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@xxxxxxxxxx>
Date: Wed Jul 15 13:31:36 2020 -0400
nfsd4: fix NULL dereference in nfsd/clients display code
Reported-by: NeilBrown <neilb@xxxxxxx>
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
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);
