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-hold…

Overview

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.

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.