[PATCH net] nfc: llcp: reject PDUs shorter than the LLCP header

[Doruk Tan Ozturk <doruk@0sec.ai>]

nfc_llcp_rx_skb() reads the two-byte LLCP header (DSAP/SSAP/PTYPE) and
dispatches by PDU type; several handlers then derive a TLV-array length as
skb->len - LLCP_HEADER_SIZE. Neither nfc_llcp_rx_skb() nor its callers
guarantee the frame is at least LLCP_HEADER_SIZE bytes, and a sub-header
PDU does reach it: digital_in_recv_dep_res() and digital_tg_recv_dep_req()
strip the DEP header with skb_pull() after only checking the DEP header
size, so a DEP I-PDU carrying a 0- or 1-byte LLCP payload is handed up as
a sub-2-byte skb.

For a CONNECT or CC PDU, nfc_llcp_recv_connect() and nfc_llcp_recv_cc()
then pass skb->len - LLCP_HEADER_SIZE to nfc_llcp_parse_connection_tlv().
For skb->len < 2 that subtraction underflows: truncated into the u16
tlv_array_len parameter it becomes ~0xFFFE, and for a CONNECT to the SDP
SAP, nfc_llcp_connect_sn() uses a size_t and underflows to SIZE_MAX. The
TLV parsers bound their walk relative to that length, so they read far
past the end of the skb.

The aggregated-frame path (nfc_llcp_recv_agf()) already drops sub-PDUs
shorter than the header. Apply the same guard once, in the dispatcher, so
every PDU type is covered.

Found by 0sec (https://0sec.ai) using automated source analysis; the
missing guard is evident from source. Compile-tested.

Fixes: d646960f7986 ("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>
---
 net/nfc/llcp_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index aed5fe1afef0..e3b3077e0e83 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1481,6 +1481,9 @@ static void nfc_llcp_rx_skb(struct nfc_llcp_local *local, struct sk_buff *skb)
 {
 	u8 dsap, ssap, ptype;
 
+	if (skb->len < LLCP_HEADER_SIZE)
+		return;
+
 	ptype = nfc_llcp_ptype(skb);
 	dsap = nfc_llcp_dsap(skb);
 	ssap = nfc_llcp_ssap(skb);
-- 
2.43.0