Re: [PATCH v4] Bluetooth: hci_event: Fix not using key encryption size when its known

Paul Menzel <pmenzel@xxxxxxxxxxxxx> · Fri, 2 May 2025 23:32:34 +0200

Dear Luiz,


Thank you for your patch. One minor comment below.

Am 02.05.25 um 22:52 schrieb Luiz Augusto von Dentz:
From: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>

This fixes the regression introduced by 50c1241e6a8a ("Bluetooth: l2cap:
Check encryption key size on incoming connection") introduced a check for

introduc*ing* a check

l2cap_check_enc_key_size which checks for hcon->enc_key_size which may
not be initialized if HCI_OP_READ_ENC_KEY_SIZE is still pending.

If the key encryption size is known, due previously reading it using
HCI_OP_READ_ENC_KEY_SIZE, then store it as part of link_key/smp_ltk
structures so the next time the encryption is changed their values are
used as conn->enc_key_size thus avoiding the racing against
HCI_OP_READ_ENC_KEY_SIZE.

Now that the enc_size is stored as part of key the information the code
then attempts to check that there is no downgrade of security if
HCI_OP_READ_ENC_KEY_SIZE returns a value smaller than what has been
previously stored.

Would the test infrastructure support easily adding a test for this?


Kind regards,

Paul


Link: https://bugzilla.kernel.org/show_bug.cgi?id=220061
Link: https://bugzilla.kernel.org/show_bug.cgi?id=220063
Fixes: 50c1241e6a8a ("Bluetooth: l2cap: Check encryption key size on incoming connection")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>
---
  include/net/bluetooth/hci_core.h |  1 +
  net/bluetooth/hci_conn.c         | 24 ++++++++++
  net/bluetooth/hci_event.c        | 76 +++++++++++++++++++-------------
  3 files changed, 70 insertions(+), 31 deletions(-)

diff --git a/include/net/bluetooth/hci_core.h b/include/net/bluetooth/hci_core.h
index 965ca1ca841c..57f6175fd1cd 100644
--- a/include/net/bluetooth/hci_core.h
+++ b/include/net/bluetooth/hci_core.h
@@ -1801,6 +1801,7 @@ struct hci_conn_params *hci_pend_le_action_lookup(struct list_head *list,
  void hci_uuids_clear(struct hci_dev *hdev);
  
  void hci_link_keys_clear(struct hci_dev *hdev);
+u8 *hci_conn_key_enc_size(struct hci_conn *conn);
  struct link_key *hci_find_link_key(struct hci_dev *hdev, bdaddr_t *bdaddr);
  struct link_key *hci_add_link_key(struct hci_dev *hdev, struct hci_conn *conn,
  				  bdaddr_t *bdaddr, u8 *val, u8 type,
diff --git a/net/bluetooth/hci_conn.c b/net/bluetooth/hci_conn.c
index d8f6aaf14703..932175d65a14 100644
--- a/net/bluetooth/hci_conn.c
+++ b/net/bluetooth/hci_conn.c
@@ -3058,3 +3058,27 @@ int hci_ethtool_ts_info(unsigned int index, int sk_proto,
  	hci_dev_put(hdev);
  	return 0;
  }
+
+u8 *hci_conn_key_enc_size(struct hci_conn *conn)
+{
+	if (conn->type == ACL_LINK) {
+		struct link_key *key;
+
+		key = hci_find_link_key(conn->hdev, &conn->dst);
+		if (!key)
+			return NULL;
+
+		return &key->pin_len;
+	} else if (conn->type == LE_LINK) {
+		struct smp_ltk *ltk;
+
+		ltk = hci_find_ltk(conn->hdev, &conn->dst, conn->dst_type,
+				   conn->role);
+		if (!ltk)
+			return NULL;
+
+		return &ltk->enc_size;
+	}
+
+	return NULL;
+}
diff --git a/net/bluetooth/hci_event.c b/net/bluetooth/hci_event.c
index 105d1446c9dc..c2a898eb5086 100644
--- a/net/bluetooth/hci_event.c
+++ b/net/bluetooth/hci_event.c
@@ -739,10 +739,17 @@ static u8 hci_cc_read_enc_key_size(struct hci_dev *hdev, void *data,
  			   handle);
  		conn->enc_key_size = 0;
  	} else {
+		u8 *key_enc_size = hci_conn_key_enc_size(conn);
+
  		conn->enc_key_size = rp->key_size;
  		status = 0;
  
-		if (conn->enc_key_size < hdev->min_enc_key_size) {
+		/* Attempt to check if the key size is too small or if it has
+		 * been downgraded from the last time it was stored as part of
+		 * the link_key.
+		 */
+		if (conn->enc_key_size < hdev->min_enc_key_size ||
+		    (key_enc_size && conn->enc_key_size < *key_enc_size)) {
  			/* As slave role, the conn->state has been set to
  			 * BT_CONNECTED and l2cap conn req might not be received
  			 * yet, at this moment the l2cap layer almost does
@@ -755,6 +762,13 @@ static u8 hci_cc_read_enc_key_size(struct hci_dev *hdev, void *data,
  			clear_bit(HCI_CONN_ENCRYPT, &conn->flags);
  			clear_bit(HCI_CONN_AES_CCM, &conn->flags);
  		}
+
+		/* Update the key encryption size with the connection one */
+		if (key_enc_size && *key_enc_size != conn->enc_key_size) {
+			bt_dev_info(hdev, "key size changed from %u to %u",
+				    *key_enc_size, conn->enc_key_size);
+			*key_enc_size = conn->enc_key_size;
+		}
  	}
  
  	hci_encrypt_cfm(conn, status);
@@ -3065,6 +3079,34 @@ static void hci_inquiry_result_evt(struct hci_dev *hdev, void *edata,
  	hci_dev_unlock(hdev);
  }
  
+static int hci_read_enc_key_size(struct hci_dev *hdev, struct hci_conn *conn)
+{
+	struct hci_cp_read_enc_key_size cp;
+	u8 *key_enc_size = hci_conn_key_enc_size(conn);
+
+	if (!read_key_size_capable(hdev)) {
+		conn->enc_key_size = HCI_LINK_KEY_SIZE;
+		return -EOPNOTSUPP;
+	}
+
+	bt_dev_dbg(hdev, "hcon %p", conn);
+
+	memset(&cp, 0, sizeof(cp));
+	cp.handle = cpu_to_le16(conn->handle);
+
+	/* If the key enc_size is already known, use it as conn->enc_key_size,
+	 * otherwise use hdev->min_enc_key_size so the likes of
+	 * l2cap_check_enc_key_size don't fail while waiting for
+	 * HCI_OP_READ_ENC_KEY_SIZE response.
+	 */
+	if (key_enc_size && *key_enc_size)
+		conn->enc_key_size = *key_enc_size;
+	else
+		conn->enc_key_size = hdev->min_enc_key_size;
+
+	return hci_send_cmd(hdev, HCI_OP_READ_ENC_KEY_SIZE, sizeof(cp), &cp);
+}
+
  static void hci_conn_complete_evt(struct hci_dev *hdev, void *data,
  				  struct sk_buff *skb)
  {
@@ -3157,23 +3199,11 @@ static void hci_conn_complete_evt(struct hci_dev *hdev, void *data,
  		if (ev->encr_mode == 1 && !test_bit(HCI_CONN_ENCRYPT, &conn->flags) &&
  		    ev->link_type == ACL_LINK) {
  			struct link_key *key;
-			struct hci_cp_read_enc_key_size cp;
  
  			key = hci_find_link_key(hdev, &ev->bdaddr);
  			if (key) {
  				set_bit(HCI_CONN_ENCRYPT, &conn->flags);
-
-				if (!read_key_size_capable(hdev)) {
-					conn->enc_key_size = HCI_LINK_KEY_SIZE;
-				} else {
-					cp.handle = cpu_to_le16(conn->handle);
-					if (hci_send_cmd(hdev, HCI_OP_READ_ENC_KEY_SIZE,
-							 sizeof(cp), &cp)) {
-						bt_dev_err(hdev, "sending read key size failed");
-						conn->enc_key_size = HCI_LINK_KEY_SIZE;
-					}
-				}
-
+				hci_read_enc_key_size(hdev, conn);
  				hci_encrypt_cfm(conn, ev->status);
  			}
  		}
@@ -3612,24 +3642,8 @@ static void hci_encrypt_change_evt(struct hci_dev *hdev, void *data,
  
  	/* Try reading the encryption key size for encrypted ACL links */
  	if (!ev->status && ev->encrypt && conn->type == ACL_LINK) {
-		struct hci_cp_read_enc_key_size cp;
-
-		/* Only send HCI_Read_Encryption_Key_Size if the
-		 * controller really supports it. If it doesn't, assume
-		 * the default size (16).
-		 */
-		if (!read_key_size_capable(hdev)) {
-			conn->enc_key_size = HCI_LINK_KEY_SIZE;
+		if (hci_read_enc_key_size(hdev, conn))
  			goto notify;
-		}
-
-		cp.handle = cpu_to_le16(conn->handle);
-		if (hci_send_cmd(hdev, HCI_OP_READ_ENC_KEY_SIZE,
-				 sizeof(cp), &cp)) {
-			bt_dev_err(hdev, "sending read key size failed");
-			conn->enc_key_size = HCI_LINK_KEY_SIZE;
-			goto notify;
-		}
  
  		goto unlock;
  	}








