Re: [PATCH 4/7] vfs: Add superblock notifications

From: David Howells <dhowells@redhat.com>
To: Jann Horn <jannh@google.com>
Cc: dhowells@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
	raven@themaw.net, linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	linux-block@vger.kernel.org, keyrings@vger.kernel.org,
	linux-security-module <linux-security-module@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/7] vfs: Add superblock notifications
Date: Wed, 29 May 2019 13:58:39 +0100	[thread overview]
Message-ID: <24577.1559134719@warthog.procyon.org.uk> (raw)
In-Reply-To: <CAG48ez2o1egR13FDd3=CgdXP_MbBsZM4SX=+aqvR6eheWddhFg@mail.gmail.com>

Jann Horn <jannh@google.com> wrote:

> It might make sense to require that the path points to the root inode
> of the superblock? That way you wouldn't be able to do this on a bind
> mount that exposes part of a shared filesystem to a container.

Why prevent that?  It doesn't prevent the container denizen from watching a
bind mount that exposes the root of a shared filesystem into a container.

It probably makes sense to permit the LSM to rule on whether a watch may be
emplaced, however.

> > +                       ret = add_watch_to_object(watch, s->s_watchers);
> > +                       if (ret == 0) {
> > +                               spin_lock(&sb_lock);
> > +                               s->s_count++;
> > +                               spin_unlock(&sb_lock);
> 
> Why do watches hold references on the superblock they're watching?

Fair point.  It was necessary at one point, but I don't think it is now.  I'll
see if I can remove it.  Note that it doesn't stop a superblock from being
unmounted and destroyed.

> > +                       }
> > +               }
> > +               up_write(&s->s_umount);
> > +               if (ret < 0)
> > +                       kfree(watch);
> > +       } else if (s->s_watchers) {
> 
> This should probably have something like a READ_ONCE() for clarity?

Note that I think I'll rearrange this to:

	} else {
		ret = -EBADSLT;
		if (s->s_watchers) {
			down_write(&s->s_umount);
			ret = remove_watch_from_object(s->s_watchers, wqueue,
						       s->s_unique_id, false);
			up_write(&s->s_umount);
		}
	}

I'm not sure READ_ONCE() is necessary, since s_watchers can only be
instantiated once and the watch list then persists until the superblock is
deactivated.  Furthermore, by the time deactivate_locked_super() is called, we
can't be calling sb_notify() on it as it's become inaccessible.

So if we see s->s_watchers as non-NULL, we should not see anything different
inside the lock.  In fact, I should be able to rewrite the above to:

	} else {
		ret = -EBADSLT;
		wlist = s->s_watchers;
		if (wlist) {
			down_write(&s->s_umount);
			ret = remove_watch_from_object(wlist, wqueue,
						       s->s_unique_id, false);
			up_write(&s->s_umount);
		}
	}

David