GHSA-q96j-3fmm-7fv4

Описание

Summary

LXD instance backup import validates project restrictions against backup/index.yaml embedded in the tar archive, but creates the actual instance from backup/container/backup.yaml extracted to the storage volume. Because these are separate, independently attacker-controlled files within the same tar archive, an attacker with instance-creation rights in a restricted project can craft a backup where index.yaml contains clean configuration (passing all restriction checks) while backup.yaml contains security.privileged=true, raw.lxc host filesystem mounts, and restricted device types. The instance is created from the unchecked backup.yaml, bypassing all project restriction enforcement.

Details

LXD projects support a restricted=true mode that enforces security boundaries on what instances within the project can do. These restrictions include blocking security.privileged=true containers, raw.lxc / raw.apparmor overrides, and device passthrough (GPU, USB, PCI, unix-char). These restrictions are intended to prevent container escape vectors regardless of user privilege level within the project.

The backup import path has two distinct configuration sources within a single tar archive:

1. backup/index.yaml - A quick-access metadata file read by backup.GetInfo() at backup/backup_info.go:68. This is the config checked against project restrictions.
2. backup/container/backup.yaml - The full instance configuration extracted to the storage volume and used for actual instance creation at api_internal.go:784.

The vulnerability exists because:

1. AllowInstanceCreation() at instances_post.go:885 validates project restrictions using only bInfo.Config from index.yaml.

2. The tar contents (including backup/container/backup.yaml) are extracted to the storage volume at generic_vfs.go:952 via unpackVolume().

3. UpdateInstanceConfig() at backup_config_utils.go:236 reads backup.yaml from storage but only syncs Name, Project, pool info, and volume UUIDs - it does not overwrite Instance.Config or Instance.Devices.

4. internalImportFromBackup() at api_internal.go:784 reads backup.yaml from the storage mount path (not index.yaml) to build the instance database record.

5. instance.CreateInternal() at api_internal.go:946 creates the instance using the config from backup.yaml. CreateInternal calls ValidConfig which validates config key format only, not project restriction compliance.

Proof of Concept

Environment setup (server admin)

These steps are performed by the LXD server administrator to set up the restricted project and grant access to the user. This represents the normal multi-tenant configuration that the exploit targets.

After this setup, the attacker can create normal unprivileged instances in restricted-project but should not be able to create privileged containers, use raw.lxc, or attach GPU/USB/unix-char devices. The exploit bypasses all of these restrictions.

Steps

1. Create an instance backup archive locally

The attacker constructs the entire backup archive locally. No access to any LXD server is needed for this step.

2. Connect to the target LXD server and import the backup

Connect to the target LXD server and confirm restricted access:

3. Escalate to full LXD admin

Start the container and use the LXD Unix socket, which was bind-mounted from the host via raw.lxc. Local connections over the Unix socket are trusted as full admin with unrestricted access across all projects.

Impact

The exploit allows full host compromise from within a restricted project. The requirement is that the user has can_view_instances, can_create_instances and can_operate_instances on the project -- standard permissions for any tenant expected to manage instances.

Possible remediation

Add a second AllowInstanceCreation (or checkInstanceRestrictions) call after backup.yaml is read from storage and before CreateInternal is called. In api_internal.go, between the ParseConfigYamlFile call (line 784) and the CreateInternal call (line 946):

Patches