N/A - CVE-2025-21942

In the Linux kernel, the following vulnerability has been resolved:

btrfs: zoned: fix extent range end unlock in cow_file_range()

Running generic/751 on the for-next branch often results in a hang like below. They are both stack by locking an extent. This suggests someone forget to unlock an extent.

INFO: task kworker/u128:1:12 blocked for more than 323 seconds.
Not tainted 6.13.0-BTRFS-ZNS+ #503
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u128:1 state:D stack:0 pid:12 tgid:12 ppid:2 flags:0x00004000
Workqueue: btrfs-fixup btrfs_work_helper [btrfs]
Call Trace:

__schedule+0x534/0xdd0
schedule+0x39/0x140
__lock_extent+0x31b/0x380 [btrfs]
? __pfx_autoremove_wake_function+0x10/0x10
btrfs_writepage_fixup_worker+0xf1/0x3a0 [btrfs]
btrfs_work_helper+0xff/0x480 [btrfs]
? lock_release+0x178/0x2c0
process_one_work+0x1ee/0x570
? srso_return_thunk+0x5/0x5f
worker_thread+0x1d1/0x3b0
? __pfx_worker_thread+0x10/0x10
kthread+0x10b/0x230
? __pfx_kthread+0x10/0x10
ret_from_fork+0x30/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30

INFO: task kworker/u134:0:184 blocked for more than 323 seconds.
Not tainted 6.13.0-BTRFS-ZNS+ #503
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u134:0 state:D stack:0 pid:184 tgid:184 ppid:2 flags:0x00004000
Workqueue: writeback wb_workfn (flush-btrfs-4)
Call Trace:

__schedule+0x534/0xdd0
schedule+0x39/0x140
__lock_extent+0x31b/0x380 [btrfs]
? __pfx_autoremove_wake_function+0x10/0x10
find_lock_delalloc_range+0xdb/0x260 [btrfs]
writepage_delalloc+0x12f/0x500 [btrfs]
? srso_return_thunk+0x5/0x5f
extent_write_cache_pages+0x232/0x840 [btrfs]
btrfs_writepages+0x72/0x130 [btrfs]
do_writepages+0xe7/0x260
? srso_return_thunk+0x5/0x5f
? lock_acquire+0xd2/0x300
? srso_return_thunk+0x5/0x5f
? find_held_lock+0x2b/0x80
? wbc_attach_and_unlock_inode.part.0+0x102/0x250
? wbc_attach_and_unlock_inode.part.0+0x102/0x250
__writeback_single_inode+0x5c/0x4b0
writeback_sb_inodes+0x22d/0x550
__writeback_inodes_wb+0x4c/0xe0
wb_writeback+0x2f6/0x3f0
wb_workfn+0x32a/0x510
process_one_work+0x1ee/0x570
? srso_return_thunk+0x5/0x5f
worker_thread+0x1d1/0x3b0
? __pfx_worker_thread+0x10/0x10
kthread+0x10b/0x230
? __pfx_kthread+0x10/0x10
ret_from_fork+0x30/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30

This happens because we have another success path for the zoned mode. When there is no active zone available, btrfs_reserve_extent() returns -EAGAIN. In this case, we have two reactions.

(1) If the given range is never allocated, we can only wait for someone
to finish a zone, so wait on BTRFS_FS_NEED_ZONE_FINISH bit and retry
afterward.

(2) Or, if some allocations are already done, we must bail out and let
the caller to send IOs for the allocation. This is because these IOs
may be necessary to finish a zone.

The commit 06f364284794 ("btrfs: do proper folio cleanup when cow_file_range() failed") moved the unlock code from the inside of the loop to the outside. So, previously, the allocated extents are unlocked just after the allocation and so before returning from the function. However, they are no longer unlocked on the case (2) above. That caused the hang issue.

Fix the issue by modifying the 'end' to the end of the allocated range. Then, we can exit the loop and the same unlock code can properly handle the case.