Re: nfs4_show_superblock considered harmful :-)

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Jun 01, 2020 at 12:01:07PM +1000, NeilBrown wrote:
> On Fri, May 29 2020, J. Bruce Fields wrote:
> 
> > On Fri, May 29, 2020 at 10:53:15AM +1000, NeilBrown wrote:
> >>  I've received a report of a 5.3 kernel crashing in
> >>  nfs4_show_superblock().
> >>  I was part way through preparing a patch when I concluded that
> >>  the problem wasn't as straight forward as I thought.
> >>
> >>  In the crash, the 'struct file *' passed to nfs4_show_superblock()
> >>  was NULL.
> >>  This file was acquired from find_any_file(), and every other caller
> >>  of find_any_file() checks that the returned value is not NULL (though
> >>  one BUGs if it is NULL - another WARNs).
> >>  But nfs4_show_open() and nfs4_show_lock() don't.
> >>  Maybe they should.  I didn't double check, but I suspect they don't
> >>  hold enough locks to ensure that the files don't get removed.
> >
> > I think the only lock held is cl_lock, acquired in states_start.
> >
> > We're starting here with an nfs4_stid that was found in the cl_stateids
> > idr.
> >
> > A struct nfs4_stid is freed by nfs4_put_stid(), which removes it from
> > that idr under cl_lock before freeing the nfs4_stid and anything it
> > points to.
> >
> > I think that was the theory....
> >
> > One possible problem is downgrades, like nfs4_stateid_downgrade.
> >
> > I'll keep mulling it over, thanks.
> 

Oops, I neglected this a while....

> I had another look at code and maybe move_to_close_lru() is the problem.
> It can clear remove the files and clear sc_file without taking
> cl_lock.  So some protection is needed against that.
> 
> I think that only applies to nfs4_show_open() - not show_lock etc.
> But I wonder it is might be best to include some extra protection
> for each different case, just in case some future code change
> allow sc_file to become NULL before the state is detached.
> 
> I'd feel more comforatable about nfs4_show_superblock() if it ignored
> nf_inode and just used nf_file - it is isn't NULL.  It looks like it
> can never be set from non-NULL to NULL.

But then that means we've always got a reference on the inode, doesn't
it?  So I still don't understand the nf_inode comment.

So maybe the NULL checks are mainly all we need.

Also it looks to me like ls_file lasts as long as the layout stateid, so
maybe it's OK.

--b.

commit 4eef57aa4fc0
Author: J. Bruce Fields <bfields@redhat.com>
Date:   Wed Jul 15 13:31:36 2020 -0400

    nfsd4: fix NULL dereference in nfsd/clients display code
    
    Reported-by: NeilBrown <neilb@suse.de>
    Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index ab5c8857ae5a..08b8376c74d7 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -507,6 +507,16 @@ find_any_file(struct nfs4_file *f)
 	return ret;
 }
 
+static struct nfsd_file *find_deleg_file(struct nfs4_file *f)
+{
+	struct nfsd_file *ret;
+
+	spin_lock(&f->fi_lock);
+	ret = nfsd_file_get(f->fi_deleg_file);
+	spin_unlock(&f->fi_lock);
+	return ret;
+}
+
 static atomic_long_t num_delegations;
 unsigned long max_delegations;
 
@@ -2444,6 +2454,8 @@ static int nfs4_show_open(struct seq_file *s, struct nfs4_stid *st)
 	oo = ols->st_stateowner;
 	nf = st->sc_file;
 	file = find_any_file(nf);
+	if (!file)
+		return 0;
 
 	seq_printf(s, "- ");
 	nfs4_show_stateid(s, &st->sc_stateid);
@@ -2481,6 +2493,8 @@ static int nfs4_show_lock(struct seq_file *s, struct nfs4_stid *st)
 	oo = ols->st_stateowner;
 	nf = st->sc_file;
 	file = find_any_file(nf);
+	if (!file)
+		return 0;
 
 	seq_printf(s, "- ");
 	nfs4_show_stateid(s, &st->sc_stateid);
@@ -2513,7 +2527,9 @@ static int nfs4_show_deleg(struct seq_file *s, struct nfs4_stid *st)
 
 	ds = delegstateid(st);
 	nf = st->sc_file;
-	file = nf->fi_deleg_file;
+	file = find_deleg_file(nf);
+	if (!file)
+		return 0;
 
 	seq_printf(s, "- ");
 	nfs4_show_stateid(s, &st->sc_stateid);