Re: [PATCH v4] Bluetooth: fix UAF in l2cap_sock_cleanup_listen() vs l2cap_conn_del()

[fourdim]

Hi,

On Sat, May 16, 2026, at 2:15 PM, Safa Karakuş wrote:
> bt_accept_dequeue() unlinks a not-yet-accepted child from the parent
> accept queue and release_sock()s it before returning, so the returned
> sk has no caller reference and is unlocked.
>
> l2cap_sock_cleanup_listen() walks these children on listening-socket
> close.  A concurrent HCI disconnect drives hci_rx_work ->
> l2cap_conn_del() which runs l2cap_chan_del() + l2cap_sock_kill() and
> frees the child sk and its l2cap_chan; cleanup_listen() then uses both:
>
>   BUG: KASAN: slab-use-after-free in l2cap_sock_kill
>     l2cap_sock_kill / l2cap_sock_cleanup_listen / __x64_sys_close
>   Freed by: l2cap_conn_del -> l2cap_sock_close_cb -> l2cap_sock_kill
>
> This is distinct from the two fixes already in this area: commit
> e83f5e24da741 ("Bluetooth: serialize accept_q access") serialises the
> accept_q list/poll and takes temporary refs inside bt_accept_dequeue(),
> and CVE-2025-39860 serialises the userspace close()/accept() race by
> calling cleanup_listen() under lock_sock() in l2cap_sock_release().
> Neither covers l2cap_conn_del() running from hci_rx_work, so this UAF
> still reproduces on current bluetooth/master.
>
> Take the reference at the source: bt_accept_dequeue() does sock_hold()
> while sk is still locked, before release_sock(); callers sock_put().
> cleanup_listen() pins the chan with l2cap_chan_hold_unless_zero() under
> a brief child sk lock (serialising vs l2cap_sock_teardown_cb()), drops
> it before l2cap_chan_lock(), and skips a duplicate l2cap_sock_kill() on
> SOCK_DEAD.  conn->lock is not taken here: cleanup_listen() runs under
> the parent sk lock and that would invert
> conn->lock -> chan->lock -> sk_lock (lockdep).
>
> KASAN/SMP: an unprivileged listen/close vs HCI-disconnect race produced
> 12 use-after-free reports per run before this change; 0, and no lockdep
> report, over 1600+ raced iterations after it on bluetooth/master.
>
> Fixes: 15f02b910562 ("Bluetooth: L2CAP: Add initial code for Enhanced 
> Credit Based Mode")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Safa Karakuş <safa.karakus@xxxxxxxxxxxx>
> ---
> Hi Luiz,
>
> v4 - rebased on current bluetooth/master (after e83f5e24d "serialize
> accept_q access"); the af_bluetooth.c hunk now sits in the reworked
> bt_accept_dequeue().  This residual (cleanup_listen vs l2cap_conn_del,
> not covered by e83f5e24d nor CVE-2025-39860) is unchanged from v3 and
> re-verified with KASAN on bluetooth/master: 12 UAF/run -> 0, no
> lockdep, over 1600+ raced iterations.
>
> Changes since v3: rebased onto e83f5e24d; commit message notes why
> e83f5e24d/CVE-2025-39860 do not cover this path.
> Changes since v2: fix at the source in bt_accept_dequeue() + chan
> lifetime via l2cap_chan_hold_unless_zero(); no conn->lock (lockdep).
> Changes since v1: consistent From/Signed-off-by.
>
>  net/bluetooth/af_bluetooth.c | 10 +++++++
>  net/bluetooth/iso.c          |  9 ++++++-
>  net/bluetooth/l2cap_sock.c   | 51 +++++++++++++++++++++++++++++++-----
>  net/bluetooth/rfcomm/sock.c  |  9 ++++++-
>  net/bluetooth/sco.c          |  9 ++++++-
>  5 files changed, 78 insertions(+), 10 deletions(-)
>
> diff --git a/net/bluetooth/af_bluetooth.c b/net/bluetooth/af_bluetooth.c
> index 9d68dd860..1a6aa3f8d 100644
> --- a/net/bluetooth/af_bluetooth.c
> +++ b/net/bluetooth/af_bluetooth.c
> @@ -340,6 +340,16 @@ struct sock *bt_accept_dequeue(struct sock 
> *parent, struct socket *newsock)
>  			if (newsock)
>  				sock_graft(sk, newsock);
> 
> +			/* Hand the caller a reference taken while sk is
> +			 * still locked.  bt_accept_unlink() just dropped
> +			 * the accept-queue reference; without this hold a
> +			 * concurrent teardown (e.g. l2cap_conn_del() ->
> +			 * l2cap_sock_kill()) could free sk between
> +			 * release_sock() and the caller using it.  Every
> +			 * caller drops this with sock_put() when done.
> +			 */
> +			sock_hold(sk);
> +
>  			release_sock(sk);
>  			if (next)
>  				sock_put(next);
> diff --git a/net/bluetooth/iso.c b/net/bluetooth/iso.c
> index 7cb2864fe..812f9002d 100644
> --- a/net/bluetooth/iso.c
> +++ b/net/bluetooth/iso.c
> @@ -751,6 +751,8 @@ static void iso_sock_cleanup_listen(struct sock *parent)
>  	while ((sk = bt_accept_dequeue(parent, NULL))) {
>  		iso_sock_close(sk);
>  		iso_sock_kill(sk);
> +		/* Drop the reference handed back by bt_accept_dequeue(). */
> +		sock_put(sk);
>  	}
> 
>  	/* If listening socket has a hcon, properly disconnect it */
> @@ -1356,8 +1358,13 @@ static int iso_sock_accept(struct socket *sock, 
> struct socket *newsock,
>  		}
> 
>  		ch = bt_accept_dequeue(sk, newsock);
> -		if (ch)
> +		if (ch) {
> +			/* Drop the bridging ref from bt_accept_dequeue();
> +			 * the grafted socket keeps ch alive from here.
> +			 */
> +			sock_put(ch);
>  			break;
> +		}
> 
>  		if (!timeo) {
>  			err = -EAGAIN;
> diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
> index cf590a67d..b34e7da8d 100644
> --- a/net/bluetooth/l2cap_sock.c
> +++ b/net/bluetooth/l2cap_sock.c
> @@ -349,8 +349,13 @@ static int l2cap_sock_accept(struct socket *sock, 
> struct socket *newsock,
>  		}
> 
>  		nsk = bt_accept_dequeue(sk, newsock);
> -		if (nsk)
> +		if (nsk) {
> +			/* Drop the bridging ref from bt_accept_dequeue();
> +			 * the grafted socket keeps nsk alive from here.
> +			 */
> +			sock_put(nsk);
>  			break;
> +		}
> 
>  		if (!timeo) {
>  			err = -EAGAIN;
> @@ -1475,22 +1480,54 @@ static void l2cap_sock_cleanup_listen(struct 
> sock *parent)
>  	BT_DBG("parent %p state %s", parent,
>  	       state_to_string(parent->sk_state));
> 
> -	/* Close not yet accepted channels */
> +	/* Close not yet accepted channels.
> +	 *
> +	 * bt_accept_dequeue() now returns sk with an extra reference held
> +	 * (taken while sk was still locked) so a concurrent l2cap_conn_del()
> +	 * -> l2cap_sock_kill() cannot free sk under us.
> +	 *
> +	 * cleanup_listen() runs under the parent sk lock, so unlike
> +	 * l2cap_sock_shutdown() we must NOT take conn->lock here: that would
> +	 * establish sk_lock -> conn->lock and invert the established
> +	 * conn->lock -> chan->lock -> sk_lock order (lockdep deadlock).
> +	 *
> +	 * Instead, briefly take the child sk lock to fetch and pin its chan.
> +	 * l2cap_conn_del() reaches the chan free only via
> +	 * l2cap_chan_del() -> l2cap_sock_teardown_cb(), which itself takes
> +	 * the child sk lock; holding it across l2cap_chan_hold_unless_zero()
> +	 * therefore guarantees the chan cannot be freed while we read and
> +	 * pin it (hold_unless_zero() additionally skips a chan already past
> +	 * its last reference).  We then drop the sk lock before taking
> +	 * chan->lock, so sk and chan locks are never held together.
> +	 */
>  	while ((sk = bt_accept_dequeue(parent, NULL))) {
> -		struct l2cap_chan *chan = l2cap_pi(sk)->chan;
> +		struct l2cap_chan *chan;
> +
> +		lock_sock_nested(sk, L2CAP_NESTING_NORMAL);
> +		chan = l2cap_chan_hold_unless_zero(l2cap_pi(sk)->chan);
> +		release_sock(sk);
> +		if (!chan) {
> +			/* l2cap_conn_del() already tearing this child down */
> +			sock_put(sk);
> +			continue;
> +		}
> 
>  		BT_DBG("child chan %p state %s", chan,
>  		       state_to_string(chan->state));
> 
> -		l2cap_chan_hold(chan);
>  		l2cap_chan_lock(chan);
> -
>  		__clear_chan_timer(chan);
>  		l2cap_chan_close(chan, ECONNRESET);
> -		l2cap_sock_kill(sk);
> -
> +		/* l2cap_conn_del() may already have killed this socket
> +		 * (it sets SOCK_DEAD); skip the duplicate to avoid a
> +		 * double sock_put()/l2cap_chan_put().
> +		 */
> +		if (!sock_flag(sk, SOCK_DEAD))
> +			l2cap_sock_kill(sk);
>  		l2cap_chan_unlock(chan);
> +
>  		l2cap_chan_put(chan);
> +		sock_put(sk);
>  	}
>  }
> 
> diff --git a/net/bluetooth/rfcomm/sock.c b/net/bluetooth/rfcomm/sock.c
> index be6639cd6..bd7d959c6 100644
> --- a/net/bluetooth/rfcomm/sock.c
> +++ b/net/bluetooth/rfcomm/sock.c
> @@ -180,6 +180,8 @@ static void rfcomm_sock_cleanup_listen(struct sock *parent)
>  	while ((sk = bt_accept_dequeue(parent, NULL))) {
>  		rfcomm_sock_close(sk);
>  		rfcomm_sock_kill(sk);
> +		/* Drop the reference handed back by bt_accept_dequeue(). */
> +		sock_put(sk);
>  	}
> 
>  	parent->sk_state  = BT_CLOSED;
> @@ -497,8 +499,13 @@ static int rfcomm_sock_accept(struct socket *sock, 
> struct socket *newsock,
>  		}
> 
>  		nsk = bt_accept_dequeue(sk, newsock);
> -		if (nsk)
> +		if (nsk) {
> +			/* Drop the bridging ref from bt_accept_dequeue();
> +			 * the grafted socket keeps nsk alive from here.
> +			 */
> +			sock_put(nsk);
>  			break;
> +		}
> 
>  		if (!timeo) {
>  			err = -EAGAIN;
> diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
> index eba44525d..f1799c6a6 100644
> --- a/net/bluetooth/sco.c
> +++ b/net/bluetooth/sco.c
> @@ -502,6 +502,8 @@ static void sco_sock_cleanup_listen(struct sock *parent)
>  	while ((sk = bt_accept_dequeue(parent, NULL))) {
>  		sco_sock_close(sk);
>  		sco_sock_kill(sk);
> +		/* Drop the reference handed back by bt_accept_dequeue(). */
> +		sock_put(sk);
>  	}
> 
>  	parent->sk_state  = BT_CLOSED;
> @@ -765,8 +767,13 @@ static int sco_sock_accept(struct socket *sock, 
> struct socket *newsock,
>  		}
> 
>  		ch = bt_accept_dequeue(sk, newsock);
> -		if (ch)
> +		if (ch) {
> +			/* Drop the bridging ref from bt_accept_dequeue();
> +			 * the grafted socket keeps ch alive from here.
> +			 */
> +			sock_put(ch);
>  			break;
> +		}
> 
>  		if (!timeo) {
>  			err = -EAGAIN;
> -- 
> 2.34.1

I reported the same issue privately to the maintainers on April 11th.
The fix looks correct for the sk-lifetime UAF.

Reported-by: Siwei Zhang <oss@xxxxxxxxxxx>
Reviewed-by: Siwei Zhang <oss@xxxxxxxxxxx>

This patch leaves the conn->chan_l list-corruption race open
(l2cap_chan_close without conn->lock). I'll send a follow-up patch
on top that addresses it.

Best,
Siwei