Re: [PATCH] nfsd: validate the nfsd_serv pointer before calling svc_wake_up

NeilBrown
2025-01-26T21:54:08Z

On Sun, 26 Jan 2025, Jeff Layton wrote:
> On Sun, 2025-01-26 at 13:39 +1100, NeilBrown wrote:
> > On Sun, 26 Jan 2025, Jeff Layton wrote:
> > > nfsd_file_dispose_list_delayed can be called from the filecache
> > > laundrette, which is shut down after the nfsd threads are shut down and
> > > the nfsd_serv pointer is cleared. If nn->nfsd_serv is NULL then there
> > > are no threads to wake.
> > >
> > > Ensure that the nn->nfsd_serv pointer is non-NULL before calling
> > > svc_wake_up in nfsd_file_dispose_list_delayed. This is safe since the
> > > svc_serv is not freed until after the filecache laundrette is cancelled.
> > >
> > > Fixes: ffb402596147 ("nfsd: Don't leave work of closing files to a work queue")
> > > Reported-by: Salvatore Bonaccorso <carnil@debian.org>
> > > Closes: https://lore.kernel.org/linux-nfs/7d9f2a8aede4f7ca9935a47e1d405643220d7946.camel@kernel.org/
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > > This is only lightly tested, but I think it will fix the bug that
> > > Salvatore reported.
> > > ---
> > > fs/nfsd/filecache.c | 11 ++++++++++-
> > > 1 file changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > > index e91c164b5ea21507659904690533a19ca43b1b64..fb2a4469b7a3c077de2dd750f43239b4af6d37b0 100644
> > > --- a/fs/nfsd/filecache.c
> > > +++ b/fs/nfsd/filecache.c
> > > @@ -445,11 +445,20 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
> > > struct nfsd_file, nf_gc);
> > > struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
> > > struct nfsd_fcache_disposal *l = nn->fcache_disposal;
> > > + struct svc_serv *serv;
> > >
> > > spin_lock(&l->lock);
> > > list_move_tail(&nf->nf_gc, &l->freeme);
> > > spin_unlock(&l->lock);
> > > - svc_wake_up(nn->nfsd_serv);
> > > +
> > > + /*
> > > + * The filecache laundrette is shut down after the
> > > + * nn->nfsd_serv pointer is cleared, but before the
> > > + * svc_serv is freed.
> > > + */
> > > + serv = nn->nfsd_serv;
> >
> > I wonder if this should be READ_ONCE() to tell the compiler that we
> > could race with clearing nn->nfsd_serv. Would the comment still be
> > needed?
> >
>
> I think we need a comment at least. The linkage between the laundrette
> and the nfsd_serv being set to NULL is very subtle. A READ_ONCE()
> doesn't convey that well, and is unnecessary here.

Why do you say "is unnecessary here" ?
If the code were
if (nn->nfsd_serv)
svc_wake_up(nn->nfsd_serv);
that would be wrong as nn->nfds_serv could be set to NULL between the
two.
And the C compile is allowed to load the value twice because the C memory
model declares that would have the same effect.
While I doubt it would actually change how the code is compiled, I think
we should have READ_ONCE() here (and I've been wrong before about what
the compiler will actually do).

Thanks,
NeilBrown