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CVE-2026-72225
jbd2: fix integer underflow in jbd2_journal_initialize_fast_commit()
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem fs/jbd2. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-72225 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.180-1 | Update the kernel and reboot. Fixed in 6.1.180-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.100-1 | Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
jbd2_journal_initialize_fast_commit() validates journal capacity by checking (journal->j_last - num_fc_blks < JBD2_MIN_JOURNAL_BLOCKS). Both j_last and num_fc_blks are unsigned, so when num_fc_blks exceeds j_last the subtraction wraps to a large value, bypassing the bounds check. The resulting underflow corrupts j_last, j_fc_first, and j_free, leading to journal abort. Fix by checking num_fc_blks against j_last before the subtraction, returning -EFSCORRUPTED.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
fs/jbd2/journal.c
- What this is
- JOURNALLING LAYER FOR BLOCK DEVICES (JBD2)MAINTAINERS section matched on fs/jbd2/ · linux-ext4@vger.kernel.org · Maintained
- Kernel module
- jbd2
Check it on the machine
grep -w CONFIG_JBD2 /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w jbd2Prints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidb
- b
- built in on some flavours, a module on others
- Build switch
- CONFIG_JBD2
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-15 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.18%, higher than 6.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/jbd2
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.10 | 5.10.266 | aa90f00932bf |
| 5.10 | 5.15.212 | e144ad0250f7 |
| 5.10 | 6.1.178 | 4b48dcb88bb9 |
| 5.10 | 6.6.145 | fb9b49618ed7 |
| 5.10 | 6.12.97 | 4450dcaadf7d |
| 5.10 | 6.18.40 | 78955fdce8ff |
| 5.10 | 7.1.5 | a58fc10adf50 |
| 5.10 | 7.2 | 289a2ca0c9b7 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The vulnerable code is reached only via the ext4 mount path (mount/fsopen syscall) when enabling JBD2 fast commit in jbd2_journal_set_features(); it is not reachable through network-facing kernel services.
- AC:LAttack complexity, low
- An attacker who can mount a crafted ext4 image fully controls journal superblock fields (s_num_fc_blks vs s_maxlen) and can reliably trigger the underflow on first-time fast-commit enable without races or rare kernel configs.
- PR:LPrivileges required, low
- Triggering ext4 mount of a malicious image requires CAP_SYS_ADMIN, which an unprivileged local user can obtain inside a user/mount namespace (e.g. unshare -Urm) to mount loop-backed ext4 with fast_commit enabled.
- UI:NUser interaction, none
- Exploitation does not require a separate victim action beyond the attacker (or their namespace) performing the mount of the prepared filesystem image; no additional user interaction is needed.
- S:UScope, unchanged
- Impact is confined to journal/ext4 corruption and I/O within the mounting kernel context; it does not cross a VM, container, or IOMMU security boundary to affect a separate authority.
- C:HConfidentiality impact, high
- Corrupted j_last/j_fc_first/j_free values drive subsequent jbd2_journal_bmap and journal I/O with attacker-influenced block indices, enabling out-of-bounds reads from the journal backing device and potential kernel memory disclosure.
- I:HIntegrity impact, high
- The same corrupted journal geometry can direct journal block allocations and writes to incorrect offsets on the backing block device, constituting exploitable metadata corruption with integrity impact beyond a simple error return.
- A:HAvailability impact, high
- The fix commit and jbd2 abort path confirm the corruption leads to journal abort and mount/operational failure; repeated mounting of malicious images can deny filesystem availability and force recovery or remount-readonly.