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

Summary

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.

Timeline

1. Sep 25, 2026, 10:22 AM UTCCVE published
2. Oct 19, 2026, 9:10 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 7366f8b6fc6aa21c4199cb5d337b023df69745b0 · fixed in 111339816138f042a59778322f941edc26903484; From 7366f8b6fc6aa21c4199cb5d337b023df69745b0 · fixed in 76e478499913dc1f368aebe85b28b7d1265985ff; From 7366f8b6fc6aa21c4199cb5d337b023df69745b0 · fixed in ff2a694f6613d54ebd77ba72583d4d94446f55bd; From 7366f8b6fc6aa21c4199cb5d337b023df69745b0 · fixed in fe967191e5851ea79818c5fe4e781c3882139218

• Linux Linux

  6.8

• KernelLinux

  From 6.8.0 · fixed in 6.12.111; From 6.13.0 · fixed in 6.18.53; From 6.19.0 · fixed in 7.2.7

References 4