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

NeilBrown
2025-01-26T02:39:58Z

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?

Otherwise:

Reviewed-by: NeilBrown <neilb@suse.de>

Thanks,
NeilBrown

> + if (serv)
> + svc_wake_up(serv);
> }
> }
>
>
> ---
> base-commit: 7541a5b8073cf0d9e2d288cac581f1aa6c11671d
> change-id: 20250125-kdevops-0989825ae8db
>
> Best regards,
> --
> Jeff Layton <jlayton@kernel.org>
>
>