CVE-2026-68187: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-68187 Overview

CVE-2026-68187 is a Linux kernel vulnerability in the transfer_args_to_stack() function in fs/exec.c. The flaw affects only CONFIG_MMU=n (nommu) builds, where the function is used by binfmt_flat and binfmt_elf_fdpic. An unsigned loop counter wraps to ULONG_MAX when bprm->p drops below PAGE_SIZE, causing the kernel to read from an attacker-influenced garbage pointer and copy PAGE_SIZE bytes of arbitrary memory into the new process stack in a non-terminating loop. The condition and decrement are unchanged since 2.6.12-rc2, making this a longstanding defect in the exec path on nommu Linux systems.

Critical Impact
A single argument or environment string of slightly over 31 pages triggers a kernel panic on nommu Linux builds, with potential for arbitrary kernel memory disclosure into user-controlled process stacks.

Affected Products

Linux kernel builds with CONFIG_MMU=n (nommu) using binfmt_flat
Linux kernel builds with CONFIG_MMU=n (nommu) using binfmt_elf_fdpic
Kernel versions inheriting the loop from 2.6.12-rc2 through the fix in the referenced stable commits

Discovery Timeline

2026-08-10 - CVE-2026-68187 published to NVD
2026-08-10 - Last updated in NVD database

Technical Details for CVE-2026-68187

Vulnerability Analysis

The defect resides in transfer_args_to_stack() in fs/exec.c. The loop uses an unsigned long index that counts downward from a starting page toward a stop value derived from bprm->p >> PAGE_SHIFT. When bprm->p falls below PAGE_SIZE, stop becomes zero, and after the index == 0 iteration the decrement wraps to ULONG_MAX. The wrapped index reads bprm->page[ULONG_MAX], returning sizeof(void *) bytes preceding the array. The corrupted pointer, effectively -1, is then passed to kmap_local_page(), and PAGE_SIZE bytes are copied from an arbitrary memory location into the stack of the process being created. The loop also fails to terminate.

Root Cause

The root cause is a numeric truncation and unsigned integer underflow in the loop counter. On systems with an MMU, bprm_set_stack_limit() and bprm_hit_stack_limit() bound bprm->p. On nommu builds these functions are empty, so valid_arg_len() is the only constraint on how far bprm->p can be pushed down. A crafted argv or envp string of just over 31 pages leaves bprm->p inside the first page and triggers the wrap. The fix counts down from MAX_ARG_PAGES so the loop terminates correctly when index reaches stop, including the stop == 0 case.

Attack Vector

A local unprivileged user on an affected nommu Linux system executes a binary via execve() while supplying a single argument or environment string of a little over 31 pages. This drives bprm->p below PAGE_SIZE, triggers the loop counter wrap during argument transfer, and causes a load access fault. The observed crash occurs in __memcpy invoked from transfer_args_to_stack during load_flat_binary, culminating in Kernel panic - not syncing: Fatal exception in interrupt. The primary consequence is denial of service, with a secondary risk of kernel memory disclosure into the new process address space before the panic.

Detection Methods for CVE-2026-68187

Indicators of Compromise

Kernel oops or panic messages referencing __memcpy, transfer_args_to_stack, and load_flat_binary or load_elf_fdpic_binary in the call stack.
Oops - load access fault entries with badaddr values in unmapped kernel virtual ranges on nommu systems.
Repeated execve failures or unexpected system reboots on embedded nommu Linux devices immediately after process creation.

Detection Strategies

Monitor dmesg and serial console output on nommu embedded systems for panics originating in bprm_execve and do_execveat_common.
Audit execve telemetry for processes launched with unusually large single argv or envp entries exceeding 31 pages (approximately 127 KB).
Correlate kernel crash dumps against the vulnerable call chain: __riscv_sys_execve -> do_execveat_common -> bprm_execve -> load_flat_binary -> transfer_args_to_stack.

Monitoring Recommendations

Centralize kernel logs from embedded and IoT nommu devices to a data lake for pattern analysis of exec-related faults.
Baseline normal execve argument sizes and alert on outliers that approach or exceed MAX_ARG_PAGES * PAGE_SIZE.
Track kernel version and CONFIG_MMU build configuration across fleet inventory to identify exposed devices.

How to Mitigate CVE-2026-68187

Immediate Actions Required

Identify all Linux devices built with CONFIG_MMU=n that use binfmt_flat or binfmt_elf_fdpic.
Apply the upstream fix that inverts the loop to count down from MAX_ARG_PAGES so termination is safe when stop == 0.
Restrict local execution privileges on affected embedded systems until patches are deployed.

Patch Information

The fix is available in the mainline and stable trees via the following commits: Kernel Git Commit 16cc4f5, Kernel Git Commit 2bc6bf7, Kernel Git Commit 55fa2c7, Kernel Git Commit c62bb00, and Kernel Git Commit dfc2a00. Rebuild affected kernels from a stable branch containing one of these commits and redeploy firmware to nommu targets.

Workarounds

Rebuild kernels with CONFIG_MMU=y where the hardware permits, since MMU builds are not affected.
Disable binfmt_flat and binfmt_elf_fdpic if the workload does not require them on affected devices.
Constrain trusted execution paths so that untrusted users cannot invoke execve with attacker-controlled argv or envp of arbitrary size.

bash

# Verify whether a running kernel is built without an MMU
zcat /proc/config.gz 2>/dev/null | grep -E '^CONFIG_MMU|^CONFIG_BINFMT_FLAT|^CONFIG_BINFMT_ELF_FDPIC'

# Confirm installed kernel version against a fixed stable release
uname -r