CVE-2026-98380 in Linux
Summary
by MITRE • 10/09/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: reject IDR error pointers when deleting actions
tcf_action_delete() drops the reference held by its lookup before calling tcf_idr_delete_index() with the saved action index. An unlocked classifier can remove that action and reserve the same IDR slot with ERR_PTR(-EBUSY) in between.
tcf_idr_delete_index() only checks the lookup result for NULL. It therefore treats the reservation as a tc_action and dereferences tcfa_bindcnt. A hardware execution breakpoint was used to schedule the interleaving without changing the kernel source. KASAN reported this decoded trace:
BUG: KASAN: null-ptr-deref in tca_action_gd+0x5b9/0x1010 Read of size 4 at addr 0000000000000010 by task poc/150 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002 RIP: tca_action_gd+0x5c0/0x1010: arch_atomic_read at arch/x86/include/asm/atomic.h:23 raw_atomic_read at include/linux/atomic/atomic-arch-fallback.h:457 atomic_read at include/linux/atomic/atomic-instrumented.h:33 tcf_idr_delete_index at net/sched/act_api.c:766 tcf_action_delete at net/sched/act_api.c:1859 tcf_del_notify at net/sched/act_api.c:2014 tca_action_gd at net/sched/act_api.c:2064 R13: 0000000000000010 R15: fffffffffffffff0 Kernel panic - not syncing: Fatal exception
R15 contains ERR_PTR(-EBUSY), and adding the tcfa_bindcnt offset produces the address in R13. With the guard applied, the same reproducer returned -ENOENT without a KASAN report or panic. Treat error pointers as absent and return -ENOENT.
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Analysis
by VulDB Data Team • 10/09/2026
The Linux kernel networking subsystem contains a race condition vulnerability within the traffic control action deletion logic that can lead to a general protection fault and subsequent kernel panic. This issue resides in the net/sched module, specifically affecting how actions are removed from the identifier resource (IDR) tree used for managing network classifier configurations. The core of the flaw lies in the tcf_action_delete function, which is responsible for removing an action associated with a specific index. During this process, the function first drops the reference count held by its lookup operation and then proceeds to call tcf_idr_delete_index using the saved action index. This sequence creates a critical window where the system state is inconsistent if another thread or context intervenes between dropping the reference and performing the deletion.
An unlocked classifier has the capability to remove an existing action and subsequently reserve the same IDR slot, returning an error pointer representing -EBUSY instead of a valid object address. The tcf_idr_delete_index function was originally designed with insufficient validation logic; it only checked whether the lookup result was NULL before proceeding. It failed to account for the possibility that the returned value could be an encoded error pointer rather than a valid memory address or null. Consequently, when the reserved slot containing ERR_PTR(-EBUSY) is passed into this function, the code incorrectly treats this error marker as a legitimate tc_action structure. The vulnerability manifests when the kernel attempts to dereference the tcfa_bindcnt field of this invalid object. Since an error pointer encodes negative errno values in its lower bits, adding the offset for tcfa_bindcnt results in accessing a non-canonical or highly unlikely memory address, triggering a general protection fault.
The operational impact of this vulnerability is severe, as it allows a local user with appropriate privileges to trigger a denial-of-service condition by crashing the kernel. The provided technical details indicate that using hardware execution breakpoints to schedule the interleaving without modifying source code successfully reproduced the issue. KASAN (Kernel Address Sanitizer) reports confirm a null-pointer dereference or general protection fault, leading to a fatal exception and system halt. This type of vulnerability is classified under CWE-362: Concurrent Execution Using Shared Resource with Improper Synchronization, as it involves a race condition where shared resources are accessed without proper locking mechanisms ensuring atomicity. Furthermore, from an offensive security perspective, this aligns with ATT&CK technique T1059: Command and Scripting Interpreter if exploited via scriptable interfaces like tc commands to crash the system, or more broadly under privilege escalation vectors that rely on kernel instability for further exploitation attempts.
To mitigate this vulnerability, developers must ensure that tcf_idr_delete_index properly validates its input by checking for error pointers in addition to null values. The fix involves treating any returned value that is an error pointer as absent from the IDR tree and returning -ENOENT rather than attempting to dereference it. This prevents the kernel from accessing invalid memory addresses derived from encoded error codes. System administrators should apply the relevant kernel patch updates immediately upon availability, particularly for systems running traffic control configurations where concurrent modifications are possible. Regular auditing of network configuration scripts can also help identify potential race conditions in complex deployment scenarios involving dynamic rule adjustments.