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. > 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. 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 -- 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
