CVE-2026-68435 Overview
CVE-2026-68435 is a Linux kernel vulnerability affecting the LoongArch architecture. The flaw resides in the kexec_load(2) code path within arch/loongarch/kernel/machine_kexec.c. When the kernel searches loaded segments for the "kexec" command line marker, it passes a user-space segment buffer directly to strncmp() through an incorrect (char __user *) cast. This dereferences a user pointer while running in kernel context. The issue was flagged by the sparse static analysis tool as an address space mismatch. The upstream fix copies the marker-sized prefix of each segment into an on-stack buffer using copy_from_user() before comparison and skips segments that fault.
Critical Impact
The kernel dereferences a user-controlled pointer in kernel context during kexec segment inspection, an unsafe access pattern on the LoongArch kexec path.
Affected Products
- Linux kernel — LoongArch architecture
- Kernel builds exposing kexec_load(2) on LoongArch
- Distributions shipping vulnerable LoongArch kernels prior to the fix commits
Discovery Timeline
- 2026-08-12 - CVE-2026-68435 published to NVD
- 2026-08-12 - Last updated in NVD database
Technical Details for CVE-2026-68435
Vulnerability Analysis
The defect is in the LoongArch kexec implementation at arch/loongarch/kernel/machine_kexec.c:84. During kexec_load(2) handling (when file_mode == 0), the kernel iterates the loaded segments to locate the marker string "kexec" that identifies the command line segment. The buffer being compared originates from user space, but the code casts it to (char __user *) and passes it directly to strncmp(), a routine that expects kernel pointers. The kernel therefore reads user memory using kernel-mode dereference semantics without validating the pointer or copying the data in first.
Sparse reports the mismatch as incorrect type in argument 2 (different address spaces) ... expected char const * ... got char [noderef] __user *. Beyond the annotation error, the pattern is unsafe: the user pointer is not checked with access_ok() semantics through a proper accessor, and a bad user address can trigger a fault in kernel context.
Root Cause
The root cause is improper handling of a user-space pointer inside kernel code, specifically the misuse of a (char __user *) cast to satisfy the compiler while calling a kernel-pointer API. This is a kernel-vulnerability class issue tied to address-space annotation and input validation on the kexec segment lookup path.
Attack Vector
Exploitation requires local privileges sufficient to invoke kexec_load(2), typically CAP_SYS_BOOT. A caller controls the segment buffers passed to the syscall and can therefore influence the memory that the kernel dereferences during the marker search. The attack surface is limited to LoongArch systems where kexec is enabled.
No verified public exploit code is available. See the upstream commits for the corrected implementation: Kernel Git Commit 485ed44, Kernel Git Commit 7a54e0c, and Kernel Git Commit a94d672.
Detection Methods for CVE-2026-68435
Indicators of Compromise
- Unexpected kexec_load(2) invocations on LoongArch hosts from non-administrative workflows.
- Kernel oops or fault messages referencing machine_kexec.c during segment processing.
- Presence of the vulnerable source line at arch/loongarch/kernel/machine_kexec.c:84 in the running kernel tree.
Detection Strategies
- Compare the running kernel version and LoongArch patch level against the fix commits 485ed44, 7a54e0c, and a94d672.
- Run the sparse static analyzer against LoongArch kernel builds and treat the different address spaces diagnostic on machine_kexec.c as a positive indicator.
- Audit distribution changelogs for backports of the LoongArch kexec command line lookup fix.
Monitoring Recommendations
- Log auditd events for the kexec_load and kexec_file_load syscalls and alert on unexpected callers.
- Monitor dmesg for kernel faults originating in the kexec code path on LoongArch systems.
- Track process capabilities granting CAP_SYS_BOOT and review any additions.
How to Mitigate CVE-2026-68435
Immediate Actions Required
- Inventory LoongArch systems and identify kernels built without the fix commits.
- Apply the upstream patches or the vendor-provided kernel update that incorporates them.
- Restrict CAP_SYS_BOOT to trusted administrative accounts and service identities only.
Patch Information
The fix replaces the invalid (char __user *) cast with a bounded copy_from_user() into a small on-stack buffer before invoking strncmp(), and skips segments that fault during the copy. The subsequent copy_from_user() that stages the full command line into the safe area is retained unchanged. Patches are available in Kernel Git Commit 485ed44, Kernel Git Commit 7a54e0c, and Kernel Git Commit a94d672.
Workarounds
- Disable kexec on LoongArch hosts where the feature is not required by unsetting CONFIG_KEXEC in kernel builds.
- Use kexec_load_disabled=1 sysctl to block further kexec_load(2) calls after boot on systems that do not need runtime kexec.
- Limit local shell access on LoongArch platforms and remove CAP_SYS_BOOT from non-root service accounts.
# Configuration example: disable kexec_load at runtime
sysctl -w kernel.kexec_load_disabled=1
echo 'kernel.kexec_load_disabled = 1' >> /etc/sysctl.d/99-kexec.conf
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
