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

Hi Vadim

this was reproduced from userspace on unmodified
linux-next (bee763d5f341) without RF hardware.

It's the peer-RX path, not a local command skb:

virtual_ncidev_write (peer NCI DATA) -> nci_rx_data_packet
-> nfc_tm_data_received -> nfc_llcp_data_received
-> rx_work -> nfc_llcp_rx_skb -> nfc_llcp_recv_connect

Bring the LLCP link up via a normal NFC-DEP activation, then send
one NCI DATA packet with a 1-byte CONNECT PDU. skb->len - 2 wraps
to 0xffffffff and the TLV walk runs off the end:

BUG: KFENCE: out-of-bounds read in nfc_llcp_recv_connect+0x9f6/0xf80
nfc_llcp_recv_connect+0x9f6 -> nfc_llcp_rx_work -> process_one_work
read 4219B past a 704B skbuff_small_head from virtual_ncidev_write
R14: 00000000ffffffff (wrapped tlv_array_len)

With the guard: rx_skb runs for all 600 short PDUs, recv_connect
reached 0 times, 0 reports.

The bound stays "<", not "<=" -- a header-only SYMM/DISC/DM is
exactly 2 bytes and must still dispatch; AGF uses "<=" only
because an AGF frame must also carry a sub-PDU. I'll drop the
"same guard as AGF" line from the commit message.

Instantiating /dev/virtual_nci needs privilege, but that's just
the syzbot transport; the 1-byte CONNECT is what a remote NFC-DEP
peer emits, and the DEP layer imposes no minimum LLCP length.
Impact is a proximity OOB read (DoS).

I can send the full reproducer if you'd like.

best
Doruk

On Sun, Jul 12, 2026 02:01 PM, Vadim Fedorenko
<[email protected]> wrote:
>
> On 11/07/2026 08:27, Doruk Tan Ozturk wrote:
> > nfc_llcp_rx_skb() reads the two-byte LLCP header (DSAP/SSAP/PTYPE) and
> > dispatches by PDU type; several handlers then derive a TLV-array length as
> > skb->len - LLCP_HEADER_SIZE. Neither nfc_llcp_rx_skb() nor its callers
> > guarantee the frame is at least LLCP_HEADER_SIZE bytes, and a sub-header
>
> that's not correct. there are 2 ways to get to nfc_llcp_rx_skb() - via
> nfc_llcp_recv_agf() or through commands/locally generated skbs. The
> first one checks against LLCP_HEADER_SIZE, while latter one creates skb
> payload with correct LLCP header size. Do you have a reproducer to
> trigger the issue?
>
>
> > PDU does reach it: digital_in_recv_dep_res() and digital_tg_recv_dep_req()
> > strip the DEP header with skb_pull() after only checking the DEP header
> > size, so a DEP I-PDU carrying a 0- or 1-byte LLCP payload is handed up as
> > a sub-2-byte skb.
> >
> > For a CONNECT or CC PDU, nfc_llcp_recv_connect() and nfc_llcp_recv_cc()
> > then pass skb->len - LLCP_HEADER_SIZE to nfc_llcp_parse_connection_tlv().
> > For skb->len < 2 that subtraction underflows: truncated into the u16
> > tlv_array_len parameter it becomes ~0xFFFE, and for a CONNECT to the SDP
> > SAP, nfc_llcp_connect_sn() uses a size_t and underflows to SIZE_MAX. The
> > TLV parsers bound their walk relative to that length, so they read far
> > past the end of the skb.
> >
> > The aggregated-frame path (nfc_llcp_recv_agf()) already drops sub-PDUs
> > shorter than the header. Apply the same guard once, in the dispatcher, so
>
> that not exactly correct, it drops skbs which are shorter or equal to
> the header, the check added in this patch is not correct then.
>
> > every PDU type is covered.
> >
> > Found by 0sec (https://0sec.ai) using automated source analysis; the
> > missing guard is evident from source. Compile-tested.
> >
> > Fixes: d646960f7986 ("NFC: Initial LLCP support")
> > Cc: [email protected]
> > Assisted-by: 0sec:claude-opus-4-8
> > Signed-off-by: Doruk Tan Ozturk <[email protected]>
> > ---
> >   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 aed5fe1afef0..e3b3077e0e83 100644
> > --- a/net/nfc/llcp_core.c
> > +++ b/net/nfc/llcp_core.c
> > @@ -1481,6 +1481,9 @@ static void nfc_llcp_rx_skb(struct nfc_llcp_local *local, struct sk_buff *skb)
> >   {
> >       u8 dsap, ssap, ptype;
> >
> > +     if (skb->len < LLCP_HEADER_SIZE)
> > +             return;
> > +
> >       ptype = nfc_llcp_ptype(skb);
> >       dsap = nfc_llcp_dsap(skb);
> >       ssap = nfc_llcp_ssap(skb);
>