[PATCH net v2] nfc: llcp: reject PDUs shorter than the LLCP header

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

Every LLCP PDU begins with a two-byte header (DSAP/SSAP + PTYPE), but the
receive path never checked that a frame is at least LLCP_HEADER_SIZE bytes
before parsing it.

A peer LLCP PDU travels: NFC-DEP frame -> nfc_tm_data_received() (target /
NCI path) or nfc_llcp_recv() (initiator data-exchange callback) ->
__nfc_llcp_recv() -> rx_work -> nfc_llcp_rx_skb() ->
nfc_llcp_recv_connect(). For a CONNECT (or CC) PDU nfc_llcp_recv_connect()
computes

	tlv_array_len = skb->len - LLCP_HEADER_SIZE;

as a size_t and hands it to the TLV walk. When skb->len is 0 or 1 the
subtraction wraps to a huge value and the walk runs far past the skb,
causing an out-of-bounds read; nfc_llcp_ptype()/nfc_llcp_ssap() likewise
read pdu->data[1] for such a short frame.

A nearby NFC device can reach this without authentication; LLCP link
activation happens automatically after NFC-DEP.

Reject PDUs shorter than the LLCP header in __nfc_llcp_recv(), the common
choke point shared by both the target (nfc_llcp_data_received()) and
initiator (nfc_llcp_recv()) receive paths, so a short skb is freed before
the rx_work worker is scheduled.

Reproduced with a KFENCE out-of-bounds read via /dev/virtual_nci on
linux-next.

Found by 0sec (https://0sec.ai) using automated source analysis.

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>
---
v2: move the check into __nfc_llcp_recv() so a short skb is dropped
    before the rx_work worker is scheduled (Vadim Fedorenko), which also
    covers the initiator nfc_llcp_recv() path. Reword the commit message
    (drop the "same guard as AGF" wording) and add a KFENCE reproduction
    note.

 net/nfc/llcp_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index aed5fe1afef0..72b6e707ad0c 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1565,6 +1565,11 @@ static void nfc_llcp_rx_work(struct work_struct *work)
 
 static void __nfc_llcp_recv(struct nfc_llcp_local *local, struct sk_buff *skb)
 {
+	if (skb->len < LLCP_HEADER_SIZE) {
+		kfree_skb(skb);
+		return;
+	}
+
 	local->rx_pending = skb;
 	timer_delete(&local->link_timer);
 	schedule_work(&local->rx_work);
-- 
2.43.0