[PATCH] Bluetooth: L2CAP: Fix use-after-free in l2cap_chan_timeout()

[PATCH] Bluetooth: L2CAP: Fix use-after-free in l2cap_chan_timeout()

[Hyunwoo Kim]

l2cap_chan_timeout() reads chan->conn without holding any lock and
without taking a reference on the connection. If l2cap_conn_del()
frees the connection concurrently, mutex_lock(&conn->lock) operates
on freed memory. The existing NULL check is insufficient as it cannot
prevent the connection from being freed after the check passes, and
the early return also leaks the channel reference held by the timer.

Fix by reading chan->conn under l2cap_chan_lock() and holding the
connection with l2cap_conn_get(). After acquiring locks in the correct
order (conn->lock then chan->lock), re-verify chan->conn in case
l2cap_conn_del() already cleaned up the channel.

Fixes: 3df91ea20e74 ("Bluetooth: Revert to mutexes from RCU list")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 net/bluetooth/l2cap_core.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index abd091155d04..a432e94281dc 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -406,13 +406,20 @@ static void l2cap_chan_timeout(struct work_struct *work)
 {
 	struct l2cap_chan *chan = container_of(work, struct l2cap_chan,
 					       chan_timer.work);
-	struct l2cap_conn *conn = chan->conn;
+	struct l2cap_conn *conn;
 	int reason;
 
 	BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
 
-	if (!conn)
+	l2cap_chan_lock(chan);
+	conn = chan->conn;
+	if (!conn) {
+		l2cap_chan_unlock(chan);
+		l2cap_chan_put(chan);
 		return;
+	}
+	l2cap_conn_get(conn);
+	l2cap_chan_unlock(chan);
 
 	mutex_lock(&conn->lock);
 	/* __set_chan_timer() calls l2cap_chan_hold(chan) while scheduling
@@ -420,6 +427,17 @@ static void l2cap_chan_timeout(struct work_struct *work)
 	 */
 	l2cap_chan_lock(chan);
 
+	/* Recheck since l2cap_conn_del() may have cleaned up the channel
+	 * while we were waiting for conn->lock.
+	 */
+	if (!chan->conn) {
+		l2cap_chan_unlock(chan);
+		l2cap_chan_put(chan);
+		mutex_unlock(&conn->lock);
+		l2cap_conn_put(conn);
+		return;
+	}
+
 	if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
 		reason = ECONNREFUSED;
 	else if (chan->state == BT_CONNECT &&
@@ -436,6 +454,7 @@ static void l2cap_chan_timeout(struct work_struct *work)
 	l2cap_chan_put(chan);
 
 	mutex_unlock(&conn->lock);
+	l2cap_conn_put(conn);
 }
 
 struct l2cap_chan *l2cap_chan_create(void)
-- 
2.43.0

Re: [PATCH] Bluetooth: L2CAP: Fix use-after-free in l2cap_chan_timeout()

[Luiz Augusto von Dentz]

Hi Hyunwoo,

On Thu, Mar 19, 2026 at 11:14 AM Hyunwoo Kim <imv4bel@gmail.com> wrote:
>
> l2cap_chan_timeout() reads chan->conn without holding any lock and
> without taking a reference on the connection. If l2cap_conn_del()
> frees the connection concurrently, mutex_lock(&conn->lock) operates
> on freed memory. The existing NULL check is insufficient as it cannot
> prevent the connection from being freed after the check passes, and
> the early return also leaks the channel reference held by the timer.
>
> Fix by reading chan->conn under l2cap_chan_lock() and holding the
> connection with l2cap_conn_get(). After acquiring locks in the correct
> order (conn->lock then chan->lock), re-verify chan->conn in case
> l2cap_conn_del() already cleaned up the channel.
>
> Fixes: 3df91ea20e74 ("Bluetooth: Revert to mutexes from RCU list")
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> ---
>  net/bluetooth/l2cap_core.c | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index abd091155d04..a432e94281dc 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -406,13 +406,20 @@ static void l2cap_chan_timeout(struct work_struct *work)
>  {
>         struct l2cap_chan *chan = container_of(work, struct l2cap_chan,
>                                                chan_timer.work);
> -       struct l2cap_conn *conn = chan->conn;
> +       struct l2cap_conn *conn;
>         int reason;
>
>         BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
>
> -       if (!conn)
> +       l2cap_chan_lock(chan);
> +       conn = chan->conn;
> +       if (!conn) {
> +               l2cap_chan_unlock(chan);
> +               l2cap_chan_put(chan);
>                 return;
> +       }
> +       l2cap_conn_get(conn);
> +       l2cap_chan_unlock(chan);
>
>         mutex_lock(&conn->lock);
>         /* __set_chan_timer() calls l2cap_chan_hold(chan) while scheduling
> @@ -420,6 +427,17 @@ static void l2cap_chan_timeout(struct work_struct *work)
>          */
>         l2cap_chan_lock(chan);
>
> +       /* Recheck since l2cap_conn_del() may have cleaned up the channel
> +        * while we were waiting for conn->lock.
> +        */
> +       if (!chan->conn) {
> +               l2cap_chan_unlock(chan);
> +               l2cap_chan_put(chan);
> +               mutex_unlock(&conn->lock);
> +               l2cap_conn_put(conn);
> +               return;
> +       }
> +
>         if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
>                 reason = ECONNREFUSED;
>         else if (chan->state == BT_CONNECT &&
> @@ -436,6 +454,7 @@ static void l2cap_chan_timeout(struct work_struct *work)
>         l2cap_chan_put(chan);
>
>         mutex_unlock(&conn->lock);
> +       l2cap_conn_put(conn);
>  }
>
>  struct l2cap_chan *l2cap_chan_create(void)
> --
> 2.43.0

Actually maybe this is because we don't use cancel_delayed_work_sync
to wait for the work to complete on l2cap_chan_del, actually that
should use disable_delayed_work_sync to prevent the timer from being
rearmed, and that should be done to all the timers since l2cap_chan is
about to be destroyed.

-- 
Luiz Augusto von Dentz

RE: Bluetooth: L2CAP: Fix use-after-free in l2cap_chan_timeout()

[bluez.test.bot]

[-- Attachment #1: Type: text/plain, Size: 2834 bytes --]

This is automated email and please do not reply to this email!

Dear submitter,

Thank you for submitting the patches to the linux bluetooth mailing list.
This is a CI test results with your patch series:
PW Link:https://patchwork.kernel.org/project/bluetooth/list/?series=1069344

---Test result---

Test Summary:
CheckPatch                    PENDING   0.43 seconds
GitLint                       PENDING   0.35 seconds
SubjectPrefix                 PASS      0.10 seconds
BuildKernel                   PASS      25.02 seconds
CheckAllWarning               PASS      27.79 seconds
CheckSparse                   PASS      26.74 seconds
BuildKernel32                 PASS      24.54 seconds
TestRunnerSetup               PASS      530.44 seconds
TestRunner_l2cap-tester       PASS      27.95 seconds
TestRunner_iso-tester         FAIL      31.28 seconds
TestRunner_bnep-tester        PASS      6.51 seconds
TestRunner_mgmt-tester        FAIL      112.54 seconds
TestRunner_rfcomm-tester      PASS      9.59 seconds
TestRunner_sco-tester         FAIL      14.31 seconds
TestRunner_ioctl-tester       PASS      10.19 seconds
TestRunner_mesh-tester        FAIL      12.46 seconds
TestRunner_smp-tester         PASS      10.69 seconds
TestRunner_userchan-tester    PASS      6.99 seconds
IncrementalBuild              PENDING   0.80 seconds

Details
##############################
Test: CheckPatch - PENDING
Desc: Run checkpatch.pl script
Output:

##############################
Test: GitLint - PENDING
Desc: Run gitlint
Output:

##############################
Test: TestRunner_iso-tester - FAIL
Desc: Run iso-tester with test-runner
Output:
BUG: KASAN: slab-use-after-free in le_read_features_complete+0x7e/0x2b0
Total: 141, Passed: 141 (100.0%), Failed: 0, Not Run: 0
##############################
Test: TestRunner_mgmt-tester - FAIL
Desc: Run mgmt-tester with test-runner
Output:
Total: 494, Passed: 489 (99.0%), Failed: 1, Not Run: 4

Failed Test Cases
Read Exp Feature - Success                           Failed       0.106 seconds
##############################
Test: TestRunner_sco-tester - FAIL
Desc: Run sco-tester with test-runner
Output:
WARNING: possible circular locking dependency detected
BUG: sleeping function called from invalid context at net/core/sock.c:3782
Total: 30, Passed: 30 (100.0%), Failed: 0, Not Run: 0
##############################
Test: TestRunner_mesh-tester - FAIL
Desc: Run mesh-tester with test-runner
Output:
Total: 10, Passed: 8 (80.0%), Failed: 2, Not Run: 0

Failed Test Cases
Mesh - Send cancel - 1                               Timed out    2.754 seconds
Mesh - Send cancel - 2                               Timed out    2.000 seconds
##############################
Test: IncrementalBuild - PENDING
Desc: Incremental build with the patches in the series
Output:



---
Regards,
Linux Bluetooth