Re: fsnotify path hooks

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Apr 20, 2021 at 2:41 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Tue, Apr 20, 2021 at 09:01:09AM +0300, Amir Goldstein wrote:
> > > One thing, whatever you end up passing to vfs_create() please make sure
> > > to retrieve mnt_userns once so permission checking and object creation
> > > line-up:
> > >
> > > int vfs_create(struct vfsmount *mnt, struct inode *dir,
> > >                struct dentry *dentry, umode_t mode, bool want_excl)
> > > {
> > >         struct user_namespace *mnt_userns;
> > >
> > >         mnt_userns = mnt_user_ns(mnt);
> > >
> > >         int error = may_create(mnt_userns, dir, dentry);
> > >         if (error)
> > >                 return error;
> > >
> > >         if (!dir->i_op->create)
> > >                 return -EACCES; /* shouldn't it be ENOSYS? */
> > >         mode &= S_IALLUGO;
> > >         mode |= S_IFREG;
> > >         error = security_inode_create(dir, dentry, mode);
> > >         if (error)
> > >                 return error;
> > >         error = dir->i_op->create(mnt_userns, dir, dentry, mode, want_excl);
> > >         if (!error)
> > >                 fsnotify_create(mnt, dir, dentry);
> > >         return error;
> > > }
> > >
> >
> > Christian,
> >
> > What is the concern here?
> > Can mnt_user_ns() change under us?
> > I am asking because Al doesn't like both mnt_userns AND path to
> > be passed to do_tuncate() => notify_change()
> > So I will need to retrieve mnt_userns again inside notify_change()
> > after it had been used for security checks in do_open().
> > Would that be acceptable to you?
>
> The mnt_userns can't change once a mnt has been idmapped and it can
> never change if the mount is visible in the filesystem already. The only
> case we've been worried about and why we did it this way is when you
> have a caller do fd = open_tree(OPEN_TREE_CLONE) and then share that
> unattached fd with multiple processes
> T1: mkdirat(fd, "dir1", 0755);
> T2: mount_setattr(fd, "",); /* changes idmapping */
> That case isn't a problem if the mnt_userns is only retrieved once for
> permission checking and operating on the inode. I think with your
> changes that still shouldn't be an issue though since the vfs_*()
> helpers encompass the permission checking anyway and for notify_change,
> we could simply add a mnt_userns field to struct iattr and pass it down.

I suppose that could work for notify_change().

Thanks,
Amir.