On 2023-08-16, Christian Brauner <brauner@xxxxxxxxxx> wrote:
> On Tue, Aug 15, 2023 at 06:46:33AM -0700, Sargun Dhillon wrote:
> > On Tue, Aug 15, 2023 at 2:30 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Aug 10, 2023 at 02:00:43AM -0700, Sargun Dhillon wrote:
> > > > We support locking certain mount attributes in the kernel. This API
> > > > isn't directly exposed to users. Right now, users can lock mount
> > > > attributes by going through the process of creating a new user
> > > > namespaces, and when the mounts are copied to the "lower privilege"
> > > > domain, they're locked. The mount can be reopened, and passed around
> > > > as a "locked mount".
> > >
> > > Not sure if that's what you're getting at but you can actually fully
> > > create these locked mounts already:
> > >
> > > P1 P2
> > > # init userns + init mountns # init userns + init mountns
> > > sudo mount --bind /foo /bar
> > > sudo mount --bind -o ro,nosuid,nodev,noexec /bar
> > >
> > > # unprivileged userns + unprivileged mountns
> > > unshare --mount --user --map-root
> > >
> > > mount --bind -oremount
> > >
> > > fd = open_tree(/bar, OPEN_TREE_CLONE)
> > >
> > > send(fd_send, P2);
> > >
> > > recv(&fd_recv, P1)
> > >
> > > move_mount(fd_recv, /locked-mnt);
> > >
> > > and now you have a fully locked mount on the host for P2. Did you mean that?
> > >
> >
> > Yep. Doing this within a program without clone / fork is awkward. Forking and
> > unsharing in random C++ programs doesn't always go super well, so in my
> > mind it'd be nice to have an API to do this directly.
> >
> > In addition, having the superblock continue to be owned by the userns that
> > its mounted in is nice because then they can toggle the other mount attributes
> > (nodev, nosuid, noexec are the ones we care about).
> >
> > > >
> > > > Locked mounts are useful, for example, in container execution without
> > > > user namespaces, where you may want to expose some host data as read
> > > > only without allowing the container to remount the mount as mutable.
> > > >
> > > > The API currently requires that the given privilege is taken away
> > > > while or before locking the flag in the less privileged position.
> > > > This could be relaxed in the future, where the user is allowed to
> > > > remount the mount as read only, but once they do, they cannot make
> > > > it read only again.
> > >
> > > s/read only/read write/
> > >
> > > >
> > > > Right now, this allows for all flags that are lockable via the
> > > > userns unshare trick to be locked, other than the atime related
> > > > ones. This is because the semantics of what the "less privileged"
> > > > position is around the atime flags is unclear.
> > >
> > > I think that atime stuff doesn't really make sense to expose to
> > > userspace. That seems a bit pointless imho.
> > >
> > > >
> > > > Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx>
> > > > ---
> > > > fs/namespace.c | 40 +++++++++++++++++++++++++++++++++++---
> > > > include/uapi/linux/mount.h | 2 ++
> > > > 2 files changed, 39 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/fs/namespace.c b/fs/namespace.c
> > > > index 54847db5b819..5396e544ac84 100644
> > > > --- a/fs/namespace.c
> > > > +++ b/fs/namespace.c
> > > > @@ -78,6 +78,7 @@ static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
> > > > struct mount_kattr {
> > > > unsigned int attr_set;
> > > > unsigned int attr_clr;
> > > > + unsigned int attr_lock;
> > >
> > > So when I originally noted down this crazy idea
> > > https://github.com/uapi-group/kernel-features
> > > I didn't envision a new struct member but rather a flag that could be
> > > raised in attr_set like MOUNT_ATTR_LOCK that would indicate for the
> > > other flags in attr_set to become locked.
> > >
> > > So if we could avoid growing the struct pointlessly I'd prefer that. Is
> > > there a reason that wouldn't work?
> > No reason. The semantics were just a little more awkward, IMHO.
> > Specifically:
> > * This attr could never be cleared, only set, which didn't seem to follow
> > the attr_set / attr_clr semantics
> > * If we ever introduced a mount_getattr call, you'd want to expose
> > each of the locked bits independently, I'd think, and exposing
> > that through one flag wouldn't give you the same fidelity.
>
> Hm, right. So it's either new flags or a new member. @Aleksa?
I like ->attr_lock more tbh, especially since they cannot be cleared.
They are implemented as mount flags internally, but conceptually locking
flags is a separate thing to setting them.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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