can: dev: can_get_echo_skb(): prevent call to kfree_skb() in hard IRQ context

In the Linux kernel, the following vulnerability has been resolved:

can: dev: can_get_echo_skb(): prevent call to kfree_skb() in hard IRQ context

If a driver calls can_get_echo_skb() during a hardware IRQ (which is often, but

not always, the case), the 'WARN_ON(in_irq)' in

net/core/skbuff.c#skb_release_head_state() might be triggered, under network

congestion circumstances, together with the potential risk of a NULL pointer

dereference.

The root cause of this issue is the call to kfree_skb() instead of

dev_kfree_skb_irq() in net/core/dev.c#enqueue_to_backlog().

This patch prevents the skb to be freed within the call to netif_rx() by

incrementing its reference count with skb_get(). The skb is finally freed by

one of the in-irq-context safe functions: dev_consume_skb_any() or

dev_kfree_skb_any(). The "any" version is used because some drivers might call

can_get_echo_skb() in a normal context.

The reason for this issue to occur is that initially, in the core network

stack, loopback skb were not supposed to be received in hardware IRQ context.

The CAN stack is an exeption.

This bug was previously reported back in 2017 in [1] but the proposed patch

never got accepted.

While [1] directly modifies net/core/dev.c, we try to propose here a

smoother modification local to CAN network stack (the assumption

behind is that only CAN devices are affected by this issue).

[1] http://lore.kernel.org/r/[email protected]