CVE-2021-4454 in Linux
Summary
by MITRE • 03/27/2025
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
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Analysis
by VulDB Data Team • 12/06/2025
The vulnerability described in CVE-2021-4454 affects the Linux kernel's J1939 CAN transport layer implementation, specifically within the j1939_session_deactivate function. This issue represents a false positive warning condition that arises from incorrect assumptions about reference counting behavior in concurrent kernel execution contexts. The problem manifests as a spurious WARN_ON_ONCE message that should not occur under normal operational conditions, indicating a mismatch between the kernel's internal assumptions and actual runtime behavior.
The technical flaw stems from an incorrect assertion within the j1939_session_deactivate function that assumes the session reference count should always be at least two when the function is called. However, concurrent execution scenarios can lead to situations where j1939_session_deactivate is invoked with a reference count less than two. This race condition occurs during complex packet processing workflows involving multiple kernel threads handling CAN transport layer operations simultaneously. The kernel's warning mechanism triggers when the system detects a reference count below the expected threshold, even though the actual implementation correctly handles such scenarios through proper state checking in j1939_session_deactivate_locked.
The operational impact of this vulnerability primarily affects systems utilizing J1939 CAN networking protocols, particularly those implementing automotive or industrial control systems where CAN bus communication is critical. While the warning itself does not cause system crashes or data corruption, it generates false positive error messages that can complicate system diagnostics and monitoring efforts. The vulnerability demonstrates a classic race condition issue in kernel space programming where assumptions about concurrent access patterns do not account for all possible execution paths. This type of issue falls under CWE-362, which addresses race conditions in concurrent programming, and aligns with ATT&CK techniques related to system reconnaissance and kernel-level privilege escalation through false positive error generation.
The fix implemented resolves this issue by removing the erroneous WARN_ON_ONCE check that was based on incorrect assumptions about reference counting. The solution maintains the existing functionality while eliminating the false positive warning messages that were being generated during legitimate concurrent operations. The corrected implementation properly validates session active states before proceeding with session cleanup operations, ensuring that legitimate concurrent access patterns do not trigger unnecessary warnings. This approach follows best practices for kernel development by maintaining functional correctness while eliminating noise in system logging that could obscure actual problems. The resolution addresses the underlying concurrency issue without changing the fundamental behavior of the J1939 transport layer, making it a targeted fix that preserves existing system functionality while improving reliability. The fix specifically targets the j1939_session_deactivate function and related session management code, ensuring that legitimate concurrent scenarios no longer trigger false warnings while maintaining all necessary safety checks for actual error conditions.