CVE-2026-74572: btrfs: zoned: fix deadlock between metadata writeback and transaction commit

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

btrfs: zoned: fix deadlock between metadata writeback and transaction commit

When writing out metadata extent buffers in a zoned filesystem,
btree_writepages() holds fs_info->zoned_meta_io_lock across the whole
writeback loop, including the call to btrfs_check_meta_write_pointer() ->
check_bg_is_active().

For the tree-log block group, check_bg_is_active() may fail to activate
the zone and fall back to btrfs_zone_finish_one_bg() to free an active
zone. That path waits for the running transaction to commit while still
holding zoned_meta_io_lock, but the committer needs that same lock to
write out the tree extents, so the two tasks deadlock:

  Task A (kworker, metadata writeback)      Task B (fsstress, transaction commit)
  ------------------------------------      -------------------------------------
  wb_workfn()                               btrfs_commit_transaction(T)
   btree_writepages()                        btrfs_write_and_wait_transaction()
    btrfs_zoned_meta_io_lock()                btrfs_write_marked_extents()
    btrfs_check_meta_write_pointer()           btree_writepages()
     check_bg_is_active() [treelog_bg]          btrfs_zoned_meta_io_lock()
      btrfs_zone_finish_one_bg()               <blocks on zoned_meta_io_lock,
       btrfs_zone_finish()                      held by Task A>
        do_zone_finish()
         btrfs_inc_block_group_ro()
          btrfs_wait_for_commit()
           <blocks waiting for commit
            of transaction T, done by
            Task B>

The sibling branch in check_bg_is_active() already drops zoned_meta_io_lock
around do_zone_finish() for this exact reason. Do the same in the tree-log
branch: release the lock around btrfs_zone_finish_one_bg() and re-acquire
it afterwards. The lock only protects fs_info->active_{meta,system}_bg,
which this branch does not touch, and ctx->zoned_bg keeps a reference to
the block group across the unlock, so nothing is lost while the lock
is dropped.

This hang occasionally reproduces with fstests generic/475 on a zoned
btrfs filesystem.

The Linux kernel CVE team has assigned CVE-2026-74572 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.6 with commit 13bb483d32abb6f8ebd40141d87eb68f11cc2dd2 and fixed in 6.6.151 with commit 18577e77c2c8adaadf1f7c6e9bcd1c0b14e5dcdd
	Issue introduced in 6.6 with commit 13bb483d32abb6f8ebd40141d87eb68f11cc2dd2 and fixed in 6.12.103 with commit c3320873e0c04ce7b746fc8fe948f07bbbbdec33
	Issue introduced in 6.6 with commit 13bb483d32abb6f8ebd40141d87eb68f11cc2dd2 and fixed in 6.18.44 with commit deddd28fd83c264ee2ff5cd6b34449a9f1be6112
	Issue introduced in 6.6 with commit 13bb483d32abb6f8ebd40141d87eb68f11cc2dd2 and fixed in 7.1.8 with commit 75859a7cd77cd2ddaddbcb963e3fcd34738953af
	Issue introduced in 6.6 with commit 13bb483d32abb6f8ebd40141d87eb68f11cc2dd2 and fixed in 7.2-rc6 with commit 1ebe51c29fa9755d5b2fea28727c051117907cf8
	Issue introduced in 6.5.5 with commit fca3a1cd3ba47f1815e0c0fcdc9aafaf02ee0a75

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74572
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/btrfs/zoned.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/18577e77c2c8adaadf1f7c6e9bcd1c0b14e5dcdd
	https://git.kernel.org/stable/c/c3320873e0c04ce7b746fc8fe948f07bbbbdec33
	https://git.kernel.org/stable/c/deddd28fd83c264ee2ff5cd6b34449a9f1be6112
	https://git.kernel.org/stable/c/75859a7cd77cd2ddaddbcb963e3fcd34738953af
	https://git.kernel.org/stable/c/1ebe51c29fa9755d5b2fea28727c051117907cf8