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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-97902 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.8 with commit 7366f8b6fc6aa21c4199cb5d337b023df69745b0 and fixed in 6.12.111 with commit 111339816138f042a59778322f941edc26903484
	Issue introduced in 6.8 with commit 7366f8b6fc6aa21c4199cb5d337b023df69745b0 and fixed in 6.18.53 with commit 76e478499913dc1f368aebe85b28b7d1265985ff
	Issue introduced in 6.8 with commit 7366f8b6fc6aa21c4199cb5d337b023df69745b0 and fixed in 7.2.7 with commit ff2a694f6613d54ebd77ba72583d4d94446f55bd
	Issue introduced in 6.8 with commit 7366f8b6fc6aa21c4199cb5d337b023df69745b0 and fixed in 7.3-rc3 with commit fe967191e5851ea79818c5fe4e781c3882139218

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-97902
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/super.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/111339816138f042a59778322f941edc26903484
	https://git.kernel.org/stable/c/76e478499913dc1f368aebe85b28b7d1265985ff
	https://git.kernel.org/stable/c/ff2a694f6613d54ebd77ba72583d4d94446f55bd
	https://git.kernel.org/stable/c/fe967191e5851ea79818c5fe4e781c3882139218