CVE-2026-97902: fs: don't return -EINVAL for successful nested thaw

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 freezeholders 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, thawsuperlocked() drops the freeze reference via freezedec() 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 bdevfreeze() -- which nests by design, see fsbdevfreeze() -- the subsequent bdevthaw() receives -EINVAL from the holder op although its freeze reference was dropped, and therefore keeps bdfsfreezecount elevated. Then device-mapper's unlockfs() ignores bdevthaw()'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 # freezeucount == 1 dmsetup suspend dut # bdfsfreezecount == 1, ucount == 2 dmsetup resume dut # ucount 2 -> 1, but thawsuper() # returns -EINVAL, so bdevthaw() # keeps bdfsfreezecount 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.

fsbdevthaw()'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 mayunfreeze() rejects that case before the reference count is touched.