[PATCH 3/4] Bluetooth: hci_sync: pin conn across hci_le_big_create_sync

[Michael Bommarito <michael.bommarito@gmail.com>]

hci_le_big_create_sync() interprets its void *data argument as a
struct hci_conn pointer and dereferences conn->iso_qos,
conn->sync_handle, conn->num_bis, conn->bis, and conn->conn_timeout
after the entry hci_conn_valid() check. As with the sibling
cmd_sync callbacks, hci_disconn_complete_evt() can retire the conn
between the validity check and the body's first deref, and the
blocking wait for HCI_EVT_LE_BIG_SYNC_ESTABLISHED extends the
race window to seconds.

A KASAN slab-use-after-free splat in cache kmalloc-8k at conn->flags
(set_bit(HCI_CONN_CREATE_BIG_SYNC, &conn->flags)) confirms the bug
on linux-next tip commit bee6ea30c487 ("Add linux-next specific
files for 20260421").

Convert hci_connect_big_sync() to the hci_cmd_sync_queue_conn_once()
helper and balance the conn pin in create_big_complete()'s
-ECANCELED short-circuit. Promote create_big_complete()'s
hci_conn_valid() + clear_bit() pair to run under hci_dev_lock so
that hci_disconn_complete_evt() cannot remove conn from
hdev->conn_hash.list between the check and the write.

Prior art: Pauli Virtanen's PATCH v2 8/8 at
https://lore.kernel.org/linux-bluetooth/e18591f264c50e15917cb8b9e5f9798d9880979d.1762100290.git.pav@iki.fi/.

Fixes: 024421cf3992 ("Bluetooth: hci_conn: Fix not setting timeout for BIG Create Sync")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 net/bluetooth/hci_sync.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/bluetooth/hci_sync.c b/net/bluetooth/hci_sync.c
index 43779375209b..47ce9ba63fe2 100644
--- a/net/bluetooth/hci_sync.c
+++ b/net/bluetooth/hci_sync.c
@@ -7265,10 +7265,16 @@ static void create_big_complete(struct hci_dev *hdev, void *data, int err)
 	bt_dev_dbg(hdev, "err %d", err);
 
 	if (err == -ECANCELED)
-		return;
+		goto done;
+
+	hci_dev_lock(hdev);
 
 	if (hci_conn_valid(hdev, conn))
 		clear_bit(HCI_CONN_CREATE_BIG_SYNC, &conn->flags);
+
+	hci_dev_unlock(hdev);
+done:
+	hci_conn_put(conn);
 }
 
 static int hci_le_big_create_sync(struct hci_dev *hdev, void *data)
@@ -7320,8 +7326,8 @@ int hci_connect_big_sync(struct hci_dev *hdev, struct hci_conn *conn)
 {
 	int err;
 
-	err = hci_cmd_sync_queue_once(hdev, hci_le_big_create_sync, conn,
-				      create_big_complete);
+	err = hci_cmd_sync_queue_conn_once(hdev, hci_le_big_create_sync, conn,
+					   create_big_complete);
 	return (err == -EEXIST) ? 0 : err;
 }
 
-- 
2.53.0