This is a note to let you know that I've just added the patch titled can: bcm: delay release of struct bcm_op after synchronize_rcu() to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: can-bcm-delay-release-of-struct-bcm_op-after-synchronize_rcu.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From d5f9023fa61ee8b94f37a93f08e94b136cf1e463 Mon Sep 17 00:00:00 2001 From: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> Date: Sat, 19 Jun 2021 13:18:13 -0300 Subject: can: bcm: delay release of struct bcm_op after synchronize_rcu() From: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> commit d5f9023fa61ee8b94f37a93f08e94b136cf1e463 upstream. can_rx_register() callbacks may be called concurrently to the call to can_rx_unregister(). The callbacks and callback data, though, are protected by RCU and the struct sock reference count. So the callback data is really attached to the life of sk, meaning that it should be released on sk_destruct. However, bcm_remove_op() calls tasklet_kill(), and RCU callbacks may be called under RCU softirq, so that cannot be used on kernels before the introduction of HRTIMER_MODE_SOFT. However, bcm_rx_handler() is called under RCU protection, so after calling can_rx_unregister(), we may call synchronize_rcu() in order to wait for any RCU read-side critical sections to finish. That is, bcm_rx_handler() won't be called anymore for those ops. So, we only free them, after we do that synchronize_rcu(). Fixes: ffd980f976e7 ("[CAN]: Add broadcast manager (bcm) protocol") Link: https://lore.kernel.org/r/20210619161813.2098382-1-cascardo@xxxxxxxxxxxxx Cc: linux-stable <stable@xxxxxxxxxxxxxxx> Reported-by: syzbot+0f7e7e5e2f4f40fa89c0@xxxxxxxxxxxxxxxxxxxxxxxxx Reported-by: Norbert Slusarek <nslusarek@xxxxxxx> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> Acked-by: Oliver Hartkopp <socketcan@xxxxxxxxxxxx> Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/can/bcm.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) --- a/net/can/bcm.c +++ b/net/can/bcm.c @@ -813,6 +813,7 @@ static int bcm_delete_rx_op(struct list_ bcm_rx_handler, op); list_del(&op->list); + synchronize_rcu(); bcm_remove_op(op); return 1; /* done */ } @@ -1538,9 +1539,13 @@ static int bcm_release(struct socket *so REGMASK(op->can_id), bcm_rx_handler, op); - bcm_remove_op(op); } + synchronize_rcu(); + + list_for_each_entry_safe(op, next, &bo->rx_ops, list) + bcm_remove_op(op); + /* remove procfs entry */ if (proc_dir && bo->bcm_proc_read) remove_proc_entry(bo->procname, proc_dir); Patches currently in stable-queue which might be from cascardo@xxxxxxxxxxxxx are queue-4.4/can-bcm-delay-release-of-struct-bcm_op-after-synchronize_rcu.patch