Subject: Re: [PATCH] nfc: llcp: fix missing return after LLCP_CLOSED check in recv_hdlc and recv_disc

From: Simon Horman <[email-masked]>
Date: Thu, 9 Apr 2026 17:45:27 +0100

On Sun, Apr 05, 2026 at 06:41:58PM +0200, Lekë Hapçiu wrote:
> From: Lekë Hapçiu <[email-masked]>
> 
> nfc_llcp_recv_hdlc() and nfc_llcp_recv_disc() both call
> nfc_llcp_sock_get() (which increments the socket reference count) and
> lock_sock() before processing incoming PDUs.  When the socket is found
> to be in state LLCP_CLOSED both functions correctly call release_sock()
> and nfc_llcp_sock_put() to undo those operations, but are missing a
> return statement:
> 
>     lock_sock(sk);
>     if (sk->sk_state == LLCP_CLOSED) {
>         release_sock(sk);
>         nfc_llcp_sock_put(llcp_sock);
>         /* ← return missing */
>     }
>     /* Falls through with lock released and reference dropped */
>     ...
>     release_sock(sk);            /* double unlock */
>     nfc_llcp_sock_put(llcp_sock); /* double put → refcount underflow */
> 
> The fall-through causes three independent bugs:
> 
>   1. Use-after-free: all llcp_sock field accesses after the LLCP_CLOSED
>      block occur with the socket lock released and the reference dropped;
>      another CPU may free the socket concurrently.
> 
>   2. Double release_sock: sk_lock.owned is already 0 — LOCKDEP reports
>      "WARNING: suspicious unlock balance detected".
> 
>   3. Double nfc_llcp_sock_put: the refcount is decremented a second time
>      at the end of the function, potentially driving it below zero
>      (refcount_t underflow), corrupting the SLUB freelist and causing a
>      subsequent use-after-free or double-free.
> 
> Both functions are reachable from any NFC P2P peer within physical
> proximity (~4 cm) without hostile NFCC firmware:
>   - nfc_llcp_recv_hdlc: triggered by sending an LLCP I, RR, or RNR PDU
>     to a SAP pair whose connection has been torn down.
>   - nfc_llcp_recv_disc: triggered by sending an LLCP DISC PDU to a SAP
>     pair that is already in LLCP_CLOSED state.
> 
> Fix: add the missing return statement in both functions so that the
> LLCP_CLOSED branch exits after cleanup.
> 
> Fixes: Introduced with nfc_llcp_recv_hdlc / nfc_llcp_recv_disc
> Signed-off-by: Lekë Hapçiu <[email-masked]>

Curiously this seems to duplicate this patch:

- [PATCH net] nfc: llcp: add missing return after LLCP_CLOSED checks
  https://lore.kernel.org/all/[email-masked]/

-- 
pw-bot: changes-requested