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(-)
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@vger.kernel.org
Signed-off-by: Safa Karakuş <safa.karakus@secunnix.com>
---
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
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@vger.kernel.org
> Signed-off-by: Safa Karakuş <safa.karakus@secunnix.com>
> ---
> 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@fourdim.xyz>
Reviewed-by: Siwei Zhang <oss@fourdim.xyz>
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
On Sat, 16 May 2026 21:15:04 +0300 Safa Karakus 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 > Feel free to add the regular KASAN uaf calltrace to help understand your fix.
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