In the Linux kernel, the following vulnerability has been resolved: nfc: llcp: drop truncated I/RR/RNR PDUs in nfc_llcp_recv_hdlc() nfc_ll

In the Linux kernel, the following vulnerability has been resolved:

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.