CVE-2025-39779
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-09-11

Last updated on: 2025-11-25

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: btrfs: subpage: keep TOWRITE tag until folio is cleaned btrfs_subpage_set_writeback() calls folio_start_writeback() the first time a folio is written back, and it also clears the PAGECACHE_TAG_TOWRITE tag even if there are still dirty blocks in the folio. This can break ordering guarantees, such as those required by btrfs_wait_ordered_extents(). That ordering breakage leads to a real failure. For example, running generic/464 on a zoned setup will hit the following ASSERT. This happens because the broken ordering fails to flush existing dirty pages before the file size is truncated. assertion failed: !list_empty(&ordered->list) :: 0, in fs/btrfs/zoned.c:1899 ------------[ cut here ]------------ kernel BUG at fs/btrfs/zoned.c:1899! Oops: invalid opcode: 0000 [#1] SMP NOPTI CPU: 2 UID: 0 PID: 1906169 Comm: kworker/u130:2 Kdump: loaded Not tainted 6.16.0-rc6-BTRFS-ZNS+ #554 PREEMPT(voluntary) Hardware name: Supermicro Super Server/H12SSL-NT, BIOS 2.0 02/22/2021 Workqueue: btrfs-endio-write btrfs_work_helper [btrfs] RIP: 0010:btrfs_finish_ordered_zoned.cold+0x50/0x52 [btrfs] RSP: 0018:ffffc9002efdbd60 EFLAGS: 00010246 RAX: 000000000000004c RBX: ffff88811923c4e0 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffffffff827e38b1 RDI: 00000000ffffffff RBP: ffff88810005d000 R08: 00000000ffffdfff R09: ffffffff831051c8 R10: ffffffff83055220 R11: 0000000000000000 R12: ffff8881c2458c00 R13: ffff88811923c540 R14: ffff88811923c5e8 R15: ffff8881c1bd9680 FS: 0000000000000000(0000) GS:ffff88a04acd0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f907c7a918c CR3: 0000000004024000 CR4: 0000000000350ef0 Call Trace: <TASK> ? srso_return_thunk+0x5/0x5f btrfs_finish_ordered_io+0x4a/0x60 [btrfs] btrfs_work_helper+0xf9/0x490 [btrfs] process_one_work+0x204/0x590 ? srso_return_thunk+0x5/0x5f worker_thread+0x1d6/0x3d0 ? __pfx_worker_thread+0x10/0x10 kthread+0x118/0x230 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x205/0x260 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Consider process A calling writepages() with WB_SYNC_NONE. In zoned mode or for compressed writes, it locks several folios for delalloc and starts writing them out. Let's call the last locked folio folio X. Suppose the write range only partially covers folio X, leaving some pages dirty. Process A calls btrfs_subpage_set_writeback() when building a bio. This function call clears the TOWRITE tag of folio X, whose size = 8K and the block size = 4K. It is following state. 0 4K 8K |/////|/////| (flag: DIRTY, tag: DIRTY) <-----> Process A will write this range. Now suppose process B concurrently calls writepages() with WB_SYNC_ALL. It calls tag_pages_for_writeback() to tag dirty folios with PAGECACHE_TAG_TOWRITE. Since folio X is still dirty, it gets tagged. Then, B collects tagged folios using filemap_get_folios_tag() and must wait for folio X to be written before returning from writepages(). 0 4K 8K |/////|/////| (flag: DIRTY, tag: DIRTY|TOWRITE) However, between tagging and collecting, process A may call btrfs_subpage_set_writeback() and clear folio X's TOWRITE tag. 0 4K 8K | |/////| (flag: DIRTY|WRITEBACK, tag: DIRTY) As a result, process B won't see folio X in its batch, and returns without waiting for it. This breaks the WB_SYNC_ALL ordering requirement. Fix this by using btrfs_subpage_set_writeback_keepwrite(), which retains the TOWRITE tag. We now manually clear the tag only after the folio becomes clean, via the xas operation.
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Meta Information
Published
2025-09-11
Last Modified
2025-11-25
Generated
2026-05-27
AI Q&A
2025-09-11
EPSS Evaluated
2026-05-25
NVD
CVE-2025-39779
Affected Vendors & Products
Showing 4 associated CPEs
Vendor Product Version / Range
linux linux_kernel From 5.15.160 (inc) to 5.16 (inc)
linux linux_kernel From 5.15.160 (inc) to 5.16 (inc)
linux linux_kernel 6.17
linux linux_kernel 6.17
Helpful Resources
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CWE ID Description
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AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability is in the Linux kernel's btrfs filesystem, specifically related to how writeback operations handle subpage folios. The function btrfs_subpage_set_writeback() clears a tag (PAGECACHE_TAG_TOWRITE) prematurely even when there are still dirty blocks in the folio. This breaks ordering guarantees required for proper writeback synchronization, causing failures such as kernel assertions and bugs during file operations, especially on zoned storage setups. The fix involves retaining the TOWRITE tag until the folio is fully cleaned to maintain correct write ordering.


How can this vulnerability impact me? :

This vulnerability can cause kernel crashes (BUGs) and assertion failures during file write operations on systems using the btrfs filesystem, particularly with zoned storage devices. It breaks the expected ordering of writebacks, potentially leading to data corruption or system instability when files are truncated or written concurrently. This can disrupt normal system operation and data integrity.


How can this vulnerability be detected on my network or system? Can you suggest some commands?

This vulnerability manifests as a kernel BUG related to btrfs on zoned setups, often triggered by running specific tests like generic/464. Detection involves monitoring kernel logs for assertion failures or BUG messages referencing fs/btrfs/zoned.c:1899. You can check kernel logs using commands such as 'dmesg | grep btrfs' or 'journalctl -k | grep btrfs' to look for related error messages or kernel oopses.


What immediate steps should I take to mitigate this vulnerability?

Immediate mitigation involves updating the Linux kernel to a version where this vulnerability is fixed, specifically one that uses the patched function btrfs_subpage_set_writeback_keepwrite() which retains the TOWRITE tag until the folio is clean. Until an update is applied, avoid workloads that trigger the bug, such as zoned btrfs setups performing concurrent writes that may cause ordering breaks.


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