[PATCH] Bluetooth: hci_sync: fix stack buffer overflow in hci_le_big_create_sync

[hkbinbin <hkbinbinbin@gmail.com>]

hci_le_big_create_sync() uses DEFINE_FLEX to allocate a
struct hci_cp_le_big_create_sync on the stack with room for 0x11 (17)
BIS entries.  However, conn->num_bis can hold up to HCI_MAX_ISO_BIS (31)
entries — validated against ISO_MAX_NUM_BIS (0x1f) in the caller
hci_conn_big_create_sync().  When conn->num_bis is between 18 and 31,
the memcpy that copies conn->bis into cp->bis writes up to 14 bytes
past the stack buffer, corrupting adjacent stack memory.

This is trivially reproducible: binding an ISO socket with
bc_num_bis = ISO_MAX_NUM_BIS (31) and calling listen() will
eventually trigger hci_le_big_create_sync() from the HCI command
sync worker, causing a KASAN-detectable stack-out-of-bounds write:

  BUG: KASAN: stack-out-of-bounds in hci_le_big_create_sync+0x256/0x3b0
  Write of size 31 at addr ffffc90000487b48 by task kworker/u9:0/71

Fix this by changing the DEFINE_FLEX count from the incorrect 0x11 to
HCI_MAX_ISO_BIS, which matches the maximum number of BIS entries that
conn->bis can actually carry.

Fixes: 42ecf1947135 ("Bluetooth: ISO: Do not emit LE BIG Create Sync if previous is pending")
Cc: stable@vger.kernel.org
Signed-off-by: hkbinbin <hkbinbinbin@gmail.com>
---
 net/bluetooth/hci_sync.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/hci_sync.c b/net/bluetooth/hci_sync.c
index 45d16639874a..b84587061ae0 100644
--- a/net/bluetooth/hci_sync.c
+++ b/net/bluetooth/hci_sync.c
@@ -7222,7 +7222,8 @@ static void create_big_complete(struct hci_dev *hdev, void *data, int err)
 
 static int hci_le_big_create_sync(struct hci_dev *hdev, void *data)
 {
-	DEFINE_FLEX(struct hci_cp_le_big_create_sync, cp, bis, num_bis, 0x11);
+	DEFINE_FLEX(struct hci_cp_le_big_create_sync, cp, bis, num_bis,
+		    HCI_MAX_ISO_BIS);
 	struct hci_conn *conn = data;
 	struct bt_iso_qos *qos = &conn->iso_qos;
 	int err;
-- 
2.51.0