[PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Ian Kent]

On Tue, 2020-01-14 at 15:25 +0800, Ian Kent wrote:
> On Mon, 2020-01-13 at 13:30 +0000, Al Viro wrote:
> > On Mon, Jan 13, 2020 at 02:03:00PM +0800, Ian Kent wrote:
> > 
> > > Oh wait, for systemd I was actually looking at:
> > > https://github.com/systemd/systemd/blob/master/src/shared/switch-root.c
> > > 
> > > > Mind you, that's not the actual systemd repo. either I probably
> > > > need to look a lot deeper (and at the actual systemd repo) to
> > > > work out what's actually being called.
> > > > 
> > > > > Sigh...  Guess I'll have to dig that Fedora KVM image out and
> > > > > try to see what it's about... ;-/  Here comes a couple of
> > > > > hours
> > > > > of build...
> > 
> > D'oh...  And yes, that would've been a bisect hazard - switch to
> > path_lookupat() later in the series gets rid of that.  Incremental
> > (to be foldede, of course):
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 1793661c3342..204677c37751 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -2634,7 +2634,7 @@ path_mountpoint(struct nameidata *nd,
> > unsigned
> > flags, struct path *path)
> >  		(err = lookup_last(nd)) > 0) {
> >  		s = trailing_symlink(nd);
> >  	}
> > -	if (!err)
> > +	if (!err && (nd->flags & LOOKUP_RCU))
> >  		err = unlazy_walk(nd);
> >  	if (!err)
> >  		err = handle_lookup_down(nd);
> 
> Ok, so I've tested with the updated patch.
> 
> The autofs connectathon tests I use function fine.
> 
> I also tested sending a SIGKILL to the daemon with about 180 active
> mounts and restarted the daemon to test the function of the ioctls
> that Al was concerned about.
> 
> While the connectathon test expired everything I had 3 mounts left
> after allowing sufficient expire time with the SIGKILL test.
> 
> Those mounts correspond to one map entry that has a mix of NFS
> vers=3 and vers=2 mount options and NFSv2 isn't supported by the
> servers I use in testing.
> 
> I'm inclined to think this is a bug in the automount mount tree
> re-connection code rather than a problem with this patch since
> all the other mounts, some simple and others with not so simple
> constructs, expired fine after automount re-connected to them.
> 
> There are two other map entries that have an NFS vers=2 option but
> they are simple mounts that will fail on attempting the automount
> because the server doesn't support v2 so they don't end up with
> mounts to reconnect to.
> 
> This particular map entry, having a mix of NFS vers=3 and vers=2
> in the offsets of the entry, will lead to a partial mount of the
> map entry which is probably not being handled properly by automount
> when re-connecting to the mounts in the tree.
> 
> So I think the patch here is fine from an autofs POV.

Umm ... unfortunately further testing shows an autofs problem.

It appears to be present in the current kernel (so far I've only
been able to check the current git head and an earlier kernel
but can't remember the version and can't check) so I must have
missed it.

I'm attempting to bisect now but managed to trash the root
file system on my VM. I'll get this done as quickly as I can.

Ian