CephFS (ceph-fuse client): A tenant with an ordinary unprivileged UID on a node that has a CephFS volume mounted via

Impact

A tenant with an ordinary unprivileged UID on a node that has a CephFS volume mounted via ceph-fuse can chmod its way to running code as root on that node, which in turn hands it whatever CephX key the mount was made with. On a shared GPU node that means the tenant inherits the mount's view of the whole file system, not just its own subtree.

Who can reach it

Any local unprivileged user on a compute node where ceph-fuse has mounted CephFS. No cluster network access is needed and no CephX credential of the attacker's own is required.

What to do

Upgrade ceph-fuse to 17.2.8 / 18.2.5 / 19.2.3 or later on every node that fuse-mounts CephFS, then remount. Where possible prefer the kernel CephFS client for tenant nodes and scope each mount's CephX key to a single subtree so a compromised mount cannot read the rest of the tree.

References