CVE-2026-90393 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix potential UAF in bpf_netns_link_update_prog
In bpf_netns_link_update_prog, the checks for old_prog and prog type are currently performed locklessly before acquiring netns_bpf_mutex. This creates a race condition that can lead to a UAF issue.
If two threads concurrently execute BPF_LINK_UPDATE on the same netns link, the following execution path can trigger a UAF:
CPU0 CPU1 bpf_netns_link_update_prog if (old_prog && old_prog != link->prog) return -EPERM; bpf_netns_link_update_prog if (old_prog && old_prog != link->prog) ... old_prog = xchg(&link->prog, new_prog); bpf_prog_put(old_prog); if (new_prog->type != link->prog->type) <-- trigger UAF
Fix this by moving the old_prog and prog->type checks inside the netns_bpf_mutex critical section. Meanwhile, use guard() to simplify lock management and avoid all the goto jumping.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel contains a race condition vulnerability within the BPF subsystem specifically in the bpf_netns_link_update_prog function that can lead to a Use-After-Free scenario when updating program links associated with network namespaces. This flaw arises from an improper synchronization mechanism where critical validation checks are performed without holding the necessary mutex locks, creating a window of opportunity for concurrent threads to manipulate shared state unsafely. The vulnerability is classified under CWE-362 which denotes Concurrent Execution using Shared Resource with Improper Synchronization resulting in Race Conditions and falls within MITRE ATT&CK technique T1059 related to Command and Scripting Interpreter execution contexts where kernel-level logic errors can be exploited for privilege escalation or denial of service.
The technical root cause lies in the sequence of operations during a BPF_LINK_UPDATE system call when targeting an existing network namespace link. The original implementation performs checks on the old program pointer and validates that the new program type matches the current link's program type before acquiring the netns_bpf_mutex. This lockless approach assumes atomicity where none exists, allowing two concurrent threads to pass these initial validity checks simultaneously. Thread CPU0 might evaluate the condition regarding the old program and determine it is safe to proceed, while concurrently thread CPU1 executes a similar check and proceeds to update the link's program pointer using an exchange operation that replaces the current program with a new one.
This concurrency leads directly to a Use-After-Free situation because once CPU1 updates the link pointer via xchg and subsequently releases its reference to the old program, the memory backing that old program may be freed if no other references exist. If CPU0 then proceeds to access or validate against what it believes is still the original program structure but which has now been replaced and potentially deallocated by CPU1, a null pointer dereference or arbitrary code execution becomes possible depending on how quickly the kernel reuses that memory region. The specific trigger occurs when checking if the new program type matches link->prog after the race condition has already altered the state of link->prog, causing access to freed memory.
The operational impact of this vulnerability is severe as it allows local attackers with sufficient privileges to execute arbitrary code within the kernel context or cause a system crash leading to denial of service conditions. By exploiting this race condition an attacker can corrupt kernel data structures potentially gaining full control over the affected Linux system. The risk is particularly acute in multi-core environments where concurrent BPF link updates are more likely, and it affects systems running recent versions of the Linux kernel that include the bpf_netns_link_update_prog implementation without the subsequent patch applied.
The resolution involves moving both the old program validation checks and the new program type verification inside the netns_bpf_mutex critical section to ensure mutual exclusion during these state-changing operations. This guarantees that only one thread can modify or inspect the link's program pointer at any given time preventing the race condition from manifesting. Additionally the fix utilizes guard macros for lock management which simplifies code structure by automatically handling unlock logic upon scope exit thereby reducing complexity and eliminating potential errors associated with manual goto-based cleanup patterns often found in kernel locking implementations.
To mitigate this vulnerability administrators should apply the latest available security patches to their Linux systems ensuring that the bpf subsystem is updated to include these synchronization fixes. Organizations relying on BPF for network monitoring or security enforcement must verify that their deployment environments are not exposed to concurrent link update operations without proper patching since unpatched kernels remain susceptible to exploitation by privileged local users aiming to escalate privileges or disrupt system availability through kernel panic induction via memory corruption.