On Tue, Oct 7, 2014 at 5:20 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > >> On Tue, Oct 7, 2014 at 4:42 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: >>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >>> >>>> On Tue, Oct 7, 2014 at 3:42 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: >>>>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >>>>> >>>>>> On Tue, Oct 7, 2014 at 2:42 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: >>>>>>> >>>>>>> I am squinting and looking this way and that but while I can imagine >>>>>>> someone more clever than I can think up some unique property of rootfs >>>>>>> that makes it a little more exploitable than just mounting a ramfs, >>>>>>> but since you have to be root to exploit those properties I think the >>>>>>> game is pretty much lost. >>>>>> >>>>>> Yes. rootfs might not be empty, it might have totally insane >>>>>> permissions, and it's globally shared, which makes it into a wonderful >>>>>> channel to pass things around that shouldn't be passed around. >>>>> >>>>> But if only root with proc mounted can reach it... I don't know. >>>> >>>> It doesn't have to be global root. It could be userns root. >>>> >>>>> There might be a case for setting MNT_LOCKED when we overmount "/" >>>>> as root but I don't yet see it. >>>>> >>>>>> Can non-root do this? You'd need to be in a userns with a "/" that >>>>>> isn't MNT_LOCKED. Can this happen on any normal setup? >>>>>> >>>>>> FWIW, I think we should unconditionally MNT_LOCKED the root on userns >>>>>> unshare, even if it's the only mount. >>>>> >>>>> To the best of my knowledge MNT_LOCKED is set uncondintially on userns >>>>> unshare. >>>> >>>> Only if list_empty(&old->mnt_expire), whatever that means, I think. >>> >>> An autofs or nfs automounted mount. Can those ever become root? >> >> Dunno. >> >> I thought that this code was what set MNT_LOCKED for things that >> should be locked: > > It does. > >> /* Don't allow unprivileged users to reveal what is under a mount */ >> if ((flag & CL_UNPRIVILEGED) && list_empty(&old->mnt_expire)) >> mnt->mnt.mnt_flags |= MNT_LOCKED; >> >> Now I'm confused. Shouldn't that be checking for submounts? Is that >> what it's doing? > > As it copies each mount (mostly submounts) it sets MNT_LOCKED. Oh. They're *all* MNT_LOCKED. Duh. > >> Anyway, I think that this should unconditionally set MNT_LOCKED on the >> thing that mounted on rootfs. > > As I read the code mnt_set_expiry is only for nfs, cifs, and afs > submounts (and perhaps proc should use them). So as they are generated > mnt_expiry should never start on the root of filesystem of the mount tree. > > Furthermore mnt_expiry is cleared when a mount is moved, and when > it is bind mounted, or propagated. > > pivot_root does look as though it may be missing the proper mnt_expiry > handling list_del_init(&old->mnt_expire), but pivot_root does have > proper MNT_LOCKED handling. pivot_root is quite broken, as noted in my other email. It's just not broken like this, I think. :) > > Looking that test was slightly off and it should be: > if ((flag & CL_UNPRIVILEGED) && > (!(flag & CL_EXPIRE) || list_empty(&old->mnt_expire)) > mnt->mnt.mnt_flags |= MNT_LOCKED; > > So on that note we might clear CL_EXPIRE when CL_UNPRIVILEGED is set > in copy_tree but I don't see that as being really needed. > > Eric -- 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
