Vulnerability: .A flaw was found in the CAN BCM networking protocol in the Linux kernel, where a local attacker can abuse a flaw in the CAN subsystem to corrupt memory, crash the system or escalate privileges. This race condition in net/can/bcm.c in the Linux kernel allows for local privilege escalation to root.
Vulnerability ID: CVE-2021-3609
CWE: CWE-362
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2022-03-03
File: net/can/bcm.c
Commit message: can: bcm: delay release of struct bcm_op after synchronize_rcu()

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@vger.kernel.org>
Reported-by: syzbot+0f7e7e5e2f4f40fa89c0@syzkaller.appspotmail.com
Reported-by: Norbert Slusarek <nslusarek@gmx.net>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Commit author: Thadeu Lima de Souza Cascardo

Vulnerable version (before fix):
 static int bcm_delete_rx_op(struct list_head *ops, struct bcm_msg_head *mh, 						  bcm_rx_handler, op);  			list_del(&op->list); 			bcm_remove_op(op); 			return 1; /* done */ 		} static int bcm_release(struct socket *sock) 					  REGMASK(op->can_id), 					  bcm_rx_handler, op);  		bcm_remove_op(op); 	}  #if IS_ENABLED(CONFIG_PROC_FS) 	/* remove procfs entry */ 	if (net->can.bcmproc_dir && bo->bcm_proc_read)

Fixed version (after fix):
 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 */ 		} static int bcm_release(struct socket *sock) 					  REGMASK(op->can_id), 					  bcm_rx_handler, 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)

Patch:
patch: @@ -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)
path: net/can/bcm.c