[PATCH] Bluetooth: hci_sync: Serialize devcoredump reset during device open

[Cen Zhang <zzzccc427@gmail.com>]

An ACTIVE devcoredump's buffer cursors must remain valid from the
packet handler's state check through its copy. hci_devcd_rx() holds
hdev->lock across that interval, and hci_devcd_reset() requires the
same lock. However, hci_dev_open_sync() invokes reset without taking
it.

A registered VHCI controller can collect a dump through its debugfs
interface while the controller is down. If a successful open overlaps
with handling a nonempty dump packet, reset can run after the receiver's
ACTIVE check and before its copy:

  hci_devcd_rx()                    hci_dev_open_sync()
    dequeue nonempty skb
    hci_dev_lock(hdev)
    handler observes ACTIVE
                                     hdev->open(hdev) succeeds
                                     hci_devcd_reset(hdev)
                                       dump.tail = NULL
                                       dump.state = IDLE
    hci_devcd_copy()
      compare against old dump.end
      memcpy(NULL, ..., skb->len)
    hci_dev_unlock(hdev)

Reset leaves dump.end pointing at the old buffer and purges only queued
packets, so it cannot discard the skb already dequeued by the receiver.
The bounds check can pass with a NULL tail, leading to a nonempty copy
to address zero. KASAN reported a 24-byte NULL write in hci_devcd_rx().

Take hdev->lock around the open-time reset. The receiver then finishes
its state check and copy using valid cursors before reset, or observes
IDLE after reset and skips the copy.

KASAN report as below:

    BUG: KASAN: null-ptr-deref in hci_devcd_rx+0x5d5/0x1a80
    Write of size 24 at addr 0000000000000000 by task kworker/u17:0/496
    
        CPU: 0 UID: 0 PID: 496 Comm: kworker/u17:0 Not tainted 
    7.2.0-rc6-pmb-bt-functional-v1+ #1 PREEMPT(lazy) 
        Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps 
    fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
    Workqueue: hci0 hci_devcd_rx
    Call Trace:
     <TASK>
     dump_stack_lvl+0x93/0xd0
     kasan_report+0xe0/0x110
     ? hci_devcd_rx+0x5d5/0x1a80
     kasan_check_range+0x105/0x1b0
     __asan_memcpy+0x3c/0x60
     hci_devcd_rx+0x5d5/0x1a80
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __pfx_hci_devcd_rx+0x10/0x10
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? lock_release+0xc8/0x280
     ? srso_alias_return_thunk+0x5/0xfbef5
     process_one_work+0x908/0x19c0
     ? __pfx_process_one_work+0x10/0x10
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? lock_is_held_type+0x8f/0x100
     ? srso_alias_return_thunk+0x5/0xfbef5
     worker_thread+0x65c/0xe40
     ? __pfx_worker_thread+0x10/0x10
     kthread+0x34f/0x460
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __pfx_kthread+0x10/0x10
     ret_from_fork+0x659/0x940
     ? __pfx_ret_from_fork+0x10/0x10
     ? srso_alias_return_thunk+0x5/0xfbef5
     ? __switch_to+0x74f/0xf80
     ? __pfx_kthread+0x10/0x10
     ret_from_fork_asm+0x1a/0x30
     </TASK>
    ==================================================================
    Disabling lock debugging due to kernel taint

Fixes: 9695ef876fd1 ("Bluetooth: Add support for hci devcoredump")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---

diff --git a/net/bluetooth/hci_sync.c b/net/bluetooth/hci_sync.c
index 1fe11d3ef1aaa8118811fb778b591e0413b4b3f0..8e4fc26b3c42d72a33393bd0b381414c4c0f927e 100644
--- a/net/bluetooth/hci_sync.c
+++ b/net/bluetooth/hci_sync.c
@@ -5454,7 +5454,9 @@ int hci_dev_open_sync(struct hci_dev *hdev)
 		goto done;
 	}
 
+	hci_dev_lock(hdev);
 	hci_devcd_reset(hdev);
+	hci_dev_unlock(hdev);
 
 	set_bit(HCI_RUNNING, &hdev->flags);
 	hci_sock_dev_event(hdev, HCI_DEV_OPEN);