> > From: Miklos Szeredi <mszeredi@xxxxxxx> > > > > Add a new filesystem flag, that results in the VFS not checking if the > > current process has enough privileges to do an mknod(). > > > > This is needed on filesystems, where an unprivileged user may be able > > to create a device node, without causing security problems. > > > > One such example is "mountlo" a loopback mount utility implemented > > with fuse and UML, which runs as an unprivileged userspace process. > > In this case the user does in fact have the right to create device > > nodes within the filesystem image, as long as the user has write > > access to the image. Since the filesystem is mounted with "nodev", > > adding device nodes is not a security concern. > > Could we enforce at do_new_mount() that if > type->fs_flags&FS_MKNOD_CHECKS_PERM then mnt_flags |= MS_NODEV? Well, the problem with that is, there will be fuse filesystems which will want devices to work and for those the capability checks will be reenabled inside ->mknod(). In fact, for backward compatibility all filesystems will have the mknod checks, except ones which explicitly request to turn it off. Since unprivileged fuse mounts always have "nodev", the only way security could be screwed up, is if a filesystem running with privileges disabled the mknod checks. I will probably add some safety guards against that into the fuse library, but of course there's no way to stop a privileged user from screwing up security anyway. If for example there's a loop mount, where the disk image file is writable by a user, and root mounts it without "nodev", the user can still create device nodes (by modifying the image) even if the mknod checks are enabled. Miklos - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
