[PATCH] cxl/port: Hold port reference until decoder release

[Dan Williams <dan.j.williams@intel.com>]

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+ #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: 40ba17afdfab ("cxl/acpi: Introduce cxl_decoder objects")
Reported-by: Ben Widawsky <ben.widawsky@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/cxl/core/port.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
index 0fc1441be014..c1681248f322 100644
--- a/drivers/cxl/core/port.c
+++ b/drivers/cxl/core/port.c
@@ -251,6 +251,7 @@ 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_endpoint_type = {
@@ -1202,7 +1203,10 @@ static struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port,
 	if (rc < 0)
 		goto err;
 
+	/* need parent to stick around to release the id */
+	get_device(&port->dev);
 	cxld->id = rc;
+
 	cxld->nr_targets = nr_targets;
 	seqlock_init(&cxld->target_lock);
 	dev = &cxld->dev;