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] https://patchwork.ozlabs.org/patch/835236/

Signed-off-by: Vincent Mailhol <redacted>
---

Changes in v3: None

Changes in v2:
 - Minor changes of link format in the changelog.
---
 drivers/net/can/dev.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
index 68834a2853c9..e291fda395a0 100644
--- a/drivers/net/can/dev.c
+++ b/drivers/net/can/dev.c

@@ -512,7 +512,11 @@ unsigned int can_get_echo_skb(struct net_device *dev, unsigned int idx)
 	if (!skb)
 		return 0;
 
-	netif_rx(skb);
+	skb_get(skb);
+	if (netif_rx(skb) == NET_RX_SUCCESS)
+		dev_consume_skb_any(skb);
+	else
+		dev_kfree_skb_any(skb);
 
 	return len;
 }

-- 
2.26.2