_port+0x18/0x90 [cxl_core]
  cxl_decoder_release+0x2a/0x60 [cxl_core]
  device_release+0x5f/0x100
  kobject_cleanup+0x80/0x1c0

The device core only guarantees parent lifetime until all children are
unregistered. If a child needs a parent to complete its ->release()
callback that child needs to hold a reference to extend the lifetime of
the parent.

Fixes: 40ba17a ("cxl/acpi: Introduce cxl_decoder objects")
Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Tested-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/164505751190.4175768.13324905271463416712.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/cxl/core/bus.c

@@ -182,6 +182,7 @@ static void cxl_decoder_release(struct device *dev)
 
 	ida_free(&port->decoder_ida, cxld->id);
 	kfree(cxld);
+	put_device(&port->dev);
 }
 
 static const struct device_type cxl_decoder_switch_type = {
@@ -481,6 +482,9 @@ cxl_decoder_alloc(struct cxl_port *port, int nr_targets, resource_size_t base,
 	if (rc < 0)
 		goto err;
 
+	/* need parent to stick around to release the id */
+	get_device(&port->dev);
+
 	*cxld = (struct cxl_decoder) {
 		.id = rc,
 		.range = {