Summary
In the Linux kernel, the following vulnerability has been resolved: btrfs: subpage: keep TOWRITE...
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:
? 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
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.
Impact
CVE-2025-39779 has a CVSS score of 5.5 (Medium). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Frequently Asked Questions
- What is CVE-2025-39779? CVE-2025-39779 is a medium-severity security vulnerability. No fixed version is listed yet.
- How severe is CVE-2025-39779? CVE-2025-39779 has a CVSS score of 5.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Is there a fix for CVE-2025-39779? No fixed version is listed for CVE-2025-39779 yet. Monitor the advisory for updates and apply mitigations in the interim.
- Is CVE-2025-39779 exploitable, and should I be worried? Whether CVE-2025-39779 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-39779 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.