CVE-2026-93255

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

btrfs: make sure EXTENT_BUFFER_READING is cleared under refs_lock

[FALSE ALERTS]
There is a bug report that the warning inside
invalidate_and_check_btree_folios() got triggered during btrfs/298:

 BTRFS info (device sdd): first mount of filesystem f9bf732a-a19b-44b9-99a7-614ddff168e2
 BTRFS info (device sdd): using crc32c checksum algorithm
 BTRFS error (device sdd): failed to find fsid cb2fdb42-b638-4f2f-badd-4127467ba674 when attempting to open seed devices
 BTRFS error (device sdd): failed to read chunk tree: -2
 ------------[ cut here ]------------
 WARNING: disk-io.c:3342 at invalidate_and_check_btree_folios+0x260/0x3c0 [btrfs], CPU#4: mount/125993
 CPU: 4 UID: 0 PID: 125993 Comm: mount Tainted: G        W  OE       7.1.0-rc7-custom+ #1 PREEMPT(full)
 Hardware name: QEMU KVM Virtual Machine, BIOS edk2-20250812-19.fc42 08/12/2025
 Call trace:
  invalidate_and_check_btree_folios+0x260/0x3c0 [btrfs] (P)
  open_ctree+0x1f50/0x23b0 [btrfs]
  btrfs_get_tree+0x89c/0xc48 [btrfs]
  vfs_get_tree+0x30/0x110
  vfs_cmd_create+0x58/0xe8
  __arm64_sys_fsconfig+0x39c/0x518
  invoke_syscall.constprop.0+0x48/0x120
  el0_svc_common.constprop.0+0x40/0xe8
  do_el0_svc+0x24/0x38
  el0_svc+0x50/0x310
  el0t_64_sync_handler+0xa0/0xe8
  el0t_64_sync+0x198/0x1a0
 ---[ end trace 0000000000000000 ]---
 BTRFS warning (device sdd): unable to release extent buffer 365985792 owner 3 gen 17 refs 3 flags 0x5

[CAUSE]
In that invalidate_and_check_btree_folios() we wait for the eb to finish
its read, then check if it's only held by us and the btree inode.

If not, then do a warning as it may be still held, and could cause
problems.

But there is a small window where the check can lead to false alerts:

    Thread A (Read endio)         |        Thread B (Unmount)
----------------------------------+-------------------------------------
 end_bbio_meta_read()             |
 | The eb has one extra ref held  |
 | by the reader, and has         |
 | EXTENT_BUFFER_READING flag set | invalidate_and_check_btree_folios()
 |                                | |
 |- clear_extent_buffer_reading() | |
 |                                | |- wait_on_bit_io();
 |                                | |  The EXTENT_BUFFER_READING flag is
 |                                | |  cleared
 |                                | |- if (refcount_read(eb->refs) > 2)
 |                                |    The eb is held by the read, us
 |                                |    and btree inode, thus it
 |                                |    will trigger the warning
 |- free_extent_buffer()          |

[FIX]
Introduce a helper, free_extent_buffer_clear_reading().

If the new parameter, @clear_reading, is set, we will hold the spinlock
at the beginning of free_extent_buffer_clear_reading() to make sure the
EXTENT_BUFFER_READING flag is cleared inside the same critical section
of decreasing refs.

Now free_extent_buffer() will just call
free_extent_buffer_clear_reading() with @clear_reading set to false, so
no behavior change.

But for end_bbio_meta_read(), it will not clear_extent_buffer_reading()
directly, but pass @clear_reading as true.

Then inside invalidate_and_check_btree_folios(), hold the refs_lock
before reading refs.
So that we eliminate the race window completely.