CVE-2026-90210 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix UAF in bpf_trampoline_multi_attach_free on update failure
When bpf_trampoline_update() fails before modify_fentry_multi()/ unregister_fentry_multi() is called, cur_image is unchanged (cur_image == old_image) and ftrace still calls into it. Freeing old_image in that case causes a UAF.
Only free old_image when it differs from cur_image.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel contains a critical use-after-free vulnerability within the BPF trampoline subsystem, specifically affecting the bpf_trampoline_multi_attach_free function during update operations. This flaw arises from an improper handling of memory deallocation when the bpf_trampoline_update routine encounters a failure condition prior to completing its full lifecycle of modifying or unregistering fentry multi structures. Under normal circumstances, the kernel maintains references to current and old image states to manage transitions between different BPF program attachments safely. However, when the update process fails early, the cur_image pointer remains unchanged and continues to point to the same memory location as the old_image pointer.
The core technical flaw lies in the unconditional freeing of the old_image structure regardless of whether it has been superseded by a new image or not. Because ftrace mechanisms continue to invoke the trampoline code through the existing cur_image reference, any attempt to free that same memory block results in a use-after-free condition. This means that subsequent accesses to the freed memory can lead to undefined behavior, potentially causing kernel panics, data corruption, or arbitrary code execution if an attacker can influence how the stale pointer is dereferenced after deallocation. The vulnerability highlights a race-like logic error where state consistency checks are insufficient before performing destructive operations on shared resources.
From a classification perspective, this issue aligns with CWE-416, which describes use-after-free vulnerabilities resulting from improper memory management practices. In terms of the MITRE ATT&CK framework for enterprise security, such flaws can be leveraged in techniques related to privilege escalation or defense evasion by exploiting kernel memory corruption to gain higher-level access or disrupt system stability. The impact is significant as it affects the integrity and availability of the operating system core, particularly in environments heavily reliant on eBPF programs for observability, networking, and security monitoring functions that depend on dynamic trampoline updates.
Mitigation strategies primarily involve applying vendor-provided kernel patches that correct the logic to verify whether cur_image differs from old_image before proceeding with deallocation operations. System administrators should ensure their systems are updated to versions where this specific conditional check is implemented correctly. Additionally, developers integrating BPF trampolines into custom modules must review their update paths to ensure they handle failure cases by preserving existing references rather than freeing them prematurely. Continuous monitoring of kernel logs for oops or panic traces can help detect exploitation attempts in real-time while patching processes are underway.