CVE-2026-45920 ext4: fix dirtyclusters double decrement on fs shutdown
A double decrement bug in the Linux kernel ext4 filesystem can trigger a kernel warning and potential instability during filesystem shutdown operations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-45920?
CVE-2026-45920 is a high-severity vulnerability in Linux kernel, published on 27 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
ext4: fix dirtyclusters double decrement on fs shutdown
fstests test generic/388 occasionally reproduces a warning in ext4_put_super() associated with the dirty clusters count:
WARNING: CPU: 7 PID: 76064 at fs/ext4/super.c:1324 ext4_put_super+0x48c/0x590 [ext4]
Tracing the failure shows that the warning fires due to an s_dirtyclusters_counter value of -1. IOW, this appears to be a spurious decrement as opposed to some sort of leak. Further tracing of the dirty cluster count deltas and an LLM scan of the resulting output identified the cause as a double decrement in the error path between ext4_mb_mark_diskspace_used() and the caller ext4_mb_new_blocks().
First, note that generic/388 is a shutdown vs. fsstress test and so produces a random set of operations and shutdown injections. In the problematic case, the shutdown triggers an error return from the ext4_handle_dirty_metadata() call(s) made from ext4_mb_mark_context(). The changed value is non-zero at this point, so ext4_mb_mark_diskspace_used() does not exit after the error bubbles up from ext4_mb_mark_context(). Instead, the former decrements both cluster counters and returns the error up to ext4_mb_new_blocks(). The latter falls into the !ar->len out path which decrements the dirty clusters counter a second time, creating the inconsistency.
To avoid this problem and simplify ownership of the cluster reservation in this codepath, lift the counter reduction to a single place in the caller. This makes it more clear that ext4_mb_new_blocks() is responsible for acquiring cluster reservation (via ext4_claim_free_clusters()) in the !delalloc case as well as releasing it, regardless of whether it ends up consumed or returned due to failure.
Weakness: CWE-415
Which versions are affected by CVE-2026-45920?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-45920?
Apply the vendor's patch.
Is CVE-2026-45920 being exploited?
There is no known exploitation of CVE-2026-45920 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-45920, DEBIAN-CVE-2026-45920, GHSA-r62x-r6m8-28mx, ROOT-OS-UBUNTU-2204-CVE-2026-45920, ROOT-OS-UBUNTU-2404-CVE-2026-45920, UBUNTU-CVE-2026-45920.