CVE-2026-98042
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Don't resurrect a scalar id dropped by collect_linked_regs()
check_cond_jmp_op() copies the compared registers into env->{false,true}_reg{1,2} before collect_linked_regs() runs and copies
those snapshots back into both branch states afterwards.
collect_linked_regs() records at most LINKED_REGS_MAX members of a linked registers group in the jump history and calls clear_scalar_id() for every member that does not fit. The compared register is not exempt from that.
As a consequence, sync_linked_regs() might adjust ranges for more registers than bpf_bt_sync_linked_regs() can propagate precision to.
Collect the linked registers before the snapshots are taken instead. This might lead to some unnecessary clear_scalar_id's, but from previous testing situations with many linked registers are extremely rare.
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Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel contains a logic flaw within its eBPF verifier subsystem related to how scalar register identifiers are managed during conditional jump operations. The vulnerability arises in the interaction between check_cond_jmp_op and collect_linked_regs functions, which handle the tracking of register states across branching paths. When evaluating a conditional jump, the verifier copies the current state of compared registers into temporary storage structures designated for false and true branch outcomes. Subsequently, the system invokes collect_linked_regs to analyze groups of linked registers that share dependencies or constraints. This function records up to a maximum number of members from each linked register group within the jump history context. For any registered member exceeding this limit, the verifier calls clear_scalar_id to discard specific scalar identity information, effectively simplifying the state representation for those registers.
The critical technical flaw is that the compared register involved in the conditional operation is not exempted from this clearing process. Because collect_linked_regs operates on a snapshot of the registers taken before its execution, it may inadvertently clear the scalar identifier for one or both of the compared registers if they are part of a large linked group. This action disrupts the subsequent synchronization phase where sync_linked_regs attempts to adjust value ranges based on the collected history. The precision propagation mechanism within bpf_bt_sync_linked_regs is not designed to handle adjustments for more registers than it can accurately track, leading to potential inconsistencies in how register states are resolved after the jump decision.
This inconsistency results in a state desynchronization where the verifier may lose critical information about variable ranges or types that were previously established. In practical terms, this could lead to incorrect assumptions by the eBPF program loader regarding the safety of certain operations performed on these registers. While immediate exploitation might be complex due to the specific conditions required involving many linked registers, such state corruption can potentially allow an attacker to bypass security checks embedded within eBPF programs. This undermines the integrity guarantees provided by the verifier, which is essential for preventing memory safety violations and privilege escalation in kernel space.
To mitigate this vulnerability, the fix involves reordering the operations so that collect_linked_regs executes before the snapshots of register states are taken. By collecting linked register information prior to capturing the state for branch evaluation, the system ensures that the clearing logic does not interfere with the integrity of the compared registers used in conditional jumps. Although this change may result in some unnecessary calls to clear_scalar_id due to the broader scope of analysis, it prevents the loss of critical scalar identity data needed for accurate range propagation and precision tracking. This adjustment aligns the verifier's behavior with its intended design, ensuring that register states remain consistent across branch boundaries.
From a classification perspective, this issue falls under CWE-691 which relates to use of insufficiently trusted or unvalidated input leading to unexpected state changes, specifically within the context of internal logic flow and data integrity. The vulnerability also touches upon aspects covered by ATT&CK technique T1059 related to command and script interpretation, as eBPF programs are interpreted by the kernel verifier before execution. Ensuring that the verifier correctly maintains register states is crucial for maintaining the security boundary between user-space applications and the Linux kernel core. Administrators should apply available kernel updates or patches that address this specific logic error in the BPF subsystem to prevent potential exploitation scenarios arising from state desynchronization during complex eBPF program verification processes.