CVE-2020-36789 Information

Description

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.cskb_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.cenqueue_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 ny\ 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/57a3ffb6-3309-3ad5-5a34-e93c3fe3614d@cetitec.com

Reference

https://git.kernel.org/stable/c/2283f79b22684d2812e5c76fc2280aae00390365 https://git.kernel.org/stable/c/248b71ce92d4f3a574b2537f9838f48e892618f4 https://git.kernel.org/stable/c/3a922a85701939624484e7f2fd07d32beed00d25 https://git.kernel.org/stable/c/451187b20431924d13fcfecc500d7cd2d9951bac https://git.kernel.org/stable/c/7e4cf2ec0ca236c3e5f904239cec6efe1f3baf22 https://git.kernel.org/stable/c/87530b557affe01c764de32dbeb58cdf47234574 https://git.kernel.org/stable/c/ab46748bf98864f9c3f5559060bf8caf9df2b41e