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.) > 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? > Returns -EINVAL if path (relative to dfd) isn't a mountmount. Requiring that chroot() only be called on mountpoints would break existing semantics, which gets us back to new systemcall instead of changing behavior of existing one. If I recall, the first line of pushback against merging the openvz code as is was "buckets of new syscalls". Pushback against adding a new system call is understandable. Why can't we fix chroot() now that we have the tools to do so? > Otherwise it disconnects path from the existing > hierarchy, attaches a permanently-empty read-only rootfs under it, > makes it the root of the mntns, and does the root refs fixup. The old > hierarchy gets thrown out. We have a chroot() syscall. We don't use it for containers because it doesn't do what we want. Does it currently do what _anybody_ wants? > Systemd could use this, too. While that's a strong argument against it, I'm willing to overlook it. Rob -- 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
