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Smack: Add support for unprivileged mounts from user namespaces
Security labels from unprivileged mounts cannot be trusted. Ideally for these mounts we would assign the objects in the filesystem the same label as the inode for the backing device passed to mount. Unfortunately it's currently impossible to determine which inode this is from the LSM mount hooks, so we settle for the label of the process doing the mount. This label is assigned to s_root, and also to smk_default to ensure that new inodes receive this label. The transmute property is also set on s_root to make this behavior more explicit, even though it is technically not necessary. If a filesystem has existing security labels, access to inodes is permitted if the label is the same as smk_root, otherwise access is denied. The SMACK64EXEC xattr is completely ignored. Explicit setting of security labels continues to require CAP_MAC_ADMIN in init_user_ns. Altogether, this ensures that filesystem objects are not accessible to subjects which cannot already access the backing store, that MAC is not violated for any objects in the fileystem which are already labeled, and that a user cannot use an unprivileged mount to gain elevated MAC privileges. sysfs, tmpfs, and ramfs are already mountable from user namespaces and support security labels. We can't rule out the possibility that these filesystems may already be used in mounts from user namespaces with security lables set from the init namespace, so failing to trust lables in these filesystems may introduce regressions. It is safe to trust labels from these filesystems, since the unprivileged user does not control the backing store and thus cannot supply security labels, so an explicit exception is made to trust labels from these filesystems. Signed-off-by:Seth Forshee <seth.forshee@canonical.com> Acked-by: