FAILED: patch "[PATCH] can: bcm: defer rx_op deallocation to workqueue to fix" failed to apply to 5.15-stable tree

[email protected]

author maintainer

2026-07-20 15:12 UTC #0 lore raw

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 68973f9db76144825e4f35dfdc80fb8279eb2d57
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072026-herbs-reliable-9eca@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 68973f9db76144825e4f35dfdc80fb8279eb2d57 Mon Sep 17 00:00:00 2001
From: Lee Jones <[email protected]>
Date: Tue, 14 Jul 2026 18:55:23 +0200
Subject: [PATCH] can: bcm: defer rx_op deallocation to workqueue to fix
thrtimer UAF

Commit f1b4e32aca08 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()") replaced synchronize_rcu() in bcm_delete_rx_op() with call_rcu() and introduced the RX_NO_AUTOTIMER flag.

However, this flag check was omitted for thrtimer in the packet rx fast-path. During BCM RX operation teardown, a concurrent RCU reader (bcm_rx_handler) can race and re-arm thrtimer via bcm_rx_update_and_send() after call_rcu() has been scheduled. Once the RCU grace period elapses, bcm_op is freed. The subsequently firing thrtimer then dereferences the deallocated op, causing a UAF.

Adding flag checks to the rx fast-path (bcm_rx_update_and_send) does not fully close the TOCTOU race and introduces latency for every CAN frame. Conversely, calling hrtimer_cancel() directly inside the RCU callback (softirq context) is fatal as hrtimer_cancel() can sleep, triggering a "scheduling while atomic" panic.

Resolve this by deferring the timer cancellation and memory free to a dedicated unbound workqueue (bcm_wq). The RCU callback now queues a work item to bcm_wq, which safely cancels both timers and deallocates memory in sleepable process context. A dedicated workqueue is used to prevent system-wide WQ saturation and is cleanly flushed/destroyed on module unload to avoid rmmod page faults.

Since the deferred work can now outlive the calling context by an unbounded amount, also take a reference on op->sk when it is assigned and drop it only once the deferred work has cancelled both timers, so a socket can no longer be freed out from under a still-armed timer whose callback (bcm_send_to_user()) dereferences op->sk.

Fixes f1b4e32aca08 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()")

Tested-by Feng Xue <[email protected]>

Tested-by Oliver Hartkopp <[email protected]>

Signed-off-by Lee Jones <[email protected]>

Signed-off-by Oliver Hartkopp <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Cc [email protected]

Signed-off-by Marc Kleine-Budde <[email protected]>

net/can/bcm.c +34 -3

index a4bef2c48a55..bdf53241bd7b 100644
@@ -58,6 +58,7 @@
 #include <linux/can/skb.h>
 #include <linux/can/bcm.h>
 #include <linux/slab.h>
+#include <linux/workqueue.h>
 #include <linux/spinlock.h>
 #include <net/can.h>
 #include <net/sock.h>

@@ -92,6 +93,8 @@ MODULE_ALIAS("can-proto-2");
 
 #define BCM_MIN_NAMELEN CAN_REQUIRED_SIZE(struct sockaddr_can, can_ifindex)
 
+static struct workqueue_struct *bcm_wq;
+
 /*
  * easy access to the first 64 bit of can(fd)_frame payload. cp->data is
  * 64 bit aligned so the offset has to be multiples of 8 which is ensured

@@ -105,6 +108,7 @@ static inline u64 get_u64(const struct canfd_frame *cp, int offset)
 struct bcm_op {
 	struct list_head list;
 	struct rcu_head rcu;
+	struct work_struct work;
 	int ifindex;
 	canid_t can_id;
 	u32 flags;

@@ -793,9 +797,12 @@ static struct bcm_op *bcm_find_op(struct list_head *ops,
 	return NULL;
 }
 
-static void bcm_free_op_rcu(struct rcu_head *rcu_head)
+static void bcm_free_op_work(struct work_struct *work)
 {
-	struct bcm_op *op = container_of(rcu_head, struct bcm_op, rcu);
+	struct bcm_op *op = container_of(work, struct bcm_op, work);
+
+	hrtimer_cancel(&op->timer);
+	hrtimer_cancel(&op->thrtimer);
 
 	if ((op->frames) && (op->frames != &op->sframe))
 		kfree(op->frames);

@@ -803,9 +810,23 @@ static void bcm_free_op_rcu(struct rcu_head *rcu_head)
 	if ((op->last_frames) && (op->last_frames != &op->last_sframe))
 		kfree(op->last_frames);
 
+	/* the last possible access to op->timer/op->thrtimer has now
+	 * happened above via hrtimer_cancel() - op->sk is no longer
+	 * needed by any pending timer callback, so drop our reference
+	 */
+	sock_put(op->sk);
+
 	kfree(op);
 }
 
+static void bcm_free_op_rcu(struct rcu_head *rcu_head)
+{
+	struct bcm_op *op = container_of(rcu_head, struct bcm_op, rcu);
+
+	INIT_WORK(&op->work, bcm_free_op_work);
+	queue_work(bcm_wq, &op->work);
+}
+
 static void bcm_remove_op(struct bcm_op *op)
 {
 	hrtimer_cancel(&op->timer);

@@ -1060,6 +1081,7 @@ static int bcm_tx_setup(struct bcm_msg_head *msg_head, struct msghdr *msg,
 
 		/* bcm_can_tx / bcm_tx_timeout_handler needs this */
 		op->sk = sk;
+		sock_hold(sk);
 		op->ifindex = ifindex;
 
 		/* initialize uninitialized (kzalloc) structure */

@@ -1221,6 +1243,7 @@ static int bcm_rx_setup(struct bcm_msg_head *msg_head, struct msghdr *msg,
 
 		/* bcm_can_tx / bcm_tx_timeout_handler needs this */
 		op->sk = sk;
+		sock_hold(sk);
 		op->ifindex = ifindex;
 
 		/* ifindex for timeout events w/o previous frame reception */

@@ -1839,11 +1862,15 @@ static int __init bcm_module_init(void)
 {
 	int err;
 
+	bcm_wq = alloc_workqueue("can-bcm-wq", WQ_UNBOUND, 0);
+	if (!bcm_wq)
+		return -ENOMEM;
+
 	pr_info("can: broadcast manager protocol\n");
 
 	err = register_pernet_subsys(&canbcm_pernet_ops);
 	if (err)
-		return err;
+		goto register_pernet_failed;
 
 	err = register_netdevice_notifier(&canbcm_notifier);
 	if (err)

@@ -1861,6 +1888,8 @@ static int __init bcm_module_init(void)
 	unregister_netdevice_notifier(&canbcm_notifier);
 register_notifier_failed:
 	unregister_pernet_subsys(&canbcm_pernet_ops);
+register_pernet_failed:
+	destroy_workqueue(bcm_wq);
 	return err;
 }

@@ -1869,6 +1898,8 @@ static void __exit bcm_module_exit(void)
 	can_proto_unregister(&bcm_can_proto);
 	unregister_netdevice_notifier(&canbcm_notifier);
 	unregister_pernet_subsys(&canbcm_pernet_ops);
+	rcu_barrier();
+	destroy_workqueue(bcm_wq);
 }
 
 module_init(bcm_module_init);

Sasha Levin

maintainer

2026-07-27 13:31 UTC #1 lore raw

PATCH 1/2 can: bcm: registration process optimization in bcm_module_init()

From: Ziyang Xuan <[email protected]>

[ Upstream commit edd1a7e42f1d2d09c5f79ecef05ae19dc669bf34 ]

Now, register_netdevice_notifier() and register_pernet_subsys() are both after can_proto_register(). It can create CAN_BCM socket and process socket once can_proto_register() successfully, so it is possible missing notifier event or proc node creation because notifier or bcm proc directory is not registered or created yet. Although this is a low probability scenario, it is not impossible.

Move register_pernet_subsys() and register_netdevice_notifier() to the front of can_proto_register(). In addition, register_pernet_subsys() and register_netdevice_notifier() may fail, check their results are necessary.

Signed-off-by Ziyang Xuan <[email protected]>

Link https://lore.kernel.org/all/823cff0ebec33fa9389eeaf8b8ded3217c32cb38.1663206163.git.william.xuanziyang@huawei.com open here

Acked-by Oliver Hartkopp <[email protected]>

Signed-off-by Marc Kleine-Budde <[email protected]>

Stable-dep-of: 68973f9db761 ("can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF")

Signed-off-by Sasha Levin <[email protected]>

---
 net/can/bcm.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

net/can/bcm.c +15 -3

index 317cd4c415ba28..361e384720cd19 100644
@@ -1832,15 +1832,27 @@ static int __init bcm_module_init(void)
 
 	pr_info("can: broadcast manager protocol\n");
 
+	err = register_pernet_subsys(&canbcm_pernet_ops);
+	if (err)
+		return err;
+
+	err = register_netdevice_notifier(&canbcm_notifier);
+	if (err)
+		goto register_notifier_failed;
+
 	err = can_proto_register(&bcm_can_proto);
 	if (err < 0) {
 		printk(KERN_ERR "can: registration of bcm protocol failed\n");
-		return err;
+		goto register_proto_failed;
 	}
 
-	register_pernet_subsys(&canbcm_pernet_ops);
-	register_netdevice_notifier(&canbcm_notifier);
 	return 0;
+
+register_proto_failed:
+	unregister_netdevice_notifier(&canbcm_notifier);
+register_notifier_failed:
+	unregister_pernet_subsys(&canbcm_pernet_ops);
+	return err;
 }
 
 static void __exit bcm_module_exit(void)

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-07-27 13:31 UTC #2 lore raw

PATCH 2/2 can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF

From: Lee Jones <[email protected]>

[ Upstream commit 68973f9db76144825e4f35dfdc80fb8279eb2d57 ]

Commit f1b4e32aca08 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()") replaced synchronize_rcu() in bcm_delete_rx_op() with call_rcu() and introduced the RX_NO_AUTOTIMER flag.

However, this flag check was omitted for thrtimer in the packet rx fast-path. During BCM RX operation teardown, a concurrent RCU reader (bcm_rx_handler) can race and re-arm thrtimer via bcm_rx_update_and_send() after call_rcu() has been scheduled. Once the RCU grace period elapses, bcm_op is freed. The subsequently firing thrtimer then dereferences the deallocated op, causing a UAF.

Adding flag checks to the rx fast-path (bcm_rx_update_and_send) does not fully close the TOCTOU race and introduces latency for every CAN frame. Conversely, calling hrtimer_cancel() directly inside the RCU callback (softirq context) is fatal as hrtimer_cancel() can sleep, triggering a "scheduling while atomic" panic.

Resolve this by deferring the timer cancellation and memory free to a dedicated unbound workqueue (bcm_wq). The RCU callback now queues a work item to bcm_wq, which safely cancels both timers and deallocates memory in sleepable process context. A dedicated workqueue is used to prevent system-wide WQ saturation and is cleanly flushed/destroyed on module unload to avoid rmmod page faults.

Since the deferred work can now outlive the calling context by an unbounded amount, also take a reference on op->sk when it is assigned and drop it only once the deferred work has cancelled both timers, so a socket can no longer be freed out from under a still-armed timer whose callback (bcm_send_to_user()) dereferences op->sk.

Fixes f1b4e32aca08 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()")

Tested-by Feng Xue <[email protected]>

Tested-by Oliver Hartkopp <[email protected]>

Signed-off-by Lee Jones <[email protected]>

Signed-off-by Oliver Hartkopp <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Cc [email protected]

Signed-off-by Marc Kleine-Budde <[email protected]>

Signed-off-by Sasha Levin <[email protected]>

---
 net/can/bcm.c | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

net/can/bcm.c +34 -3

index 361e384720cd19..a61430c0356a4e 100644
@@ -58,6 +58,7 @@
 #include <linux/can/skb.h>
 #include <linux/can/bcm.h>
 #include <linux/slab.h>
+#include <linux/workqueue.h>
 #include <linux/spinlock.h>
 #include <net/sock.h>
 #include <net/net_namespace.h>

@@ -89,6 +90,8 @@ MODULE_ALIAS("can-proto-2");
 
 #define BCM_MIN_NAMELEN CAN_REQUIRED_SIZE(struct sockaddr_can, can_ifindex)
 
+static struct workqueue_struct *bcm_wq;
+
 /*
  * easy access to the first 64 bit of can(fd)_frame payload. cp->data is
  * 64 bit aligned so the offset has to be multiples of 8 which is ensured

@@ -102,6 +105,7 @@ static inline u64 get_u64(const struct canfd_frame *cp, int offset)
 struct bcm_op {
 	struct list_head list;
 	struct rcu_head rcu;
+	struct work_struct work;
 	int ifindex;
 	canid_t can_id;
 	u32 flags;

@@ -740,9 +744,12 @@ static struct bcm_op *bcm_find_op(struct list_head *ops,
 	return NULL;
 }
 
-static void bcm_free_op_rcu(struct rcu_head *rcu_head)
+static void bcm_free_op_work(struct work_struct *work)
 {
-	struct bcm_op *op = container_of(rcu_head, struct bcm_op, rcu);
+	struct bcm_op *op = container_of(work, struct bcm_op, work);
+
+	hrtimer_cancel(&op->timer);
+	hrtimer_cancel(&op->thrtimer);
 
 	if ((op->frames) && (op->frames != &op->sframe))
 		kfree(op->frames);

@@ -750,9 +757,23 @@ static void bcm_free_op_rcu(struct rcu_head *rcu_head)
 	if ((op->last_frames) && (op->last_frames != &op->last_sframe))
 		kfree(op->last_frames);
 
+	/* the last possible access to op->timer/op->thrtimer has now
+	 * happened above via hrtimer_cancel() - op->sk is no longer
+	 * needed by any pending timer callback, so drop our reference
+	 */
+	sock_put(op->sk);
+
 	kfree(op);
 }
 
+static void bcm_free_op_rcu(struct rcu_head *rcu_head)
+{
+	struct bcm_op *op = container_of(rcu_head, struct bcm_op, rcu);
+
+	INIT_WORK(&op->work, bcm_free_op_work);
+	queue_work(bcm_wq, &op->work);
+}
+
 static void bcm_remove_op(struct bcm_op *op)
 {
 	hrtimer_cancel(&op->timer);

@@ -1008,6 +1029,7 @@ static int bcm_tx_setup(struct bcm_msg_head *msg_head, struct msghdr *msg,
 
 		/* bcm_can_tx / bcm_tx_timeout_handler needs this */
 		op->sk = sk;
+		sock_hold(sk);
 		op->ifindex = ifindex;
 
 		/* initialize uninitialized (kzalloc) structure */

@@ -1173,6 +1195,7 @@ static int bcm_rx_setup(struct bcm_msg_head *msg_head, struct msghdr *msg,
 
 		/* bcm_can_tx / bcm_tx_timeout_handler needs this */
 		op->sk = sk;
+		sock_hold(sk);
 		op->ifindex = ifindex;
 
 		/* ifindex for timeout events w/o previous frame reception */

@@ -1830,11 +1853,15 @@ static int __init bcm_module_init(void)
 {
 	int err;
 
+	bcm_wq = alloc_workqueue("can-bcm-wq", WQ_UNBOUND, 0);
+	if (!bcm_wq)
+		return -ENOMEM;
+
 	pr_info("can: broadcast manager protocol\n");
 
 	err = register_pernet_subsys(&canbcm_pernet_ops);
 	if (err)
-		return err;
+		goto register_pernet_failed;
 
 	err = register_netdevice_notifier(&canbcm_notifier);
 	if (err)

@@ -1852,6 +1879,8 @@ static int __init bcm_module_init(void)
 	unregister_netdevice_notifier(&canbcm_notifier);
 register_notifier_failed:
 	unregister_pernet_subsys(&canbcm_pernet_ops);
+register_pernet_failed:
+	destroy_workqueue(bcm_wq);
 	return err;
 }

@@ -1860,6 +1889,8 @@ static void __exit bcm_module_exit(void)
 	can_proto_unregister(&bcm_can_proto);
 	unregister_netdevice_notifier(&canbcm_notifier);
 	unregister_pernet_subsys(&canbcm_pernet_ops);
+	rcu_barrier();
+	destroy_workqueue(bcm_wq);
 }
 
 module_init(bcm_module_init);

Signature · 1 line

2.53.0