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

[Simon Horman <horms@kernel.org>]

On Tue, Sep 15, 2026 at 07:54:27PM +0100, Aamir Ahmed wrote:
> 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 <elb12345@hotmail.co.uk>
> ---
> 
> 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/
>     
>     Built net/nfc/llcp_core.o on v7.3-rc1 with KASAN and W=1, no warnings.
>     No NFC hardware; not runtime-tested.

Reviewed-by: Simon Horman <horms@kernel.org>