6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Aamir Ahmed commit 273f9d667cde649f8de9d72b1303cc2f4b658c50 upstream. 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 Reviewed-by: Simon Horman Link: https://patch.msgid.link/AS8P251MB0001789BBF04B72745C7D96BC8BA2@AS8P251MB0001.EURP251.PROD.OUTLOOK.COM Signed-off-by: David Heidelberg Signed-off-by: Greg Kroah-Hartman --- net/nfc/llcp_core.c | 3 +++ 1 file changed, 3 insertions(+) --- a/net/nfc/llcp_core.c +++ b/net/nfc/llcp_core.c @@ -1101,6 +1101,9 @@ static void nfc_llcp_recv_hdlc(struct nf 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);