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

Jeff Layton
2025-01-26T22:48:21Z

On Mon, 2025-01-27 at 08:53 +1100, NeilBrown wrote:
> 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).
>
>

It's unnecessary because the outcome of either case is acceptable.

When racing with shutdown, either it's NULL and the laundrette won't
call svc_wake_up(), or it's non-NULL and it will. In the non-NULL case,
the call to svc_wake_up() will be a no-op because the threads are shut
down.

The vastly common case in this code is that this pointer will be non-
NULL, because the server is running (i.e. not racing with shutdown). I
don't see the need in making all of those accesses volatile.
--
Jeff Layton <jlayton@kernel.org>