# CVE-2020-36789: linux: In the Linux kernel, the following vulnerability has been resolved:  can: dev: can_get_echo_skb(): prevent call to kf...

CVE-2020-36789 is fixed in linux on 1 platform, first by DEBIAN-CVE-2020-36789, released April 17, 2025. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2020-36789
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0019
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 17, 2025

## 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.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/57a3ffb6-3309-3ad5-5a34-e93c3fe3614d@cetitec.com

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.9.9-1 | [DEBIAN-CVE-2020-36789](https://patchblox.com/security/data/fix/linux/debian-cve-2020-36789.md) | Apr 17, 2025 |
| Linux | linux Debian 13.5 (trixie) | 5.9.9-1 | [DEBIAN-CVE-2020-36789](https://patchblox.com/security/data/fix/linux/debian-cve-2020-36789.md) | Apr 17, 2025 |
| Linux | linux Debian 14 | 5.9.9-1 | [DEBIAN-CVE-2020-36789](https://patchblox.com/security/data/fix/linux/debian-cve-2020-36789.md) | Apr 17, 2025 |

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