CVE-2026-98223 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
mm: filemap: retain mapped dropbehind folios
Fault-around can map ready dropbehind folios without going through the normal page-cache lookup that clears dropbehind. A mapping represents a competing cached user, so retain the folio instead of forcibly unmapping it when writeback completes.
For a mapped folio, folio_unmap_invalidate() can call unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep. Retaining mapped folios avoids this path when folio_end_dropbehind() runs in non-preemptible task context.
Tal was able to trigger a sleeping-in-atomic warning due to this [1].
Unmapped dropbehind folios continue through the existing invalidation path.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel vulnerability identified involves a race condition within the memory management subsystem, specifically affecting how file-backed pages are handled during writeback operations and fault-around processes. The core issue stems from an inconsistency in how drop-behind folios are managed when they are already mapped into user space. Drop-behind is a read-ahead optimization technique where the kernel pre-fetches subsequent blocks of data to improve performance for sequential reads. However, under certain conditions involving fault-around mechanisms, ready drop-behind folios can be mapped without undergoing the standard page-cache lookup process that typically clears their drop-behind status. This creates a scenario where the kernel incorrectly assumes these pages are not actively referenced by user-space applications, leading to improper handling during subsequent writeback completion events.
The technical flaw manifests when the system attempts to invalidate or unmap these folios after writeback finishes. Normally, unmapping a mapped folio requires calling folio_unmap_invalidate(), which in turn invokes unmap_mapping_folio(). This function acquires the i_mmap_rwsem semaphore and may sleep while performing its operations. The critical error occurs because this sequence can be triggered from non-preemptible task contexts where sleeping is strictly prohibited by kernel safety rules. When a mapped drop-behind folio is retained incorrectly, it bypasses the normal invalidation path that would safely handle unmapping in an appropriate context. Instead, the system attempts to perform operations requiring sleep while holding atomic locks or running in interrupt context, violating fundamental concurrency constraints within the Linux kernel architecture.
This flaw results in a sleeping-in-atomic warning being triggered by developers and testers who can reproduce the specific timing conditions required for this race condition. While immediate exploitation leading to arbitrary code execution is unlikely due to the nature of the error manifesting as a stability issue rather than memory corruption, it poses significant risks to system reliability. The primary operational impact includes kernel panics or soft lockups that degrade system availability and performance. These events can cause service interruptions in production environments, particularly under heavy I/O loads where fault-around optimizations are frequently utilized. The vulnerability highlights the complexity of managing cached data structures across different execution contexts within high-performance storage stacks.
Mitigation strategies primarily involve applying kernel patches that correct the logic for retaining mapped drop-behind folios. Developers must ensure that when a mapping represents a competing cached user, the system retains the folio rather than forcibly unmaping it during writeback completion if doing so would violate atomic context constraints. For unmapped drop-behind folios, the existing invalidation path remains appropriate and should continue to be used without modification. System administrators can mitigate risk by keeping their Linux kernels updated with the latest security patches that address this specific memory management logic error. Additionally, monitoring kernel logs for sleeping-in-atomic warnings can help identify systems potentially affected by similar concurrency issues in other subsystems until comprehensive patching is applied across all infrastructure components.
This vulnerability aligns with CWE classifications related to improper locking and race conditions within concurrent programming environments. Specifically, it reflects weaknesses associated with incorrect synchronization mechanisms that allow operations requiring sleep to execute in non-preemptible contexts. From a threat modeling perspective using the MITRE ATT&CK framework, this issue relates to techniques involving resource exhaustion or denial of service through system instability rather than direct privilege escalation. Understanding these underlying mechanics helps security teams prioritize patching efforts based on workload characteristics and I/O patterns that might trigger the problematic code paths more frequently in their specific deployment environments.