[SRU][J][PATCH 0/1] CVE-2025-39779

[Cengiz Can]

https://ubuntu.com/security/CVE-2025-39779

[ Impact ]

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.

[ Fix ]

jammy/linux: backported from b1511360c8ac

[ Test Plan ]

Build and boot tested.

[ Where Problems Could Occur ]

A regression in this fix would affect btrfs on systems where the page size is
larger than the filesystem block size (subpage mode), such as 64K-page arm64
or ppc64le hosts running a 4K-block btrfs, and it primarily touches the
writeback path for zoned devices and compressed writes. A bad fix could leave
the TOWRITE tag set too long or clear it incorrectly, causing writeback to
stall or ordered extents to be mishandled during heavy concurrent
writepages() activity. Systems not using btrfs, or running btrfs where the
page size equals the block size (for example, standard 4K-page x86_64 with the
default 4K block size), are not exercised by this change.

[ Other Info ]

Kybele flow-v12-16-g3521c8c5. Reference: 75f4970b/v1