[PATCH v4] Bluetooth: fix UAF in l2cap_sock_cleanup_listen() vs l2cap_conn_del()
From: Safa Karakuş
Date: Sat May 16 2026 - 14:15:44 EST
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