btrfs Subpage Vulnerability in Linux Kernel

CVE-2025-39779

What is CVE-2025-39779?

A vulnerability in the Linux kernel's btrfs file system can lead to a critical ordering issue during write operations. This problem occurs when the TOWRITE tag is improperly cleared while there are still dirty blocks present in a folio. As a result, concurrent processes can fail to wait for the writeback of dirty pages, causing inconsistencies, particularly during operations that require strict ordering guarantees. The defect can trigger kernel assertions, leading to system crashes under specific workloads, especially in zoned setups. This issue has been resolved by ensuring that the TOWRITE tag is retained until the folio is clean, thereby preserving proper writeback order.

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