[PATCH v1] Bluetooth: hci_core: use skb_get() instead of skb_clone() for req_skb

BT enable fails intermittently with -ETIMEDOUT (-110).  The kernel log
shows the HCI Read Local Version command was sent and the firmware
replied with status 0x00 (logged by hci_req_cmd_complete() BT_DBG),
but the waiter in __hci_cmd_sync_sk() never woke up and timed out
after 10 s:

  bluetooth hci0: Opcode 0xfc00              // __hci_cmd_sync_sk
  bluetooth hci0: opcode 0xfc00 plen 1       // hci_cmd_sync_add
  bluetooth hci0: skb len 4                  // hci_cmd_sync_alloc
  bluetooth hci0: length 1                   // hci_req_sync_run
  Bluetooth: hci0 cmd_cnt 1 cmd queued 1     // hci_cmd_work
  Bluetooth: hci0 type 1 len 4               // hci_send_frame
  Bluetooth: opcode 0xfc00 status 0x00       // hci_req_cmd_complete
  <-- req_skb NULL: req_complete_skb not set,
      hci_cmd_sync_complete() never called,
      req_status stays HCI_REQ_PEND            -->
  <-- 10 s later: wait_event_interruptible_timeout expires -->
  bluetooth hci0: end: err -110              // __hci_cmd_sync_sk

The root cause is that hci_send_cmd_sync() clones the sent command
into hdev->req_skb so that hci_req_cmd_complete() can locate the
registered completion callback.  Under memory pressure this
skb_clone() fails, leaving hdev->req_skb NULL.  The firmware reply
is received and processed, but hci_req_cmd_complete() finds NULL
req_skb, so hci_cmd_sync_complete() is never called, req_status
stays HCI_REQ_PEND, and the waiter times out with -ETIMEDOUT.

req_skb is only used to read bt_cb(skb)->hci callbacks and opcode --
it is never modified.  Replace skb_clone() with skb_get(), which
simply increments the reference count of hdev->sent_cmd without
allocating new memory and therefore cannot fail.

This issue was first observed as a use-after-free in ttyport_close()
when ttyport_open() failed, which was investigated in an earlier
patch series [1].  That investigation led to the discovery of the
true root cause described above.

[1] https://lore.kernel.org/all/[email protected]/

Fixes: 2615fd9a7c25 ("Bluetooth: hci_sync: Fix overwriting request callback")
Cc: [email protected]
Signed-off-by: Xin Chen <[email protected]>
---
 net/bluetooth/hci_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
index 509c820a693d..35a1be57e386 100644
--- a/net/bluetooth/hci_core.c
+++ b/net/bluetooth/hci_core.c
@@ -4093,7 +4093,7 @@ static int hci_send_cmd_sync(struct hci_dev *hdev, struct sk_buff *skb)
 	if (READ_ONCE(hdev->req_status) == HCI_REQ_PEND &&
 	    !hci_dev_test_and_set_flag(hdev, HCI_CMD_PENDING)) {
 		kfree_skb(hdev->req_skb);
-		hdev->req_skb = skb_clone(hdev->sent_cmd, GFP_KERNEL);
+		hdev->req_skb = skb_get(hdev->sent_cmd);
 	}
 
 	return err;
-- 
2.43.0