nfc: llcp: bound the connect_sn TLV walk to the skb

commit 55c68ac upstream.

Commit 27256cd ("nfc: llcp: bound SNL TLV parsing to the skb and
add length checks") fixed the unbounded TLV walk in nfc_llcp_recv_snl(),
and commit d8bd2de ("nfc: llcp: fix OOB read and u8 offset wrap in
TLV parsers") subsequently bounded nfc_llcp_parse_gb_tlv() and
nfc_llcp_parse_connection_tlv(). One sibling parser sharing the same
pattern remains unbounded: nfc_llcp_connect_sn().

nfc_llcp_connect_sn() walks a TLV list, reading a two-byte header
(type, length) followed by length bytes of value, without checking that
the two header bytes or the declared length stay within the buffer. It
returns a pointer to a service name of up to 255 bytes that may point
past the end of the skb; it is subsequently consumed by memcmp() in
nfc_llcp_sock_from_sn(). In addition tlv_array_len was computed as
"skb->len - LLCP_HEADER_SIZE" in size_t, so a CONNECT/CC frame shorter
than the LLCP header underflows to a huge length and the walk runs far
past the buffer.

nfc_llcp_connect_sn() is reachable from nfc_llcp_recv_connect() and
nfc_llcp_recv_cc(), i.e. from received CONNECT and CC PDUs. A nearby
NFC device can reach this without authentication; LLCP link activation
happens automatically after NFC-DEP, and the nfc_llcp_rx_skb()
dispatcher applies no minimum-length guard.

Walk the TLV list by pointer, bounded by skb_tail_pointer(skb), and
validate each declared length before use, matching the approach already
used for nfc_llcp_recv_snl(). Starting the walk at
&skb->data[LLCP_HEADER_SIZE] against the tail pointer also removes the
size_t underflow for short frames.

Found by 0sec automated security-research tooling (https://0sec.ai).

Fixes: d646960 ("NFC: Initial LLCP support")
Cc: stable@vger.kernel.org
Assisted-by: 0sec:claude-opus-4-8
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260709131229.44477-1-doruk@0sec.ai
Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: peaktwilight

net/nfc/llcp_core.c

@@ -847,21 +847,23 @@ static struct nfc_llcp_sock *nfc_llcp_sock_get_sn(struct nfc_llcp_local *local,
 static const u8 *nfc_llcp_connect_sn(const struct sk_buff *skb, size_t *sn_len)
 {
 	u8 type, length;
-	const u8 *tlv = &skb->data[2];
-	size_t tlv_array_len = skb->len - LLCP_HEADER_SIZE, offset = 0;
+	const u8 *tlv = &skb->data[LLCP_HEADER_SIZE];
+	const u8 *tlv_end = skb_tail_pointer(skb);
 
-	while (offset < tlv_array_len) {
+	while (tlv + 2 < tlv_end) {
 		type = tlv[0];
 		length = tlv[1];
 
+		if (tlv + 2 + length > tlv_end)
+			break;
+
 		pr_debug("type 0x%x length %d\n", type, length);
 
 		if (type == LLCP_TLV_SN) {
 			*sn_len = length;
 			return &tlv[2];
 		}
 
-		offset += length + 2;
 		tlv += length + 2;
 	}