nfc: llcp: reject PDUs shorter than the LLCP header

commit 95674f506c upstream.

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.

nfc_llcp_rx_skb() reads the header via nfc_llcp_ptype()/nfc_llcp_dsap()/
nfc_llcp_ssap(), which dereference pdu->data[0] and pdu->data[1], and a
CONNECT or CC PDU then computes

	tlv_array_len = skb->len - LLCP_HEADER_SIZE;

as a size_t and hands it to the TLV walk. When the frame is shorter than
the header the subtraction wraps to a huge value and the walk runs far
past the buffer, an out-of-bounds read.

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

Guard the common receive choke point __nfc_llcp_recv(), shared by both the
target (nfc_llcp_data_received()) and initiator (nfc_llcp_recv()) paths, so
a short skb is dropped before the rx_work worker parses it. Use
pskb_may_pull() rather than a skb->len test so the two header bytes are
guaranteed to sit in the skb linear area even for a non-linear skb,
matching how the sibling NCI and HCI receive paths validate their headers.

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

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

Fixes: d646960f79 ("NFC: Initial LLCP support")
Cc: stable@vger.kernel.org
Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20260714164631.75068-1-doruk@0sec.ai
Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/nfc/llcp_core.c

@@ -1552,6 +1552,11 @@ 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 (!pskb_may_pull(skb, LLCP_HEADER_SIZE)) {
+		kfree_skb(skb);
+		return;
+	}
+
 	local->rx_pending = skb;
 	del_timer(&local->link_timer);
 	schedule_work(&local->rx_work);