CVE-2025-40209 btrfs: fix memory leak of qgroup_list in btrfs_add_qgroup_relation
An unprivileged user can cause a kernel memory leak in btrfs by triggering invalid qgroup relations, potentially leading to denial of service.
- 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-2025-40209?
CVE-2025-40209 is a memory corruption vulnerability in Linux, published on 21 Nov 2025. 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:
btrfs: fix memory leak of qgroup_list in btrfs_add_qgroup_relation
When btrfs_add_qgroup_relation() is called with invalid qgroup levels (src >= dst), the function returns -EINVAL directly without freeing the preallocated qgroup_list structure passed by the caller. This causes a memory leak because the caller unconditionally sets the pointer to NULL after the call, preventing any cleanup.
The issue occurs because the level validation check happens before the mutex is acquired and before any error handling path that would free the prealloc pointer. On this early return, the cleanup code at the 'out' label (which includes kfree(prealloc)) is never reached.
In btrfs_ioctl_qgroup_assign(), the code pattern is:
prealloc = kzalloc(sizeof(*prealloc), GFP_KERNEL); ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst, prealloc); prealloc = NULL; // Always set to NULL regardless of return value ... kfree(prealloc); // This becomes kfree(NULL), does nothing
When the level check fails, 'prealloc' is never freed by either the callee or the caller, resulting in a 64-byte memory leak per failed operation. This can be triggered repeatedly by an unprivileged user with access to a writable btrfs mount, potentially exhausting kernel memory.
Fix this by freeing prealloc before the early return, ensuring prealloc is always freed on all error paths.
Which versions are affected by CVE-2025-40209?
How do I fix CVE-2025-40209?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2025-40209 being exploited?
There is no known exploitation of CVE-2025-40209 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40209, DEBIAN-CVE-2025-40209, GHSA-2833-8jvf-3588, ROOT-OS-DEBIAN-13-CVE-2025-40209, UBUNTU-CVE-2025-40209.