Here are some more thoughts on the ubd-control device: The current patch provides a ubd-control device for processes with suitable permissions (i.e. root) to create, start, stop, and fetch information about devices. There is no isolation between devices created by one process and those created by another. Therefore two processes that do not trust each other cannot both use UBD without potential interference. There is also no isolation for containers. I think it would be a mistake to keep the ubd-control interface in its current form since the current global/root model is limited. Instead I suggest: - Creating a device returns a new file descriptor instead of a global dev_id. The device can be started/stopped/configured through this (and only through this) per-device file descriptor. The device is not visible to other processes through ubd-control so interference is not possible. In order to give another process control over the device the fd can be passed (e.g. SCM_RIGHTS). Now multiple applications/containers/etc can use ubd-control without interfering with each other. The security model still requires root though since devices can be malicious. FUSE allows unprivileged mounts (see fuse_allow_current_process()). Only processes with the same uid as the FUSE daemon can access such mounts (in the default configuration). This prevents security issues while still allowing unprivileged use cases. I suggest adapting the FUSE security model to block devices: - Devices can be created without CAP_SYS_ADMIN but they have an 'unprivileged' flag set to true. - Unprivileged devices are not probed for partitions and LVM doesn't touch them. This means the kernel doesn't access these devices via code paths that might be exploitable. - When another process with a different uid from ubdsrv opens an unprivileged device, -EACCES is returned. This protects other uids from the unprivileged device. - When another process with a different uid from ubdsrv opens a _privileged_ device there is no special access check because ubdsrv is privileged. With these changes UBD can be used by unprivileged processes and containers. I think it's worth discussing the details and having this model from the start so UBD can be used in a wide range of use cases. Stefan
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