CVE-2025-39779: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-39779 is a vulnerability in the Linux kernel's BTRFS filesystem subsystem, discovered and disclosed on September 11, 2025. The issue specifically affects the BTRFS subpage handling mechanism where the TOWRITE tag management in the btrfssubpageset_writeback() function can lead to ordering guarantee violations (NVD).

The vulnerability occurs when btrfssubpagesetwriteback() calls foliostartwriteback() and prematurely clears the PAGECACHETAGTOWRITE tag while dirty blocks still exist in the folio. This breaks ordering guarantees required by btrfswaitorderedextents(). The issue manifests particularly in zoned setups, where it can cause assertion failures due to improper dirty page flushing before file size truncation. The vulnerability has been assigned a CVSS v3.1 base score of 5.5 with vector CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (Red Hat).

The vulnerability can lead to system crashes and potential data integrity issues in BTRFS filesystems. When triggered, it causes kernel BUG assertions and system instability, particularly affecting systems using zoned storage setups. The impact is primarily focused on availability, with no direct confidentiality or integrity compromises indicated (Red Hat).

The issue has been fixed by implementing btrfssubpagesetwritebackkeepwrite(), which maintains the TOWRITE tag until the folio becomes clean. The tag is now manually cleared only after the folio is cleaned, using xas operations (NVD).