On Tue, Oct 7, 2014 at 2:50 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > >> On Tue, Oct 7, 2014 at 2:26 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: >>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >>> >>>> Why should MNT_LOCKED on submounts be enforced? >>>> >>>> Is it because, if you retain a reference to the detached tree, then >>>> you can see under the submounts? >>> >>> Yes. MNT_DETACH is a recursive operation that detaches all of the mount >>> and all of it's submounts. Which means you can see under the submounts >>> if you have a reference to a detached mount. >>> >>>> If so, let's fix *that*. Because >>>> otherwise the whole model of pivot_root + detach will break. >>> >>> I am not certain what you are referring to. pivot_root doesn't >>> manipulate the mount tree so you can see under anything. >>> >>> What I believe is the appropriate fix is to fail umount2(...,MNT_DETACH) >>> if there are any referenced mount points being detached that have a >>> locked submount. >> >> Most of the container-using things do, roughly: >> >> Unshare userns and mountns >> Mount some new stuff >> pivot_root to the new stuff >> MNT_DETACH the old. >> >> That last step will almost always fail if you make this change. > > I don't think so. > > I expect I could add full busy detection of normal umounts and those > applications would not fail. > > What I am proposing is a more targeted version of busy detection that > looks at each mount in the set that detach will unmount. For each mount > if it is busy with non-submount references and it has at least one > locked submount fail the detach with -EBUSY. > > Do you really think we have userspace references to the one or more of the > mounts under old? > I suspect that we have a userspace reference to old itself. --Andy > Eric -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
