[SRU Focal/linux-hwe-5.13 1/2] can: bcm: delay release of struct bcm_op after synchronize_rcu()

[Thadeu Lima de Souza Cascardo]

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@canonical.com
Cc: linux-stable <stable at vger.kernel.org>
Reported-by: syzbot+0f7e7e5e2f4f40fa89c0 at syzkaller.appspotmail.com
Reported-by: Norbert Slusarek <nslusarek at gmx.net>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
Acked-by: Oliver Hartkopp <socketcan at hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl at pengutronix.de>
(cherry picked from commit d5f9023fa61ee8b94f37a93f08e94b136cf1e463)
CVE-2021-3609
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 net/can/bcm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/can/bcm.c b/net/can/bcm.c
index f3e4d9528fa3..0928a39c4423 100644
--- a/net/can/bcm.c
+++ b/net/can/bcm.c
@@ -785,6 +785,7 @@ static int bcm_delete_rx_op(struct list_head *ops, struct bcm_msg_head *mh,
 						  bcm_rx_handler, op);
 
 			list_del(&op->list);
+			synchronize_rcu();
 			bcm_remove_op(op);
 			return 1; /* done */
 		}
@@ -1533,9 +1534,13 @@ static int bcm_release(struct socket *sock)
 					  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);
+
 #if IS_ENABLED(CONFIG_PROC_FS)
 	/* remove procfs entry */
 	if (net->can.bcmproc_dir && bo->bcm_proc_read)
-- 
2.30.2