Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx): > Hi various people who care about user-space NFS servers and/or > security-relevant APIs. > > I propose the following set of new syscalls: > > int credfd_create(unsigned int flags): returns a new credfd that > corresponds to current's creds. > > int credfd_activate(int fd, unsigned int flags): Change current's > creds to match the creds stored in fd. To be clear, this changes both > the "subjective" and "objective" (aka real_cred and cred) because > there aren't any real semantics for what happens when userspace code > runs with real_cred != cred. Is there a URL where I can find the motivation, and why the existing features can't be used? My guess would be, uid 100000 is root in a container, and you want him to be able to send a request to a root daemon on the host, on behalf of uid 100005 in the container, over which 100000 has privilege. (Which is sort of what we need for the cgmanager proxy; there we do it by checking checking that 100000 is mapped to 0 in the requestor's uid_map, and that 100005 is mapped in that uid_map) The credfd would be useful there, especially combined with a credfd_access(credfd, fd, perms) call. But I'd like to hear exactly how nfs and ganesha would use these. What all would be assiciated with the credfd? Everything that is in the kernel cred? > Rules: > > - credfd_activate fails (-EINVAL) if fd is not a credfd. > - credfd_activate fails (-EPERM) if the fd's userns doesn't match > current's userns. credfd_activate is not intended to be a substitute > for setns. > - credfd_activate will fail (-EPERM) if LSM does not allow the > switch. This probably needs to be a new selinux action -- > dyntransition is too restrictive. > > > Optional: > - credfd_create always sets cloexec, because the alternative is silly. > - credfd_activate fails (-EINVAL) if dumpable. This is because we > don't want a privileged daemon to be ptraced while impersonating > someone else. > - optional: both credfd_create and credfd_activate fail if > !ns_capable(CAP_SYS_ADMIN) or perhaps !capable(CAP_SETUID). > > The first question: does this solve Ganesha's problem? > > The second question: is this safe? I can see two major concerns. The > bigger concern is that having these syscalls available will allow > users to exploit things that were previously secure. For example, > maybe some configuration assumes that a task running as uid==1 can't > switch to uid==2, even with uid 2's consent. Similar issues happen > with capabilities. If CAP_SYS_ADMIN is not required, then this is no > longer really true. > > Alternatively, something running as uid == 0 with heavy capability > restrictions in a mount namespace (but not a uid namespace) could pass > a credfd out of the namespace. This could break things like Docker > pretty badly. CAP_SYS_ADMIN guards against this to some extent. But > I think that Docker is already totally screwed if a Docker root task > can receive an O_DIRECTORY or O_PATH fd out of the container, so it's > not entirely clear that the situation is any worse, even without > requiring CAP_SYS_ADMIN. > > The second concern is that it may be difficult to use this correctly. > There's a reason that real_cred and cred exist, but it's not really > well set up for being used. > > As a simple way to stay safe, Ganesha could only use credfds that have > real_uid == 0. > > --Andy -- 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
