On Wed, Oct 8, 2014 at 4:38 PM, Serge Hallyn <serge.hallyn@xxxxxxxxxx> wrote: > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx): >> On Wed, Oct 8, 2014 at 2:36 PM, Rob Landley <rob@xxxxxxxxxxx> wrote: >> > On 10/08/14 14:31, Andy Lutomirski wrote: >> >> On Wed, Oct 8, 2014 at 12:23 PM, Eric W. Biederman >> >> <ebiederm@xxxxxxxxxxxx> wrote: >> >>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: >> >>>>> Maybe we want to say that rootfs should not be used if we are going to >> >>>>> create containers... >> >>> >> >>> Today it is an assumption of the vfs that rootfs is mounted. With >> >>> rootfs mounted and pivot_root at the base of the mount stack you can >> >>> make as minimal of a set of mounts as the vfs allows. >> >>> >> >>> Removing rootfs from the vfs requires an audit of everything that >> >>> manipulates mounts. It is not remotely a local excercise. >> >> >> >> Would it be a less invasive audit to allow different mount namespaces >> >> to have different rootfses? >> > >> > I.E. The same way different namespaces have different init tasks? >> > >> > The abstraction containers has implemented here should be logically >> > consistent. >> > >> >>>> Could we have an extra rootfs-like fs that is always completely empty, >> >>>> doesn't allow any writes, and can sit at the bottom of container >> >>>> namespace hierarchies? If so, and if we add a new syscall that's like >> >>>> pivot_root (or unshare) but prunes the hierarchy, then we could switch >> >>>> to that rootfs then. >> >>> >> >>> Or equally have something that guarantees that rootfs is empty and >> >>> read-only at the time the normal root filesystem is mounted. That is >> >>> certainly a much more localized change if we want to go there. >> >>> >> >>> I am half tempted to suggest that mount --move /some/path / be updated >> >>> to make the old / just go away (perhaps to be replaced with a read-only >> >>> empty rootfs). That gets us into figuring out if we break userspace >> >>> which is a big challenge. >> >> >> >> Hence my argument for a new syscall or entirely new operation. >> > >> > I'm still waiting for somebody to explain to my why chroot() shouldn't >> > be changed to do this instead of adding a new syscall. (At least when >> > mount namespace support is enabled.) >> >> Because chroot has no effect on the namespace at all. If you fork and >> the child chroots, the parent isn't chrooted. And, more importantly >> for my example, is a process has it's cwd as /foo, and then it forks >> and the child chroots, then parent's ".." isn't changed as a result of >> the chroot. >> >> > >> >> mount(2) and friends are way too multiplexed right now. I just found >> >> yet another security bug due to the insanely complicated semantics of >> >> the vfs syscalls. (Yes, a different one from the one yesterday.) >> > >> > As the guy who rewrote busybox mount 3 times, and who just implemented a >> > brand new one (toybox) from scratch: >> > >> > It's a bit fiddly, yes. >> > >> >> A new operation kills several birds with one stone. It could look like: >> >> >> >> int mntns_change_root(int dfd, const char *path, int flags); >> >> >> >> return -EPERM if chrooted. >> > >> > Really? >> >> Now that CVE-2014-7970 is public: what the heck is pivot_root supposed >> to do if the caller is chrooted? The current behavior is obviously >> incorrect (it leaks memory), but it's not entirely clear to me what >> should happen. I think it should either be disallowed or should have >> well-defined semantics. >> >> For simplicity, if a new syscall for this is added, then I think that >> the caller-is-chrooted case should be disallowed. If someone needs it >> and can articulate what the semantics should be, then I have no >> problem with allowing it going forward. > > It's not that I'd have a need for that, but rather if for some > reason I started out chrooted due to some bogus initramfs, I'd > prefer to not have to feel like a criminial and escape the chroot > first. You already can't create a userns if you're chrooted (even if you have global privilege). --Andy -- 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
