CVE-2025-39759

In the Linux kernel, the following vulnerability has been resolved:

btrfs: qgroup: fix race between quota disable and quota rescan ioctl

Theres a race between a task disabling quotas and another running the rescan ioctl that can result in a use-after-free of qgroup records from the fs_info->qgroup_tree rbtree.

This happens as follows:

1. Task A enters btrfs_ioctl_quota_rescan() -> btrfs_qgroup_rescan();

2. Task B enters btrfs_quota_disable() and calls btrfs_qgroup_wait_for_completion(), which does nothing because at that point fs_info->qgroup_rescan_running is false (it wasnt set yet by task A);

3. Task B calls btrfs_free_qgroup_config() which starts freeing qgroups from fs_info->qgroup_tree without taking the lock fs_info->qgroup_lock;

4. Task A enters qgroup_rescan_zero_tracking() which starts iterating the fs_info->qgroup_tree tree while holding fs_info->qgroup_lock, but task B is freeing qgroup records from that tree without holding the lock, resulting in a use-after-free.

Fix this by taking fs_info->qgroup_lock at btrfs_free_qgroup_config(). Also at btrfs_qgroup_rescan() dont start the rescan worker if quotas were already disabled.

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/2fd0f5ceb997f90f4332ccbab6c7e907e6b2d0eb
• https://git.kernel.org/stable/c/7cda0fdde5d9890976861421d207870500f9aace
• https://git.kernel.org/stable/c/b172535ccba12f0cf7d23b3b840989de47fc104d
• https://git.kernel.org/stable/c/c38028ce0d0045ca600b6a8345a0ff92bfb47b66
• https://git.kernel.org/stable/c/dd0b28d877b293b1d7f8727a7de08ae36b6b9ef0
• https://git.kernel.org/stable/c/e1249667750399a48cafcf5945761d39fa584edf
• https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html