CVE-2026-46256 in Linux
Summary
by MITRE • 06/03/2026
In the Linux kernel, the following vulnerability has been resolved:
NFS/localio: prevent direct reclaim recursion into NFS via nfs_writepages
LOCALIO is an NFS loopback mount optimization that avoids using the network for READ, WRITE and COMMIT if the NFS client and server are determined to be on the same system. But because LOCALIO is still fundamentally "just NFS loopback mount" it is susceptible to recursion deadlock via direct reclaim, e.g.: NFS LOCALIO down to XFS and then back into NFS via nfs_writepages.
Fix LOCALIO's potential for direct reclaim deadlock by ensuring that all its page cache allocations are done from GFP_NOFS context.
Thanks to Ben Coddington for pointing out commit ad22c7a043c2 ("xfs: prevent stack overflows from page cache allocation").
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Analysis
by VulDB Data Team • 06/04/2026
The vulnerability involves a critical recursion issue within the Linux kernel's NFS implementation that can lead to system deadlock during memory allocation operations. This flaw specifically affects the LOCALIO optimization mechanism which is designed to improve performance by avoiding network traffic when NFS client and server reside on the same system. The LOCALIO feature essentially creates a loopback mount that bypasses normal network communication for read, write, and commit operations, but this optimization introduces a dangerous dependency chain that can result in recursive memory allocation scenarios.
The technical flaw stems from improper memory allocation context handling within the LOCALIO subsystem. When direct reclaim operations occur during page cache management, the system can enter a recursive loop where NFS writepage operations attempt to allocate memory while already holding locks, creating a deadlock condition. This occurs because the LOCALIO implementation does not properly isolate its memory allocation operations from the direct reclaim context, allowing the allocation mechanism to trigger further reclaim operations. The vulnerability manifests when XFS filesystem operations interact with NFS through the LOCALIO path, creating a circular dependency where memory allocation attempts cause additional reclaim operations that ultimately fail due to the recursive nature of the lock acquisition.
The operational impact of this vulnerability is severe as it can cause complete system hangs or crashes when memory pressure occurs during NFS operations. Systems using LOCALIO optimizations are particularly susceptible to this issue, especially when running workloads that generate significant I/O activity or when memory pressure is high. The deadlock condition prevents the system from making forward progress, effectively rendering the affected NFS mounts unusable until the system is rebooted. This vulnerability affects any Linux system running kernel versions that include the problematic LOCALIO implementation and can be exploited by malicious actors or triggered by legitimate system workloads that create memory pressure conditions.
The fix addresses this issue by ensuring that all page cache allocations within the LOCALIO subsystem are performed from GFP_NOFS context, which prevents the allocation from triggering direct reclaim operations. This change isolates the LOCALIO memory operations from the normal reclaim path, breaking the recursive dependency that leads to deadlock conditions. The solution aligns with established kernel memory management practices and follows the principle of avoiding filesystem operations during memory allocation that could cause recursive lock acquisitions. This approach is consistent with the broader kernel development strategy for preventing stack overflows and memory allocation deadlocks, as demonstrated in commit ad22c7a043c2 referenced in the fix description. The mitigation ensures that LOCALIO operations maintain their performance benefits while eliminating the risk of system-wide deadlocks during memory pressure scenarios. This fix represents a standard defensive programming approach that prevents recursive memory allocation patterns that could compromise system stability.