[PATCH net v2] nfc: llcp: drop truncated I/RR/RNR PDUs in nfc_llcp_recv_hdlc()

nfc_llcp_recv_hdlc() reads the sequence byte skb->data[2], via
nfc_llcp_ns()/nfc_llcp_nr(), before any length check. The receive path
only guarantees the two-byte LLCP header -- __nfc_llcp_recv() checks it
with pskb_may_pull() and nfc_llcp_recv_agf() admits two-byte inner PDUs
-- so a two-byte I, RR or RNR PDU reads one byte of uninitialised skb
tailroom. The byte becomes N(R)/N(S); a peer can already set those with
a well-formed PDU, so this is acting on uninitialised memory, not new
peer control.

Guard the read with pskb_may_pull(), as commit 95674f506c63 ("nfc: llcp:
reject PDUs shorter than the LLCP header") did for the two-byte header,
so the sequence byte is present and linear before it is read. RR and RNR
PDUs are LLCP_HEADER_SIZE + LLCP_SEQUENCE_SIZE bytes and an I PDU is
longer, so no valid frame is rejected; a truncated PDU is malformed, so
return without a DM reply.

Fixes: d646960f7986 ("NFC: Initial LLCP support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Aamir Ahmed <redacted>
---

Notes:
    v2:
      - scope to nfc_llcp_recv_hdlc() only; the recv_dm() half duplicated
        Lekë Hapçiu's pending fix (20260729011547.19191-2-snowwlake@icloud.com)
      - return silently instead of answering DM(NOCONN) as v1 did, which would
        close an established peer connection (Sashiko)
      - guard with pskb_may_pull(), matching commit 95674f506c63
      - add Assisted-by, target net, Cc the right lists, reword the changelog
    v1: https://lore.kernel.org/netdev/AS8P251MB0001E8602F36054CDD8979A2C8B32@AS8P251MB0001.EURP251.PROD.OUTLOOK.COM/ (local)
    
    Built net/nfc/llcp_core.o on v7.3-rc1 with KASAN and W=1, no warnings.
    No NFC hardware; not runtime-tested.

 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 cac1b54..c553123 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c

@@ -1074,6 +1074,9 @@ static void nfc_llcp_recv_hdlc(struct nfc_llcp_local *local,
 	struct sock *sk;
 	u8 dsap, ssap, ptype, ns, nr;
 
+	if (!pskb_may_pull(skb, LLCP_HEADER_SIZE + LLCP_SEQUENCE_SIZE))
+		return;
+
 	ptype = nfc_llcp_ptype(skb);
 	dsap = nfc_llcp_dsap(skb);
 	ssap = nfc_llcp_ssap(skb);

-- 
2.53.0.windows.1