[PATCH v2 07/39] mount: attach mappings to mounts

[Tycho Andersen]

On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > +static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt)
> > +{
> > +	return mnt->mnt_user_ns;
> > +}
> 
> I think you might want a READ_ONCE() here. Right now it seems ok, since the
> mnt_user_ns can't change, but if we ever allow it to change (and I see you have
> a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public tree
> :D), the pattern of,
> 
>         user_ns = mnt_user_ns(path->mnt);
>         if (mnt_idmapped(path->mnt)) {
>                 uid = kuid_from_mnt(user_ns, uid);
>                 gid = kgid_from_mnt(user_ns, gid);
>         }
> 
> could race.

Actually, isn't a race possible now?

kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
WRITE_ONCE(m->mnt.mnt_flags, flags);
kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);

So maybe it should be:

         if (mnt_idmapped(path->mnt)) {
                 barrier();
                 user_ns = mnt_user_ns(path->mnt);
                 uid = kuid_from_mnt(user_ns, uid);
                 gid = kgid_from_mnt(user_ns, gid);
         }

since there's no data dependency between mnt_idmapped() and
mnt_user_ns()?

Tycho