CVE-2026-97902

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

fs: don't return -EINVAL for successful nested thaw

Commit 7366f8b6fc6a ("fs: handle freezing from multiple devices")

replaced the freeze_holders bitmask with per-holder counters to allow

nested freezes. In the bitmask version, a thaw that released a shared

hold while another holder remained returned 0. Since the rework,

thaw_super_locked() drops the freeze reference via freeze_dec() but

then returns -EINVAL when other freezers remain, misinforming the

caller: the thaw did succeed, the superblock just stays frozen for the

remaining holders.

This breaks bdev-initiated freezing. When a filesystem is frozen with

FIFREEZE and additionally frozen via bdev_freeze() -- which nests by

design, see fs_bdev_freeze() -- the subsequent bdev_thaw() receives

-EINVAL from the holder op although its freeze reference was dropped,

and therefore keeps bd_fsfreeze_count elevated. Then device-mapper's

unlock_fs() ignores bdev_thaw()'s return value, so nothing rebalances

the count. After the user's FITHAW and umount, the block device can

never be mounted again:

dm-1: Can't mount, blockdev is frozen

There is no way for userspace to drop the leaked count; only

destroying the block device (or a reboot) recovers the device.

Reproducer (any kernel since v6.8):

dmsetup create dut --table "0 $(blockdev --getsz "$DEV") linear $DEV 0"

mkfs.ext4 /dev/mapper/dut

mount /dev/mapper/dut /mnt

fsfreeze --freeze /mnt # freeze_ucount == 1

dmsetup suspend dut # bd_fsfreeze_count == 1, ucount == 2

dmsetup resume dut # ucount 2 -> 1, but thaw_super()

# returns -EINVAL, so bdev_thaw()

# keeps bd_fsfreeze_count at 1

fsfreeze --unfreeze /mnt # filesystem thaws fine

umount /mnt

mount /dev/mapper/dut /mnt # EBUSY, forever

The same happens with fsfreeze held across an LVM snapshot of the

origin volume.

fs_bdev_thaw()'s documentation already describes the intended

semantics: "If this function returns zero it doesn't mean that the

filesystem is unfrozen as it may have been frozen multiple times".

Restore them by returning 0 when a nested thaw drops its hold while

other freezers remain. Thawing without holding a freeze still fails

with -EINVAL as may_unfreeze() rejects that case before the reference

count is touched.