On Tue, Nov 18, 2014 at 7:21 AM, Seth Forshee <seth.forshee@xxxxxxxxxxxxx> wrote: > On Wed, Nov 12, 2014 at 10:22:54AM -0600, Seth Forshee wrote: >> On Wed, Nov 12, 2014 at 02:09:15PM +0100, Miklos Szeredi wrote: >> > On Tue, Nov 11, 2014 at 09:37:10AM -0600, Eric W. Biederman wrote: >> > >> > > > Maybe I'm being dense, but can someone give a concrete example of such an >> > > > attack? >> > > >> > > There are two variants of things at play here. >> > > >> > > There is the classic if you don't freeze your context at open time when >> > > you pass that file descriptor to another process unexpected things can >> > > happen. >> > > >> > > An essentially harmless but extremely confusing example is what happens >> > > to a partial read when it stops halfway through a uid value and the next >> > > read on the same file descriptor is from a process in a different user >> > > namespace. Which uid value should be returned to userspace. >> > >> > Fuse device doesn't currently do partial reads, so that's a non-issue. >> > >> > > Now if I am in a nefarious mood I can create a unprivileged user >> > > namespace, open /dev/fuse and mount a fuse filesystem. Pass the file >> > > descriptor to /dev/fuse to a processes that is in the default user >> > > namespace (and thus can use any uid/gid). With that file desctipor >> > > report that there is a setuid 0 exectuable on that file system. >> > >> > Yes, and this would also be prevented by MNT_NOSUID, which would be a good idea >> > anyway. I just don't see the reason we'd want to allow clearing MNT_NOSUID in a >> > private namespace. >> > >> > So we don't currently see a use case for relaxing either the MNT_NOSUID >> > restriction or for relaxing the requirement on the user namespace the fuse >> > server is in. Is that correct? >> > >> > If so, we should leave both restrictions in place since that allows the greatest >> > flexibility in the future, is either of those needs to be relaxed. >> >> I'm not aware of specific use cases for either at this point. However, >> Andy's patch [1] will limit suid to the set of namespaces where the user >> who mounted the filesystem already has privileges. Enforcing MNT_NOSUID >> will require enforcement in the vfs, and in that case we definitely need >> to decide whether the policy is to implicitly add the flag or fail the >> mount attempt if the flag is not present [2]. > > I asked around a bit, and it turns out there are use cases for nested > containers (i.e. a container within a container) where the rootfs for > the outer container mounts a filesystem containing the rootfs for the > inner container. If that mount is nosuid then suid utilities like ping > aren't going to work in the inner container. > > So since there's a use case for suid in a userns mount and we have what > we belive are sufficient protections against using this as a vector to > get privileges outside the container, I'm planning to move ahead without > the MNT_NOSUID restriction. Any objections? Are you talking about MNT_NOSUID the flag or my ns-dependent thing? --Andy > > Thanks, > Seth > -- Andy Lutomirski AMA Capital Management, LLC -- 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
