linux-kernel - Re: [PATCH v2 1/2] nfc: llcp: avoid double release/put on LLCP_CLOSED in nfc_llcp_recv

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Message-ID: <CAFgAp7gP_yk7nF_AN+B_DRDJW--ytCKKQToG2m6y4h_SLBBaLA@mail.gmail.com>
Date: Wed, 17 Dec 2025 22:05:27 +0900
From: くさあさ <pioooooooooip@...il.com>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: linux-nfc@...ts.01.org, Paolo Abeni <pabeni@...hat.com>, 
	Jakub Kicinski <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] nfc: llcp: avoid double release/put on LLCP_CLOSED
 in nfc_llcp_recv_disc()

Hi Krzysztof,

Sorry about that — my previous response might not have made it to the
list/thread.
Replying here to address your concerns before sending v3.

1) DM_DISC reply after LLCP_CLOSED
This is not a new behavior introduced by my change. In the old code, the
LLCP_CLOSED branch did release_sock() and nfc_llcp_sock_put(), but it did not
return/goto, so execution continued and still reached nfc_llcp_send_dm(...,
LLCP_DM_DISC) afterwards. The disc patch only removes the redundant
CLOSED-branch
cleanup so release_sock()/nfc_llcp_sock_put() are performed exactly once via the
common exit path, while keeping the existing DM_DISC reply behavior.

2) Initial refcount / double free concern
nfc_llcp_recv_disc()/recv_hdlc() take an extra reference via nfc_llcp_sock_get()
(sock_hold()). The issue is the mismatched put/unlock: the CLOSED branch drops
the reference and releases the lock, and then the common exit path does the same
again. This is a refcount/locking imbalance regardless of whether it immediately
frees the object, and it may become a UAF depending on timing/refcounting.

Regarding your formatting notes: I will wrap commit messages per
submitting-patches,
use a 12-char sha in Fixes, and run scripts/checkpatch.pl (and --strict) and fix
reported warnings before sending v3.

Best regards,
Qianchang

On Wed, Dec 17, 2025 at 9:57 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>
> On 17/12/2025 13:46, Qianchang Zhao wrote:
> > nfc_llcp_sock_get() takes a reference on the LLCP socket via sock_hold().
> >
> > In nfc_llcp_recv_disc(), when the socket is already in LLCP_CLOSED state, the
> > code used to perform release_sock() and nfc_llcp_sock_put() in the CLOSED branch
> > but then continued execution and later performed the same cleanup again on the
> > common exit path. This results in refcount imbalance (double put) and unbalanced
>
> Please wrap commit message according to Linux coding style / submission
> process (neither too early nor over the limit):
> https://elixir.bootlin.com/linux/v6.4-rc1/source/Documentation/process/submitting-patches.rst#L597
>
> > lock release.
> >
> > Remove the redundant CLOSED-branch cleanup so that release_sock() and
> > nfc_llcp_sock_put() are performed exactly once via the common exit path, while
> > keeping the existing DM_DISC reply behavior.
> >
> > Fixes: d646960f7986fefb460a2b062d5ccc8ccfeacc3a ("NFC: Initial LLCP support")
>
> 12 char sha.
>
> Please run scripts/checkpatch.pl on the patches and fix reported
> warnings. After that, run also 'scripts/checkpatch.pl --strict' on the
> patches and (probably) fix more warnings. Some warnings can be ignored,
> especially from --strict run, but the code here looks like it needs a
> fix. Feel free to get in touch if the warning is not clear.
>
>
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Qianchang Zhao <pioooooooooip@...il.com>
> > ---
> >  net/nfc/llcp_core.c | 5 -----
> >  1 file changed, 5 deletions(-)
> >
> > diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
> > index beeb3b4d2..ed37604ed 100644
> > --- a/net/nfc/llcp_core.c
> > +++ b/net/nfc/llcp_core.c
> > @@ -1177,11 +1177,6 @@ static void nfc_llcp_recv_disc(struct nfc_llcp_local *local,
> >
> >       nfc_llcp_socket_purge(llcp_sock);
> >
> > -     if (sk->sk_state == LLCP_CLOSED) {
> > -             release_sock(sk);
> > -             nfc_llcp_sock_put(llcp_sock);
>
> You did not answer my previous review. You also did not answer my
> concerns from earlier private report. Please respond before you send
> again v3.
>
> > -     }
> > -
> >       if (sk->sk_state == LLCP_CONNECTED) {
> >               nfc_put_device(local->dev);
> >               sk->sk_state = LLCP_CLOSED;
>
>
> Best regards,
> Krzysztof