CVE-2026-92518 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
riscv, bpf: Fix kernel stack corruption in tailcall with CFI
When CONFIG_CFI_CLANG is enabled, prog->bpf_func already skips the kcfi instruction during setup. Including it again in the tailcall jump offset causes it to jump over an extra 4 bytes, skipping the stack pointer adjustment, which will result in kernel stack corruption.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel's Berkeley Packet Filter subsystem contains a critical implementation flaw related to Control Flow Integrity enforcement on RISC-V architectures when compiled with Clang and the CONFIG_CFI_CLANG configuration option enabled. This vulnerability arises from an incorrect handling of function pointers during tail call operations, which are used in eBPF programs to invoke other BPF functions without returning to the caller immediately. The root cause lies in how the kernel calculates the jump offset for these indirect calls when CFI is active. Specifically, the bpf_func pointer already accounts for and skips over the kcfi instruction inserted by Clang during program setup. However, the logic governing tail call jumps erroneously includes this adjustment again when computing the final target address.
This double counting of the skip offset results in a miscalculated jump destination that is four bytes ahead of the intended entry point of the called function. On RISC-V architectures, these initial bytes typically contain instructions necessary for proper stack frame setup or other architectural-specific prologue operations. By jumping over this critical code segment, specifically skipping the instruction responsible for adjusting the stack pointer, the kernel fails to allocate sufficient space on the current execution stack for local variables and return addresses required by the callee function. This deviation from expected control flow leads directly to kernel stack corruption, as subsequent writes to what should be valid stack memory locations may overwrite adjacent data structures or critical state information due to misaligned offsets.
The operational impact of this vulnerability is severe, potentially leading to arbitrary code execution if an attacker can craft malicious eBPF programs that trigger these tail calls under specific conditions. Kernel stack corruption often manifests as unpredictable system crashes, data leakage through memory disclosure vulnerabilities, or complete privilege escalation from user space to kernel mode. Since BPF programs are frequently used for network filtering and observability tasks within containers and cloud environments, the ability to exploit this flaw could compromise the isolation boundaries of virtualized workloads. The issue is particularly insidious because it only occurs when CFI is enabled, a security feature designed specifically to prevent such control flow hijacking attacks, yet its implementation here inadvertently creates an exploitable condition due to incorrect offset arithmetic.
Mitigation strategies primarily involve applying the upstream kernel patch that corrects the tail call jump offset calculation logic for RISC-V systems with Clang-based CFI support. System administrators should ensure their kernels are updated to versions where this fix is included, particularly if they utilize eBPF features on RISC-V hardware or emulated environments. For organizations unable to immediately update kernel binaries, disabling CONFIG_CFI_CLANG in the build configuration serves as a temporary workaround, although this reduces overall system security posture by removing control flow integrity protections. Long-term remediation requires rigorous code review processes for architecture-specific BPF implementations and automated testing suites that verify stack alignment and function entry points under CFI-enabled builds to prevent similar arithmetic errors in future kernel releases. This vulnerability is categorized under CWE-787 Out-of-bounds Write as it involves writing to incorrect memory locations due to improper pointer arithmetic, and aligns with ATT&CK techniques related to privilege escalation via exploitation of software vulnerabilities within the operating system layer.