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.