Re: [PATCH net v3] nfc: llcp: reject PDUs shorter than the LLCP header

On 14.07.2026 17:46, Doruk Tan Ozturk wrote:
> 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: [email protected]
> Suggested-by: David Laight <[email protected]>
> Signed-off-by: Doruk Tan Ozturk <[email protected]>
> ---
> 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);

Reviewed-by: Vadim Fedorenko <[email protected]>