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

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()?