Linux Kernel CVE-2025-68359

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

btrfs: fix double free of qgroup record after failure to add delayed ref head

In the previous code it was possible to incur into a double kfree() scenario when calling add_delayed_ref_head(). This could happen if the record was reported to already exist in the btrfs_qgroup_trace_extent_nolock() call, but then there was an error later on add_delayed_ref_head(). In this case, since add_delayed_ref_head() returned an error, the caller went to free the record. Since add_delayed_ref_head() couldn't set this kfree'd pointer to NULL, then kfree() would have acted on a non-NULL 'record' object which was pointing to memory already freed by the callee.

The problem comes from the fact that the responsibility to kfree the object is on both the caller and the callee at the same time. Hence, the fix for this is to shift the ownership of the 'qrecord' object out of the add_delayed_ref_head(). That is, we will never attempt to kfree() the given object inside of this function, and will expect the caller to act on the 'qrecord' object on its own. The only exception where the 'qrecord' object cannot be kfree'd is if it was inserted into the tracing logic, for which we already have the 'qrecord_inserted_ret' boolean to account for this. Hence, the caller has to kfree the object only if add_delayed_ref_head() reports not to have inserted it on the tracing logic.

As a side-effect of the above, we must guarantee that 'qrecord_inserted_ret' is properly initialized at the start of the function, not at the end, and then set when an actual insert happens. This way we avoid 'qrecord_inserted_ret' having an invalid value on an early exit.

The documentation from the add_delayed_ref_head() has also been updated to reflect on the exact ownership of the 'qrecord' object.

The Linux kernel btrfs subsystem is affected on builds that include the qgroup accounting code prior to the stable fixes referenced at git.kernel.org (commits 364685c4c2d9c9f4408d95451bcf42fdeebc3ebb, 725e46298876a2cc1f1c3fb22ba69d29102c3ddf, and 7617680769e3119dfb3b43a2b7c287ce2242211c). No specific version numbers or CPE strings were provided in the input, and the three commit hashes indicate the fix was backported to multiple stable branches; exact affected version ranges should be confirmed against each stable tree's changelog. Only deployments using btrfs are exposed, and the qgroup/quota feature path is the trigger.