CVE-2026-72427 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Fix effective prog array index with BPF_F_PREORDER
replace_effective_prog() and purge_effective_progs() located the slot in the effective array by walking the program hlist and counting entries linearly. That count does not match the array layout: compute_effective_ progs() places BPF_F_PREORDER programs at the front (ancestor cgroup first, attach order within a cgroup) and the rest after them (descendant cgroup first). So when a preorder program is present, the linear hlist position no longer equals the program's index in the effective array.
For replace_effective_prog() (bpf_link_update()) this overwrote the wrong slot, corrupting the effective order. For purge_effective_progs(), it could dummy out a slot belonging to a different program and leave the detached program in the array while bpf_prog_put() drops its reference, i.e. a use-after-free.
Fix both by replaying compute_effective_progs()'s placement (including the per-cgroup preorder reversal) in a shared effective_prog_pos() helper. Identify the entry by its struct bpf_prog_list pointer rather than by (prog, link) value, so the lookup resolves to exactly the attachment the syscall selected even when the same bpf_prog is attached to several cgroups in the hierarchy.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical flaw in the Linux kernel's eBPF (extended Berkeley Packet Filter) subsystem that affects how program arrays are managed during BPF link updates and program purging operations. This issue specifically impacts the handling of BPF_F_PREORDER flag functionality within the effective program array structure, creating a fundamental mismatch between the logical positioning of programs and their actual storage locations.
The technical root cause stems from inconsistent array indexing mechanisms within the BPF subsystem's internal data structures. The replace_effective_prog() and purge_effective_progs() functions were designed to locate target slots in the effective program array by performing linear traversal of the program hash list and counting entries sequentially. However, this approach fails to account for the actual layout algorithm implemented by compute_effective_progs(), which deliberately places BPF_F_PREORDER programs at the beginning of the array following a specific ordering rule that prioritizes ancestor cgroups first, followed by descendant cgroups in attachment order within each group.
This mismatch creates severe operational consequences that extend beyond simple data corruption. When replace_effective_prog() attempts to update a program array entry, it writes to an incorrect memory location due to the misaligned indexing, thereby corrupting the effective program ordering sequence. More critically, purge_effective_progs() suffers from a use-after-free condition where the function incorrectly identifies and clears a slot belonging to a different program while leaving the detached program reference intact in the array structure, creating a dangerous state where freed memory may be accessed later.
The vulnerability directly relates to CWE-121 and CWE-415 categories related to buffer overflow conditions and double free issues, while also mapping to ATT&CK techniques involving privilege escalation through kernel exploitation. The fix implemented addresses this by introducing a shared helper function called effective_prog_pos() that replicates the exact placement logic from compute_effective_progs(), including the per-cgroup preorder reversal behavior. This approach ensures consistent indexing across all operations and prevents the linear hash list traversal errors.
The solution specifically addresses how program entries are identified within the system by shifting from value-based lookups using (prog, link) pairs to pointer-based resolution through struct bpf_prog_list pointers. This change ensures that even when identical BPF programs are attached to multiple cgroups throughout the hierarchy, the lookup mechanism correctly resolves to the specific attachment point selected by the syscall rather than potentially matching any instance of the same program. This refinement prevents scenarios where operations might inadvertently target the wrong program entry within the effective array structure.
The operational impact of this vulnerability extends to all systems running Linux kernels with BPF support, particularly those utilizing complex cgroup hierarchies with multiple BPF program attachments. The use-after-free condition creates potential for privilege escalation attacks and system instability, while the incorrect replacement behavior could lead to unexpected program execution flows that compromise security policies and network filtering rules implemented through eBPF programs. Organizations should prioritize updating their kernel versions to address this vulnerability, as it represents a fundamental flaw in how BPF programs are managed within the kernel's security framework.
The fix demonstrates proper kernel security engineering principles by maintaining consistency between different subsystem components and ensuring that all array access operations respect the same underlying data layout rules. The shared helper function approach eliminates code duplication while providing a centralized mechanism for program positioning that can be easily audited and maintained. This solution aligns with industry best practices for kernel security updates and reflects the importance of maintaining consistent state management across concurrent operations in critical system components.