[PATCH net 1/2] nfc: nci: fix u8 underflow in nci_store_general_bytes_nfc_dep

nci_store_general_bytes_nfc_dep() computes the General Bytes length by
subtracting a fixed header offset from the peer-supplied atr_res_len
(POLL) or atr_req_len (LISTEN) field:

    ndev->remote_gb_len = min_t(__u8,
        atr_res_len - NFC_ATR_RES_GT_OFFSET,   /* offset = 15 */
        NFC_ATR_RES_GB_MAXSIZE);

Both length fields are __u8.  When a malicious NFC-DEP peer sends an
ATR_RES/ATR_REQ whose length is smaller than the fixed offset (< 15
or < 14 respectively), the subtraction wraps:

    atr_res_len = 0  ->  (u8)(0 - 15) = 241
    min_t(__u8, 241, NFC_ATR_RES_GB_MAXSIZE=47) = 47

The subsequent memcpy then reads 47 bytes beyond the valid activation
parameter data into ndev->remote_gb[].  This buffer is later fed to
nfc_llcp_parse_gb_tlv() as a TLV array.

Reject the frame with NCI_STATUS_RF_PROTOCOL_ERROR when the length is
below the required offset.  The existing caller already logs and
continues for other helpers that return a non-OK status from this
switch, so no change is required on the caller side.

Fixes: 767f19ae698e ("NFC: Implement NCI dep_link_up and dep_link_down")
Cc: [email protected]
Signed-off-by: Lekë Hapçiu <[email protected]>
---
 net/nfc/nci/ntf.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/nfc/nci/ntf.c b/net/nfc/nci/ntf.c
index f5c9a8ab7ec1..263f76a2450f 100644
--- a/net/nfc/nci/ntf.c
+++ b/net/nfc/nci/ntf.c
@@ -653,6 +653,9 @@ static int nci_store_general_bytes_nfc_dep(struct nci_dev *ndev,
 	switch (ntf->activation_rf_tech_and_mode) {
 	case NCI_NFC_A_PASSIVE_POLL_MODE:
 	case NCI_NFC_F_PASSIVE_POLL_MODE:
+		if (ntf->activation_params.poll_nfc_dep.atr_res_len <
+		    NFC_ATR_RES_GT_OFFSET)
+			return NCI_STATUS_RF_PROTOCOL_ERROR;
 		ndev->remote_gb_len = min_t(__u8,
 			(ntf->activation_params.poll_nfc_dep.atr_res_len
 						- NFC_ATR_RES_GT_OFFSET),
@@ -665,6 +668,9 @@ static int nci_store_general_bytes_nfc_dep(struct nci_dev *ndev,
 
 	case NCI_NFC_A_PASSIVE_LISTEN_MODE:
 	case NCI_NFC_F_PASSIVE_LISTEN_MODE:
+		if (ntf->activation_params.listen_nfc_dep.atr_req_len <
+		    NFC_ATR_REQ_GT_OFFSET)
+			return NCI_STATUS_RF_PROTOCOL_ERROR;
 		ndev->remote_gb_len = min_t(__u8,
 			(ntf->activation_params.listen_nfc_dep.atr_req_len
 						- NFC_ATR_REQ_GT_OFFSET),
-- 
2.51.0