CVE-2026-31458 Overview
A NULL pointer dereference vulnerability has been discovered in the Linux kernel's Data Access MONitor (DAMON) subsystem. The vulnerability exists in the mm/damon/sysfs component where multiple sysfs command paths dereference contexts_arr[0] without first verifying that kdamond->contexts->nr == 1. This allows a privileged user to trigger a kernel panic by setting nr_contexts to 0 via sysfs while DAMON is running.
Critical Impact
Privileged users can cause a kernel panic through NULL pointer dereference, resulting in system denial of service and potential system instability.
Affected Products
- Linux kernel with DAMON subsystem enabled
- Systems with /sys/kernel/mm/damon/admin/kdamonds/ sysfs interface accessible
- Linux kernel versions prior to the security patches
Discovery Timeline
- 2026-04-22 - CVE CVE-2026-31458 published to NVD
- 2026-04-23 - Last updated in NVD database
Technical Details for CVE-2026-31458
Vulnerability Analysis
The vulnerability resides in the DAMON sysfs interface, specifically in the damon_sysfs_handle_cmd() function. When processing various state commands through the sysfs interface, the kernel code assumes that at least one context exists in the contexts_arr array. However, a privileged user can manipulate the nr_contexts value to 0 through the sysfs interface while DAMON is actively running.
The following state commands are affected by this vulnerability:
- update_schemes_stats
- update_schemes_tried_regions
- update_schemes_tried_bytes
- update_schemes_effective_quotas
- update_tuned_intervals
When any of these commands are issued after nr_contexts has been set to 0, the kernel attempts to access contexts_arr[0], which results in a NULL pointer dereference and subsequent kernel panic.
Root Cause
The root cause is a missing bounds check in the DAMON sysfs command handler. The code paths for handling multiple sysfs state commands directly access contexts_arr[0] without first verifying that kdamond->contexts->nr equals 1 or is greater than 0. This represents an insufficient input validation flaw where the sysfs interface allows modification of critical state variables without proper synchronization or bounds checking in the command processing logic.
Attack Vector
The attack requires privileged access to the system (typically root) to manipulate the sysfs interface. An attacker can exploit this vulnerability through the following sequence:
- Start DAMON monitoring using a tool like damo start
- Navigate to /sys/kernel/mm/damon/admin/kdamonds/0
- Write 0 to contexts/nr_contexts to empty the contexts directory
- Write any of the affected state commands to the state file
This sequence causes the kernel to dereference a NULL pointer, resulting in a kernel panic and system crash. While this requires local privileged access, it can be used to cause denial of service on systems where an attacker has obtained elevated privileges.
Detection Methods for CVE-2026-31458
Indicators of Compromise
- Unexpected kernel panics with backtraces pointing to DAMON sysfs functions
- System logs showing NULL pointer dereference errors in mm/damon/sysfs module
- Abnormal access patterns to /sys/kernel/mm/damon/admin/kdamonds/ directory
- Repeated writes to nr_contexts files followed by state file modifications
Detection Strategies
- Monitor sysfs access to DAMON-related paths for suspicious write patterns
- Implement kernel crash dump analysis to identify NULL pointer dereferences in DAMON subsystem
- Deploy auditd rules to track access to /sys/kernel/mm/damon/ directories
- Review kernel logs for DAMON-related warnings or errors preceding system instability
Monitoring Recommendations
- Enable kernel crash dump collection (kdump) for post-incident analysis
- Configure audit rules for sysfs operations in the DAMON namespace
- Monitor for unusual sequences of sysfs writes from privileged processes
- Implement system stability monitoring to detect unexpected kernel panics
How to Mitigate CVE-2026-31458
Immediate Actions Required
- Apply the latest kernel security patches from the official kernel.org repository
- Restrict access to the DAMON sysfs interface to only necessary administrative accounts
- Review and limit root access on systems where DAMON is enabled
- Consider disabling DAMON if not required for system operations until patching is complete
Patch Information
The Linux kernel maintainers have released patches to address this vulnerability. The fix adds proper guards for all commands (except OFF) at the entry point of damon_sysfs_handle_cmd(), ensuring that contexts->nr is verified before accessing contexts_arr[0].
Patches are available from the following kernel git commits:
- Kernel Git Commit 1bfe9fb
- Kernel Git Commit 1e8da79
- Kernel Git Commit 708033c
- Kernel Git Commit aba5460
- Kernel Git Commit bbe03ad
Workarounds
- Restrict sysfs access permissions to DAMON directories using filesystem ACLs
- Implement SELinux or AppArmor policies to limit which processes can write to DAMON sysfs
- Disable the DAMON subsystem if it is not required for system operations
- Monitor and audit privileged user activities on affected systems
# Restrict access to DAMON sysfs interface
chmod 700 /sys/kernel/mm/damon/admin/kdamonds/
# Add audit rule to monitor DAMON sysfs access
auditctl -w /sys/kernel/mm/damon/ -p wa -k damon_sysfs_access
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
