CVE-2025-37752 Overview
A vulnerability has been discovered in the Linux kernel's network traffic control subsystem, specifically within the Stochastic Fairness Queueing (SFQ) scheduler (sch_sfq). The flaw involves insufficient validation of the limit parameter during qdisc configuration updates, allowing an attacker with local access to trigger an array-index-out-of-bounds condition that can lead to system crashes and potential privilege escalation.
Critical Impact
Local attackers can exploit improper limit validation in the SFQ scheduler to cause kernel crashes via UBSAN array-index-out-of-bounds errors, potentially leading to denial of service or privilege escalation on affected Linux systems.
Affected Products
- Linux Kernel (multiple versions)
- Debian Linux 11.0
- Linux Kernel 6.15-rc1
Discovery Timeline
- 2025-05-01 - CVE-2025-37752 published to NVD
- 2025-11-04 - Last updated in NVD database
Technical Details for CVE-2025-37752
Vulnerability Analysis
The vulnerability resides in the SFQ (Stochastic Fairness Queueing) network scheduler module (net/sched/sch_sfq.c). The core issue is that direct validation of the limit parameter on user-supplied data is insufficient because the limit value can be indirectly modified based on changes to other related parameters such as flows, depth, and divisor.
When an attacker configures a qdisc with specific parameter combinations, the limit can be updated to an invalid value after the initial validation check has passed. This results in an array index that exceeds the bounds of the struct sfq_head[128] array, causing the kernel to access memory outside the allocated buffer.
The crash manifests during the sfq_link function call within sfq_dec, which is triggered during dequeue operations. The syzkaller fuzzer discovered this issue when specific configurations caused index 65535 to be used against an array that only supports 128 entries.
Root Cause
The root cause is the placement of the limit validation check in the configuration process. The original implementation validated the limit parameter early in the configuration update flow, before other parameters (such as flows, depth, and divisor) had been processed. Since these parameters can indirectly affect the computed limit value, the early validation was bypassed in certain edge cases.
The fix moves the limit validation to the end of the configuration update process, ensuring all parameter changes are accounted for before the final bounds check is performed. This comprehensive validation catches scenarios where the limit is indirectly updated through parameter interdependencies.
Attack Vector
The attack requires local access to the system with sufficient privileges to configure network qdiscs using the tc (traffic control) utility. An attacker can craft specific qdisc configurations that exploit the validation timing issue.
The vulnerability can be triggered through malicious traffic control configurations such as:
tc qdisc add dev dummy0 handle 1: root sfq limit 2 flows 1 depth 1
tc qdisc add dev dummy0 handle 1: root sfq limit 2 flows 1 divisor 1
These configurations cause the limit to be computed in a way that results in an out-of-bounds array index (65535) when the scheduler attempts to process packets, leading to kernel panic or undefined behavior when the sfq_link function is called within sfq_dec during dequeue operations.
Detection Methods for CVE-2025-37752
Indicators of Compromise
- Kernel crash logs containing UBSAN: array-index-out-of-bounds in net/sched/sch_sfq.c messages
- Unexpected system reboots or kernel panics during network traffic processing
- Call traces referencing sfq_link, sfq_dec, sfq_dequeue, or sfq_reset functions
- Unusual tc qdisc configurations with low limit, flows, depth, or divisor values
Detection Strategies
- Monitor kernel logs for UBSAN violations specifically mentioning sch_sfq.c or the SFQ scheduler
- Deploy kernel runtime integrity monitoring to detect unexpected crashes in network subsystem components
- Audit tc command usage for suspicious qdisc configurations targeting dummy or other network interfaces
- Implement syscall monitoring for RTM_NEWQDISC netlink messages with unusual parameter combinations
Monitoring Recommendations
- Enable UBSAN (Undefined Behavior Sanitizer) in kernel builds to detect array bounds violations before exploitation
- Configure system logging to capture detailed kernel crash information including full stack traces
- Monitor for repeated network scheduler reconfigurations that may indicate exploitation attempts
- Use SentinelOne's kernel-level monitoring capabilities to detect anomalous traffic control operations
How to Mitigate CVE-2025-37752
Immediate Actions Required
- Update the Linux kernel to a patched version that includes the limit validation fix
- Restrict access to the tc traffic control utility to trusted administrators only
- Monitor systems for signs of exploitation attempts via kernel log analysis
- Consider disabling the SFQ scheduler module if not required for operations
Patch Information
Multiple patches have been released across various kernel branches to address this vulnerability. The fix relocates the limit validation to occur after all configuration parameters have been processed, preventing indirect limit manipulation from bypassing bounds checks.
Patches are available through the following kernel git commits:
Debian users should refer to the Debian LTS Security Announcement for distribution-specific update instructions.
Workarounds
- Limit access to network namespace and traffic control capabilities using Linux capabilities (CAP_NET_ADMIN)
- Use SELinux or AppArmor policies to restrict which processes can invoke tc commands
- Remove or blacklist the sch_sfq kernel module if SFQ scheduling is not required: echo "blacklist sch_sfq" >> /etc/modprobe.d/blacklist.conf
- Monitor and alert on any qdisc configurations with unusually low parameter values
# Blacklist SFQ scheduler module if not needed
echo "blacklist sch_sfq" >> /etc/modprobe.d/blacklist.conf
# Remove currently loaded module (if possible)
modprobe -r sch_sfq
# Restrict tc command to root only
chmod 700 /sbin/tc
# Verify current qdisc configurations
tc qdisc show
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
