Medium · published May 21, 2024
In the Linux kernel, the following vulnerability has been resolved:
cxl/port: Fix delete_endpoint() vs parent unregistration race
The CXL subsystem, at cxl_mem ->probe() time, establishes a lineage of
ports (struct cxl_port objects) between an endpoint and the root of a
CXL topology. Each port including the endpoint port is attached to the
cxl_port driver.
Given that setup, it follows that when either any port in that lineage
goes through a cxl_port ->remove() event, or the memdev goes through a
cxl_mem ->remove() event. The hierarchy below the removed port, or the
entire hierarchy if the memdev is removed needs to come down.
The delete_endpoint() callback is careful to check whether it is being
called to tear down the hierarchy, or if it is only being called to
teardown the memdev because an ancestor port is going through
->remove().
That care needs to take the device_lock() of the endpoint's parent.
Which requires 2 bugs to be fixed:
1/ A reference on the parent is needed to prevent use-after-free
scenarios like this signature:
BUG: spinlock bad magic on CPU#0, kworker/u56:0/11
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20230524-3.fc38 05/24/2023
Workqueue: cxl_port detach_memdev [cxl_core]
RIP: 0010:spin_bug+0x65/0xa0
Call Trace:
do_raw_spin_lock+0x69/0xa0
__mutex_lock+0x695/0xb80
delete_endpoint+0xad/0x150 [cxl_core]
devres_release_all+0xb8/0x110
device_unbind_cleanup+0xe/0x70
device_release_driver_internal+0x1d2/0x210
detach_memdev+0x15/0x20 [cxl_core]
process_one_work+0x1e3/0x4c0
worker_thread+0x1dd/0x3d0
2/ In the case of RCH topologies, the parent device that needs to be
locked is not always @port->dev as returned by cxl_mem_find_port(), use
endpoint->dev.parent instead.