CVE-2024-35985 in Linux
Summary
by MITRE • 05/20/2024
In the Linux kernel, the following vulnerability has been resolved:
sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf()
It was possible to have pick_eevdf() return NULL, which then causes a NULL-deref. This turned out to be due to entity_eligible() returning falsely negative because of a s64 multiplcation overflow.
Specifically, reweight_eevdf() computes the vlag without considering the limit placed upon vlag as update_entity_lag() does, and then the scaling multiplication (remember that weight is 20bit fixed point) can overflow. This then leads to the new vruntime being weird which then causes the above entity_eligible() to go side-ways and claim nothing is eligible.
Thus limit the range of vlag accordingly.
All this was quite rare, but fatal when it does happen.
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Analysis
by VulDB Data Team • 05/20/2024
The vulnerability CVE-2024-35985 resides within the Linux kernel's scheduler component, specifically in the EAVDF (Enhanced Earliest Eligible Virtual Deadline First) scheduling algorithm implementation. This issue manifests in the sched/eevdf subsystem where the reweight_eevdf() function fails to properly constrain the vlag parameter, leading to a critical out-of-bounds condition that can result in system crashes or denial of service. The problem stems from a fundamental mathematical overflow issue during the computation of scheduling parameters that affects the core scheduling logic.
The technical flaw occurs when entity_eligible() function returns incorrect results due to s64 multiplication overflow within the reweight_eevdf() function. This overflow condition specifically affects the scaling multiplication operation where weight values are represented as 20-bit fixed point numbers. When the computation exceeds the bounds of the s64 data type, it produces unexpected results that corrupt the virtual runtime calculations. The corrupted vruntime values then cause the entity_eligible() function to incorrectly determine that no scheduling entities are eligible for execution, leading to a NULL return from pick_eevdf() function.
The operational impact of this vulnerability is severe despite its rarity, as it can cause complete system instability when triggered. The NULL dereference condition resulting from the corrupted scheduling logic can lead to kernel panics or system crashes, effectively rendering the affected system unusable. This vulnerability particularly affects systems under heavy scheduling load where the specific mathematical conditions for overflow are more likely to occur, making it a critical concern for production environments running Linux kernel versions containing this flaw.
The root cause analysis reveals that reweight_eevdf() does not properly consider the same vlag limits that update_entity_lag() enforces, creating a mismatch in the boundary conditions for virtual lag calculations. This mismatch allows the scaling multiplication operations to overflow and produce values that fall outside the expected parameter ranges. The vulnerability aligns with CWE-191 Integer Underflow (Wrap or Wraparound) and CWE-190 Integer Overflow (Wrap or Wraparound) classifications, while also demonstrating characteristics of ATT&CK technique T1499.004 for Network Denial of Service. The fix requires implementing proper range limiting for vlag values to prevent the overflow conditions that trigger the cascading failure in the scheduling algorithm.
Mitigation strategies should focus on applying the kernel patch that properly constrains the vlag parameter range in reweight_eevdf() function. System administrators should prioritize updating to kernel versions containing the fix, particularly in production environments where scheduler stability is critical. Monitoring for unusual scheduling behavior or system instability may serve as early indicators of potential exploitation, though the rarity of occurrence makes proactive patching the most reliable defense mechanism. Organizations should also consider implementing additional system stability monitoring to detect any anomalies in scheduling performance that might indicate the presence of this vulnerability before it leads to system crashes.