Re: [PATCH] nfsd: properly tear down server when write_ports fails

[NeilBrown]

On Tue, 12 Dec 2023, Jeff Layton wrote:
> When the initial write to the "portlist" file fails, we'll currently put
> the reference to the nn->nfsd_serv, but leave the pointer intact. This
> leads to a UAF if someone tries to write to "portlist" again.
> 
> Simple reproducer, from a host with nfsd shut down:
> 
>     # echo "foo 2049" > /proc/fs/nfsd/portlist
>     # echo "foo 2049" > /proc/fs/nfsd/portlist
> 
> The kernel will oops on the second one when it trips over the dangling
> nn->nfsd_serv pointer. There is a similar bug in __write_ports_addfd.
> 
> This patch fixes it by adding some extra logic to nfsd_put to ensure
> that nfsd_last_thread is called prior to putting the reference when the
> conditions are right.
> 
> Fixes: 9f28a971ee9f ("nfsd: separate nfsd_last_thread() from nfsd_put()")
> Closes: https://issues.redhat.com/browse/RHEL-19081
> Reported-by: Zhi Li <yieli@xxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> This should probably go to stable, but we'll need to backport for v6.6
> since older kernels don't have nfsd_nl_rpc_status_get_done. We should
> just be able to drop that hunk though.
> ---
>  fs/nfsd/nfsctl.c | 32 ++++++++++++++++++++++++++++----
>  fs/nfsd/nfsd.h   |  8 +-------
>  fs/nfsd/nfssvc.c |  2 +-
>  3 files changed, 30 insertions(+), 12 deletions(-)

This is much the same as

https://lore.kernel.org/linux-nfs/20231030011247.9794-2-neilb@xxxxxxx/

It seems that didn't land.  Maybe I dropped the ball...

NeilBrown


> 
> diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> index 3e15b72f421d..1ceccf804e44 100644
> --- a/fs/nfsd/nfsctl.c
> +++ b/fs/nfsd/nfsctl.c
> @@ -61,6 +61,30 @@ enum {
>  	NFSD_MaxReserved
>  };
>  
> +/**
> + * nfsd_put - put the reference to the nfsd_serv for given net
> + * @net: the net namespace for the serv
> + * @err: current error for the op
> + *
> + * When putting a reference to the nfsd_serv from a control operation
> + * we must first call nfsd_last_thread if all of these are true:
> + *
> + * - the configuration operation is going fail
> + * - there are no running threads
> + * - there are no successfully configured ports
> + *
> + * Otherwise, just put the serv reference.
> + */
> +static inline void nfsd_put(struct net *net, int err)
> +{
> +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> +	struct svc_serv *serv = nn->nfsd_serv;
> +
> +	if (err < 0 && !nn->nfsd_serv->sv_nrthreads && !nn->keep_active)
> +		nfsd_last_thread(net);
> +	svc_put(serv);
> +}
> +
>  /*
>   * write() for these nodes.
>   */
> @@ -709,7 +733,7 @@ static ssize_t __write_ports_addfd(char *buf, struct net *net, const struct cred
>  	    !nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
>  		svc_get(nn->nfsd_serv);
>  
> -	nfsd_put(net);
> +	nfsd_put(net, err);
>  	return err;
>  }
>  
> @@ -748,7 +772,7 @@ static ssize_t __write_ports_addxprt(char *buf, struct net *net, const struct cr
>  	if (!nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
>  		svc_get(nn->nfsd_serv);
>  
> -	nfsd_put(net);
> +	nfsd_put(net, 0);
>  	return 0;
>  out_close:
>  	xprt = svc_find_xprt(nn->nfsd_serv, transport, net, PF_INET, port);
> @@ -757,7 +781,7 @@ static ssize_t __write_ports_addxprt(char *buf, struct net *net, const struct cr
>  		svc_xprt_put(xprt);
>  	}
>  out_err:
> -	nfsd_put(net);
> +	nfsd_put(net, err);
>  	return err;
>  }
>  
> @@ -1687,7 +1711,7 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
>  int nfsd_nl_rpc_status_get_done(struct netlink_callback *cb)
>  {
>  	mutex_lock(&nfsd_mutex);
> -	nfsd_put(sock_net(cb->skb->sk));
> +	nfsd_put(sock_net(cb->skb->sk), 0);
>  	mutex_unlock(&nfsd_mutex);
>  
>  	return 0;
> diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> index f5ff42f41ee7..3aa8cd2c19ac 100644
> --- a/fs/nfsd/nfsd.h
> +++ b/fs/nfsd/nfsd.h
> @@ -113,13 +113,6 @@ int		nfsd_pool_stats_open(struct inode *, struct file *);
>  int		nfsd_pool_stats_release(struct inode *, struct file *);
>  void		nfsd_shutdown_threads(struct net *net);
>  
> -static inline void nfsd_put(struct net *net)
> -{
> -	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> -
> -	svc_put(nn->nfsd_serv);
> -}
> -
>  bool		i_am_nfsd(void);
>  
>  struct nfsdfs_client {
> @@ -153,6 +146,7 @@ struct nfsd_net;
>  enum vers_op {NFSD_SET, NFSD_CLEAR, NFSD_TEST, NFSD_AVAIL };
>  int nfsd_vers(struct nfsd_net *nn, int vers, enum vers_op change);
>  int nfsd_minorversion(struct nfsd_net *nn, u32 minorversion, enum vers_op change);
> +void nfsd_last_thread(struct net *net);
>  void nfsd_reset_versions(struct nfsd_net *nn);
>  int nfsd_create_serv(struct net *net);
>  
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index fe61d9bbcc1f..d6939e23ffcf 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -542,7 +542,7 @@ static struct notifier_block nfsd_inet6addr_notifier = {
>  /* Only used under nfsd_mutex, so this atomic may be overkill: */
>  static atomic_t nfsd_notifier_refcount = ATOMIC_INIT(0);
>  
> -static void nfsd_last_thread(struct net *net)
> +void nfsd_last_thread(struct net *net)
>  {
>  	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
>  	struct svc_serv *serv = nn->nfsd_serv;
> 
> ---
> base-commit: a39b6ac3781d46ba18193c9dbb2110f31e9bffe9
> change-id: 20231211-nfsd-fixes-d9f21d5c12d7
> 
> Best regards,
> -- 
> Jeff Layton <jlayton@xxxxxxxxxx>
> 
>