Affected: Linux

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

Bluetooth: hci_core: Fix use-after-free in vhci_flush()

syzbot reported use-after-free in vhci_flush() without repro. [0]

From the splat, a thread close()d a vhci file descriptor while its device was being used by iotcl() on another thread.

Once the last fd refcnt is released, vhci_release() calls hci_unregister_dev(), hci_free_dev(), and kfree() for struct vhci_data, which is set to hci_dev->dev->driver_data.

The problem is that there is no synchronisation after unlinking hdev from hci_dev_list in hci_unregister_dev(). There might be another thread still accessing the hdev which was fetched before the unlink operation.

We can use SRCU for such synchronisation.

Let's run hci_dev_reset() under SRCU and wait for its completion in hci_unregister_dev().

Another option would be to restore hci_dev->destruct(), which was removed in commit 587ae086f6e4 ("Bluetooth: Remove unused hci-destruct cb"). However, this would not be a good solution, as we should not run hci_unregister_dev() while there are in-flight ioctl() requests, which could lead to another data-race KCSAN splat.

Note that other drivers seem to have the same problem, for exmaple, virtbt_remove().

[0]: BUG: KASAN: slab-use-after-free in skb_queue_empty_lockless include/linux/skbuff.h:1891 [inline] BUG: KASAN: slab-use-after-free in skb_queue_purge_reason+0x99/0x360 net/core/skbuff.c:3937 Read of size 8 at addr ffff88807cb8d858 by task syz.1.21…