Re: allowing for a completely cached umount(2) pathwalk

[Christian Brauner <brauner@kernel.org>]

On Fri, Apr 14, 2023 at 02:21:00PM +0000, Trond Myklebust wrote:
> 
> 
> > On Apr 14, 2023, at 09:41, Christian Brauner <brauner@kernel.org> wrote:
> > 
> > On Fri, Apr 14, 2023 at 06:06:38AM -0400, Jeff Layton wrote:
> >> On Fri, 2023-04-14 at 03:43 +0100, Al Viro wrote:
> >>> On Fri, Apr 14, 2023 at 08:41:03AM +1000, NeilBrown wrote:
> >>> 
> >>>> The path name that appears in /proc/mounts is the key that must be used
> >>>> to find and unmount a filesystem.  When you do that "find"ing you are
> >>>> not looking up a name in a filesystem, you are looking up a key in the
> >>>> mount table.
> >>> 
> >>> No.  The path name in /proc/mounts is *NOT* a key - it's a best-effort
> >>> attempt to describe the mountpoint.  Pathname resolution does not work
> >>> in terms of "the longest prefix is found and we handle the rest within
> >>> that filesystem".
> >>> 
> >>>> We could, instead, create an api that is given a mount-id (first number
> >>>> in /proc/self/mountinfo) and unmounts that.  Then /sbin/umount could
> >>>> read /proc/self/mountinfo, find the mount-id, and unmount it - all
> >>>> without ever doing path name lookup in the traditional sense.
> >>>> 
> >>>> But I prefer your suggestion.  LOOKUP_MOUNTPOINT could be renamed
> >>>> LOOKUP_CACHED, and it only finds paths that are in the dcache, never
> >>>> revalidates, at most performs simple permission checks based on cached
> >>>> content.
> >>> 
> >>> umount /proc/self/fd/42/barf/something
> >>> 
> >> 
> >> Does any of that involve talking to the server? I don't necessarily see
> >> a problem with doing the above. If "something" is in cache then that
> >> should still work.
> >> 
> >> The main idea here is that we want to avoid communicating with the
> >> backing store during the umount(2) pathwalk.
> >> 
> >>> Discuss.
> >>> 
> >>> OTON, umount-by-mount-id is an interesting idea, but we'll need to decide
> >>> what would the right permissions be for it.
> >>> 
> >>> But please, lose the "mount table is a mapping from path prefix to filesystem"
> >>> notion - it really, really is not.  IIRC, there are systems that work that way,
> >>> but it's nowhere near the semantics used by any Unices, all variants of Linux
> >>> included.
> >> 
> >> I'm not opposed to something by umount-by-mount-id either. All of this
> >> seems like something that should probably rely on CAP_SYS_ADMIN.
> > 
> > The permission model needs to account for the fact that mount ids are
> > global and as such you could in principle unmount any mount in any mount
> > namespace. IOW, you can circumvent lookup restrictions completely.
> > 
> > So we could resolve the mnt-id to an FMODE_PATH and then very roughly
> > with no claim to solving everything:
> > 
> > may_umount_by_mnt_id(struct path *opath)
> > {
> > struct path root;
> > bool reachable;
> > 
> > // caller in principle able to circumvent lookup restrictions
> >        if (!may_cap_dac_readsearch())
> > return false;
> > 
> > // caller can mount in their mountns
> > if (!may_mount())
> > return false;
> > 
> > // target mount and caller in the same mountns
> > if (!check_mnt())
> > return false;
> > 
> > // caller could in principle reach mount from it's root
> > get_fs_root(current->fs, &root);
> >        reachable = is_path_reachable(real_mount(opath->mnt), opath->dentry, &root);
> > path_put(&root);
> > 
> > return reachable;
> > }
> > 
> > However, that still means that we have laxer restrictions on unmounting
> > by mount-id then on unmount with lookup as for lookup just having
> > CAP_DAC_READ_SEARCH isn't enough. Usually - at least for filesytems
> > without custom permission handlers - we also establish that the inode
> > can be mapped into the caller's idmapping.
> > 
> > So that would mean that unmounting by mount-id would allow you to
> > unmount mounts in cases where you wouldn't with umount. That might be
> > fine though as that's ultimately the goal here in a way.
> > 
> > One could also see a very useful feature in this where you require
> > capable(CAP_DAC_READ_SEARCH) and capable(CAP_SYS_ADMIN) and then allow
> > unmounting any mount in the system by mount-id. This would obviously be
> > very useful for privileged service managers but I haven't thought this
> > Through.
> 
> That is exactly why having a separate syscall to do the lookup of the mount-id is good: it provides separation of privilege.
> 
> The conversion of mount-id to an O_PATH file descriptor is just akin to a path lookup, so only needs CAP_DAC_READ_SEARCH (since you require privilege only to bypass the ACL directory read and lookup restrictions). The resulting O_PATH file descriptor has no special properties that require any further privilege.
> 
> Then use that resulting file descriptor for the umount, which normally requires CAP_SYS_ADMIN.

There's a difference between unmounting directly by providing a mount id
and getting an O_PATH file descriptor from a mnt-id. If you can simply
unmount by mount-id it's useful for users that have CAP_DAC_READ_SEARCH
in a container. Without it you likely need to require
capable(CAP_DAC_READ_SEARCH) aka system level privileges just like
open_to_handle_at() which makes this interface way less generic and
usable. Otherwise you'd be able to get an O_PATH fd to something that
you wouldn't be able to access through normal path lookup.