CVE-2026-97420 in Linux
Summary
by MITRE • 09/24/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: NUL-terminate replaced sysctl value
When writing to sysctls, proc_sys_call_handler() guarantees that the buffer passed to proc handlers is NUL-terminated. If bpf_sysctl_set_new_value() replaces the pending sysctl value, it can hand a replacement buffer directly to proc handlers. However, the helper currently copies only buf_len bytes into that buffer without appending a NUL terminator, leaving downstream parsers vulnerable to out-of-bounds access.
Fix this by appending a '\0' after the replaced value to restore the expected sysctl semantics. Since the helper already rejects buf_len greater than PAGE_SIZE - 1, there is always room for the extra byte.
Reproduced in a QEMU x86_64 guest booted with KASAN while exercising the sysctl replacement path with a cgroup/sysctl BPF program. The reproducer targets `/proc/sys/net/core/flow_limit_cpu_bitmap`, fills the original user write buffer with non-zero bytes, and overrides the sysctl value so the replacement buffer lacks a terminating NUL. Under that setup, the pre-fix kernel reported:
BUG: KASAN: slab-out-of-bounds in strnchrnul+0x72/0x90 Read of size 1 at addr ffff88800de57000 by task repro_patch3/66 CPU: 0 UID: 0 PID: 66 Comm: repro_patch3 Not tainted 7.1.0-rc3-00269-g8370ca1f87cc #6 PREEMPT(lazy) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 Call Trace: <TASK> dump_stack_lvl+0x68/0xa0 print_report+0xcb/0x5e0 ? __virt_addr_valid+0x21d/0x3f0 ? strnchrnul+0x72/0x90 ? strnchrnul+0x72/0x90 kasan_report+0xca/0x100 ? strnchrnul+0x72/0x90 strnchrnul+0x72/0x90 bitmap_parse+0x37/0x2e0 flow_limit_cpu_sysctl+0xc6/0x840 ? __pfx_flow_limit_cpu_sysctl+0x10/0x10 ? __kvmalloc_node_noprof+0x5ba/0x870 proc_sys_call_handler+0x31d/0x480 ? __pfx_proc_sys_call_handler+0x10/0x10 ? selinux_file_permission+0x39f/0x500 ? lock_is_held_type+0x9e/0x120 vfs_write+0x98e/0x1000 ... </TASK> The buggy address is located 0 bytes to the right of allocated 4096-byte region [ffff88800de56000, ffff88800de57000)
With this fix applied, rerunning the same sysctl-targeted path yields no corresponding KASAN reports.
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Analysis
by VulDB Data Team • 09/24/2026
The Linux kernel's BPF subsystem contains a critical memory safety vulnerability within the bpf_sysctl_set_new_value helper function that results in an out-of-bounds read condition when manipulating system control parameters via eBPF programs. This flaw stems from a violation of fundamental string handling conventions expected by the proc filesystem layer, specifically regarding NUL-termination requirements for buffer data passed to sysctl handlers. The vulnerability affects systems where BPF programs are permitted to intercept and modify sysctl values through cgroup or other attached hooks, creating a potential vector for information disclosure or kernel instability depending on how downstream parsers utilize the malformed input.
The technical root cause lies in the interaction between proc_sys_call_handler and custom BPF-driven value replacements. Under normal operation, when user space writes to a sysctl entry, the proc filesystem layer ensures that the buffer passed to specific handler functions is NUL-terminated, allowing string parsing routines like strnchrnul to safely scan for delimiters without exceeding allocated memory boundaries. However, when bpf_sysctl_set_new_value replaces the pending value with data provided by an eBPF program, it performs a direct copy of buf_len bytes into the replacement buffer but fails to append a NUL terminator at index buf_len. This omission leaves the final byte of the allocation uninitialized or containing stale kernel memory content, violating the contract assumed by subsequent string processing functions that expect valid C-style strings within the allocated region.
The operational impact manifests as a slab-out-of-bounds read error when downstream parsers attempt to process the unterminated buffer. As demonstrated in reproduction scenarios using KASAN on QEMU x86_64 environments targeting /proc/sys/net/core/flow_limit_cpu_bitmap, the kernel triggers a BUG report indicating an invalid memory access at strnchrnul. This function is commonly used for scanning strings up to a specified length and relies on encountering a NUL byte or reaching the limit; without proper termination, it may read beyond the allocated 4096-byte region into adjacent slab cache metadata or other kernel structures. While immediate exploitation for arbitrary code execution is unlikely due to the read-only nature of this specific flaw, successful exploitation could lead to denial of service through kernel panic or potentially leak sensitive kernel memory contents if the out-of-bounds data contains cryptographic keys, credentials, or internal state information.
From a classification perspective, this vulnerability aligns with CWE-170 Improper Null Termination and CWE-125 Out-of-bounds Read within the Common Weakness Enumeration framework. In terms of attack vectors, it relates to ATT&CK technique T1046 Network Service Discovery if used for enumeration or potentially T1003 OS Credential Dumping if memory contents are exfiltrated through subsequent exploitation chains involving privilege escalation. The vulnerability specifically impacts configurations where unprivileged users can attach BPF programs that interact with sysctl interfaces, highlighting the risks associated with allowing user-space controlled data to influence kernel-level string processing without rigorous boundary checks and semantic validation.
The remediation involves modifying bpf_sysctl_set_new_value to explicitly append a NUL character after copying buf_len bytes into the replacement buffer. This fix is safe because the helper already enforces that buf_len does not exceed PAGE_SIZE minus one, ensuring there is always at least one byte available in the allocated memory region for the terminator without causing an overflow of its own. By restoring standard sysctl semantics where all processed buffers are guaranteed to be NUL-terminated, the kernel prevents downstream parsers from accessing invalid memory regions. System administrators should apply this patch immediately on affected kernels and review BPF program permissions to ensure that only trusted applications can modify critical system parameters through eBPF hooks, thereby reducing the attack surface for potential abuse of this logic error.