netdev - Re: [PATCH] nfsd: fix double fget() bug in __write_ports

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Message-id: <168548566376.23533.14778348024215909777@noble.neil.brown.name>
Date: Wed, 31 May 2023 08:27:43 +1000
From: "NeilBrown" <neilb@...e.de>
To: "Dan Carpenter" <dan.carpenter@...aro.org>
Cc: "Stanislav Kinsbursky" <skinsbursky@...allels.com>,
 "Chuck Lever" <chuck.lever@...cle.com>, "Jeff Layton" <jlayton@...nel.org>,
 "Trond Myklebust" <trond.myklebust@...merspace.com>,
 "Anna Schumaker" <anna@...nel.org>, "J. Bruce Fields" <bfields@...hat.com>,
 linux-nfs@...r.kernel.org, netdev@...r.kernel.org,
 kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] nfsd: fix double fget() bug in __write_ports_addfd()

On Mon, 29 May 2023, Dan Carpenter wrote:
> The bug here is that you cannot rely on getting the same socket
> from multiple calls to fget() because userspace can influence
> that.  This is a kind of double fetch bug.
> 
> The fix is to delete the svc_alien_sock() function and insted do
> the checking inside the svc_addsock() function.

Hi,
 I definitely agree with the change to pass the 'net' into
 svc_addsock(), and check the the fd has the correct net.

 I'm not sure I agree with the removal of the svc_alien_sock() test.  It
 is best to perform sanity tests before allocation things, and
 nfsd_create_serv() can create a new 'serv' - though most often it just
 incs the refcount.

 Maybe instead svc_alien_sock() could return the struct socket (if
 successful), and it could be passed to svc_addsock()???

 I would probably then change the name of svc_alien_sock()

Thanks,
NeilBrown


> 
> Fixes: 3064639423c4 ("nfsd: check passed socket's net matches NFSd superblock's one")
> Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
> ---
> Based on static analysis and untested.  This goes through the NFS tree. 
> Inspired by CVE-2023-1838.
> 
>  include/linux/sunrpc/svcsock.h |  7 +++----
>  fs/nfsd/nfsctl.c               |  7 +------
>  net/sunrpc/svcsock.c           | 23 +++++------------------
>  3 files changed, 9 insertions(+), 28 deletions(-)
> 
> diff --git a/include/linux/sunrpc/svcsock.h b/include/linux/sunrpc/svcsock.h
> index d16ae621782c..a7116048a4d4 100644
> --- a/include/linux/sunrpc/svcsock.h
> +++ b/include/linux/sunrpc/svcsock.h
> @@ -61,10 +61,9 @@ int		svc_recv(struct svc_rqst *, long);
>  void		svc_send(struct svc_rqst *rqstp);
>  void		svc_drop(struct svc_rqst *);
>  void		svc_sock_update_bufs(struct svc_serv *serv);
> -bool		svc_alien_sock(struct net *net, int fd);
> -int		svc_addsock(struct svc_serv *serv, const int fd,
> -					char *name_return, const size_t len,
> -					const struct cred *cred);
> +int		svc_addsock(struct svc_serv *serv, struct net *net,
> +			    const int fd, char *name_return, const size_t len,
> +			    const struct cred *cred);
>  void		svc_init_xprt_sock(void);
>  void		svc_cleanup_xprt_sock(void);
>  struct svc_xprt *svc_sock_create(struct svc_serv *serv, int prot);
> diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> index e0e98b40a6e5..1489e0b703b4 100644
> --- a/fs/nfsd/nfsctl.c
> +++ b/fs/nfsd/nfsctl.c
> @@ -698,16 +698,11 @@ static ssize_t __write_ports_addfd(char *buf, struct net *net, const struct cred
>  		return -EINVAL;
>  	trace_nfsd_ctl_ports_addfd(net, fd);
>  
> -	if (svc_alien_sock(net, fd)) {
> -		printk(KERN_ERR "%s: socket net is different to NFSd's one\n", __func__);
> -		return -EINVAL;
> -	}
> -
>  	err = nfsd_create_serv(net);
>  	if (err != 0)
>  		return err;
>  
> -	err = svc_addsock(nn->nfsd_serv, fd, buf, SIMPLE_TRANSACTION_LIMIT, cred);
> +	err = svc_addsock(nn->nfsd_serv, net, fd, buf, SIMPLE_TRANSACTION_LIMIT, cred);
>  
>  	if (err >= 0 &&
>  	    !nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 46845cb6465d..e4184e40793c 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -1474,22 +1474,6 @@ static struct svc_sock *svc_setup_socket(struct svc_serv *serv,
>  	return svsk;
>  }
>  
> -bool svc_alien_sock(struct net *net, int fd)
> -{
> -	int err;
> -	struct socket *sock = sockfd_lookup(fd, &err);
> -	bool ret = false;
> -
> -	if (!sock)
> -		goto out;
> -	if (sock_net(sock->sk) != net)
> -		ret = true;
> -	sockfd_put(sock);
> -out:
> -	return ret;
> -}
> -EXPORT_SYMBOL_GPL(svc_alien_sock);
> -
>  /**
>   * svc_addsock - add a listener socket to an RPC service
>   * @serv: pointer to RPC service to which to add a new listener
> @@ -1502,8 +1486,8 @@ EXPORT_SYMBOL_GPL(svc_alien_sock);
>   * Name is terminated with '\n'.  On error, returns a negative errno
>   * value.
>   */
> -int svc_addsock(struct svc_serv *serv, const int fd, char *name_return,
> -		const size_t len, const struct cred *cred)
> +int svc_addsock(struct svc_serv *serv, struct net *net, const int fd,
> +		char *name_return, const size_t len, const struct cred *cred)
>  {
>  	int err = 0;
>  	struct socket *so = sockfd_lookup(fd, &err);
> @@ -1514,6 +1498,9 @@ int svc_addsock(struct svc_serv *serv, const int fd, char *name_return,
>  
>  	if (!so)
>  		return err;
> +	err = -EINVAL;
> +	if (sock_net(so->sk) != net)
> +		goto out;
>  	err = -EAFNOSUPPORT;
>  	if ((so->sk->sk_family != PF_INET) && (so->sk->sk_family != PF_INET6))
>  		goto out;
> -- 
> 2.39.2
> 
>