CVE-2026-64256 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
xfs: don't wrap around quota ids in dqiterate
LOLLM noticed that q_id is an unsigned 32-bit variable. If it happens to be set to XFS_DQ_ID_MAX due to a filesystem that actually has a dquot for ID_MAX, then this addition will truncate to zero and the iteration starts over. Fix this by casting to u64.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability in question affects the Linux kernel's XFS filesystem implementation where a critical flaw exists in the quota iteration mechanism. This issue stems from improper handling of unsigned 32-bit variables during quota identifier processing, creating a potential path for unexpected behavior and system instability. The problem manifests when the q_id variable reaches its maximum value of XFS_DQ_ID_MAX, which represents the upper boundary for quota identifiers within the XFS filesystem.
The technical implementation flaw occurs in the dqiterate function where quota identifiers are processed in a loop structure. When the variable q_id equals XFS_DQ_ID_MAX and an addition operation is performed to increment the identifier, the unsigned 32-bit arithmetic causes integer overflow behavior. This overflow results in the value wrapping around to zero due to the truncation properties of 32-bit unsigned integer operations, effectively resetting the iteration process and causing the system to start examining quota entries from the beginning rather than continuing the logical sequence.
This vulnerability has significant operational impact on systems utilizing XFS filesystems with high quota identifier values. The wraparound behavior can lead to infinite loops during quota iteration processes, potentially causing system hangs, resource exhaustion, or denial of service conditions. Attackers could theoretically exploit this by creating specific filesystem conditions that trigger the overflow scenario, though the practical exploitation requires precise control over filesystem quota management and may be limited by system configuration constraints.
The fix implemented addresses this issue by casting the problematic variable to a 64-bit unsigned integer type, specifically u64, which provides sufficient range to accommodate the maximum quota identifier values without overflow conditions. This approach follows established security practices for preventing integer overflow vulnerabilities and aligns with common mitigation strategies recommended in industry standards such as those referenced in CWE-190, which catalogs integer overflow and wraparound issues. The solution ensures that quota iteration processes maintain proper continuity regardless of the specific quota identifier values present in the filesystem.
From an operational security perspective, this vulnerability demonstrates the importance of careful variable sizing and overflow protection in kernel-level code, particularly when dealing with boundary conditions and iterative processes. The fix represents a straightforward but critical improvement to the XFS subsystem's robustness, preventing potential system instability while maintaining backward compatibility with existing quota management functionality. This type of vulnerability is commonly classified under ATT&CK techniques related to privilege escalation and system stability compromise, as it affects core filesystem operations that are fundamental to system security and reliability.