[PATCH net v3] 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.

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: d646960f7986 ("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>
---
v3: use pskb_may_pull() so the guard also covers non-linear skbs and
    guarantees the header bytes are in the linear area (David Laight).
v2: move the guard into __nfc_llcp_recv() so both the target and
    initiator receive paths are covered by a single check.

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index aed5fe1afef0..e3b2627cb089 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 (!pskb_may_pull(skb, LLCP_HEADER_SIZE)) {
+		kfree_skb(skb);
+		return;
+	}
+
 	local->rx_pending = skb;
 	timer_delete(&local->link_timer);
 	schedule_work(&local->rx_work);
-- 
2.43.0