Date:   Fri, 13 Dec 2019 06:49:34 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
Cc:     Maciej Żenczykowski <maze@...gle.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Sean Tranchetti <stranche@...eaurora.org>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux SCTP <linux-sctp@...r.kernel.org>
Subject: Re: [PATCH] net: introduce ip_local_unbindable_ports sysctl

On Tue, Nov 26, 2019 at 04:13:13PM -0800, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <maze@...gle.com>
> 
> and associated inet_is_local_unbindable_port() helper function:
> use it to make explicitly binding to an unbindable port return
> -EPERM 'Operation not permitted'.
> 
> Autobind doesn't honour this new sysctl since:
>   (a) you can simply set both if that's the behaviour you desire
>   (b) there could be a use for preventing explicit while allowing auto
>   (c) it's faster in the relatively critical path of doing port selection
>       during connect() to only check one bitmap instead of both
> 
> Various ports may have special use cases which are not suitable for
> use by general userspace applications. Currently, ports specified in
> ip_local_reserved_ports sysctl will not be returned only in case of
> automatic port assignment, but nothing prevents you from explicitly
> binding to them - even from an entirely unprivileged process.
> 
> In certain cases it is desirable to prevent the host from assigning the
> ports even in case of explicit binds, even from superuser processes.
> 
> Example use cases might be:
>  - a port being stolen by the nic for remote serial console, remote
>    power management or some other sort of debugging functionality
>    (crash collection, gdb, direct access to some other microcontroller
>    on the nic or motherboard, remote management of the nic itself).
>  - a transparent proxy where packets are being redirected: in case
>    a socket matches this connection, packets from this application
>    would be incorrectly sent to one of the endpoints.
> 
> Initially I wanted to solve this problem via the simple one line:
> 
> static inline bool inet_port_requires_bind_service(struct net *net, unsigned short port) {
> -       return port < net->ipv4.sysctl_ip_prot_sock;
> +       return port < net->ipv4.sysctl_ip_prot_sock || inet_is_local_reserved_port(net, port);
> }
> 
> However, this doesn't work for two reasons:
>   (a) it changes userspace visible behaviour of the existing local
>       reserved ports sysctl, and there appears to be enough documentation
>       on the internet talking about setting it to make this a bad idea
>   (b) it doesn't prevent privileged apps from using these ports,
>       CAP_BIND_SERVICE is relatively likely to be available to, for example,
>       a recursive DNS server so it can listed on port 53, which also needs
>       to do src port randomization for outgoing queries due to security
>       reasons (and it thus does manual port binding).
> 
> If we *know* that certain ports are simply unusable, then it's better
> nothing even gets the opportunity to try to use them.  This way we at
> least get a quick failure, instead of some sort of timeout (or possibly
> even corruption of the data stream of the non-kernel based use case).
> 
> Test:
>   vm:~# cat /proc/sys/net/ipv4/ip_local_unbindable_ports
> 
>   vm:~# python -c 'import socket; s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM, 0); s.bind(("::", 3967))'
>   vm:~# python -c 'import socket; s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM, 0); s.bind(("::", 3967))'
>   vm:~# echo 3967 > /proc/sys/net/ipv4/ip_local_unbindable_ports
>   vm:~# cat /proc/sys/net/ipv4/ip_local_unbindable_ports
>   3967
>   vm:~# python -c 'import socket; s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM, 0); s.bind(("::", 3967))'
>   socket.error: (1, 'Operation not permitted')
>   vm:~# python -c 'import socket; s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM, 0); s.bind(("::", 3967))'
>   socket.error: (1, 'Operation not permitted')
> 
> Cc: Sean Tranchetti <stranche@...eaurora.org>
> Cc: Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Linux SCTP <linux-sctp@...r.kernel.org>
> Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
> ---
>  Documentation/networking/ip-sysctl.txt | 13 +++++++++++++
>  include/net/ip.h                       | 12 ++++++++++++
>  include/net/netns/ipv4.h               |  1 +
>  net/ipv4/af_inet.c                     |  4 ++++
>  net/ipv4/sysctl_net_ipv4.c             | 18 ++++++++++++++++--
>  net/ipv6/af_inet6.c                    |  2 ++
>  net/sctp/socket.c                      |  5 +++++
>  7 files changed, 53 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index fd26788e8c96..7129646a18bd 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -940,6 +940,19 @@ ip_local_reserved_ports - list of comma separated ranges
>  
>  	Default: Empty
>  
> +ip_local_unbindable_ports - list of comma separated ranges
> +	Specify the ports which are not directly bind()able.
> +
> +	Usually you would use this to block the use of ports which
> +	are invalid due to something outside of the control of the
> +	kernel.  For example a port stolen by the nic for serial
> +	console, remote power management or debugging.
> +
> +	There's a relatively high chance you will also want to list
> +	these ports in 'ip_local_reserved_ports' to prevent autobinding.
> +
> +	Default: Empty
> +
>  ip_unprivileged_port_start - INTEGER
>  	This is a per-namespace sysctl.  It defines the first
>  	unprivileged port in the network namespace.  Privileged ports
> diff --git a/include/net/ip.h b/include/net/ip.h
> index 02d68e346f67..14b99bf59ffc 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -346,6 +346,13 @@ static inline bool inet_is_local_reserved_port(struct net *net, unsigned short p
>  	return test_bit(port, net->ipv4.sysctl_local_reserved_ports);
>  }
>  
> +static inline bool inet_is_local_unbindable_port(struct net *net, unsigned short port)
> +{
> +	if (!net->ipv4.sysctl_local_unbindable_ports)
> +		return false;
> +	return test_bit(port, net->ipv4.sysctl_local_unbindable_ports);
> +}
> +
>  static inline bool sysctl_dev_name_is_allowed(const char *name)
>  {
>  	return strcmp(name, "default") != 0  && strcmp(name, "all") != 0;
> @@ -362,6 +369,11 @@ static inline bool inet_is_local_reserved_port(struct net *net, unsigned short p
>  	return false;
>  }
>  
> +static inline bool inet_is_local_unbindable_port(struct net *net, unsigned short port)
> +{
> +	return false;
> +}
> +
>  static inline bool inet_port_requires_bind_service(struct net *net, unsigned short port)
>  {
>  	return port < PROT_SOCK;
> diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> index c0c0791b1912..6a235651925d 100644
> --- a/include/net/netns/ipv4.h
> +++ b/include/net/netns/ipv4.h
> @@ -197,6 +197,7 @@ struct netns_ipv4 {
>  
>  #ifdef CONFIG_SYSCTL
>  	unsigned long *sysctl_local_reserved_ports;
> +	unsigned long *sysctl_local_unbindable_ports;
>  	int sysctl_ip_prot_sock;
>  #endif
>  
> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> index 2fe295432c24..b26046431612 100644
> --- a/net/ipv4/af_inet.c
> +++ b/net/ipv4/af_inet.c
> @@ -494,6 +494,10 @@ int __inet_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len,
>  		goto out;
>  
>  	snum = ntohs(addr->sin_port);
> +	err = -EPERM;
> +	if (snum && inet_is_local_unbindable_port(net, snum))
> +		goto out;
> +
>  	err = -EACCES;
>  	if (snum && inet_port_requires_bind_service(net, snum) &&
>  	    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index fcb2cd167f64..fd363b57a653 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -745,6 +745,13 @@ static struct ctl_table ipv4_net_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= proc_do_large_bitmap,
>  	},
> +	{
> +		.procname	= "ip_local_unbindable_ports",
> +		.data		= &init_net.ipv4.sysctl_local_unbindable_ports,
> +		.maxlen		= 65536,
> +		.mode		= 0644,
> +		.proc_handler	= proc_do_large_bitmap,
> +	},
>  	{
>  		.procname	= "ip_no_pmtu_disc",
>  		.data		= &init_net.ipv4.sysctl_ip_no_pmtu_disc,
> @@ -1353,11 +1360,17 @@ static __net_init int ipv4_sysctl_init_net(struct net *net)
>  
>  	net->ipv4.sysctl_local_reserved_ports = kzalloc(65536 / 8, GFP_KERNEL);
>  	if (!net->ipv4.sysctl_local_reserved_ports)
> -		goto err_ports;
> +		goto err_reserved_ports;
> +
> +	net->ipv4.sysctl_local_unbindable_ports = kzalloc(65536 / 8, GFP_KERNEL);
> +	if (!net->ipv4.sysctl_local_unbindable_ports)
> +		goto err_unbindable_ports;
>  
>  	return 0;
>  
> -err_ports:
> +err_unbindable_ports:
> +	kfree(net->ipv4.sysctl_local_reserved_ports);
> +err_reserved_ports:
>  	unregister_net_sysctl_table(net->ipv4.ipv4_hdr);
>  err_reg:
>  	if (!net_eq(net, &init_net))
> @@ -1370,6 +1383,7 @@ static __net_exit void ipv4_sysctl_exit_net(struct net *net)
>  {
>  	struct ctl_table *table;
>  
> +	kfree(net->ipv4.sysctl_local_unbindable_ports);
>  	kfree(net->ipv4.sysctl_local_reserved_ports);
>  	table = net->ipv4.ipv4_hdr->ctl_table_arg;
>  	unregister_net_sysctl_table(net->ipv4.ipv4_hdr);
> diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
> index 60e2ff91a5b3..3c83e3200543 100644
> --- a/net/ipv6/af_inet6.c
> +++ b/net/ipv6/af_inet6.c
> @@ -292,6 +292,8 @@ static int __inet6_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len,
>  		return -EINVAL;
>  
>  	snum = ntohs(addr->sin6_port);
> +	if (snum && inet_is_local_unbindable_port(net, snum))
> +		return -EPERM;
>  	if (snum && inet_port_requires_bind_service(net, snum) &&
>  	    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
>  		return -EACCES;
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 0b485952a71c..d1c93542419d 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -384,6 +384,9 @@ static int sctp_do_bind(struct sock *sk, union sctp_addr *addr, int len)
>  		}
>  	}
>  
> +	if (snum && inet_is_local_unbindable_port(net, snum))
> +		return -EPERM;
> +
>  	if (snum && inet_port_requires_bind_service(net, snum) &&
>  	    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
>  		return -EACCES;
> @@ -1061,6 +1064,8 @@ static int sctp_connect_new_asoc(struct sctp_endpoint *ep,
>  		if (sctp_autobind(sk))
>  			return -EAGAIN;
>  	} else {
> +		if (inet_is_local_unbindable_port(net, ep->base.bind_addr.port))
> +			return -EPERM;
>  		if (inet_port_requires_bind_service(net, ep->base.bind_addr.port) &&
>  		    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
>  			return -EACCES;
> -- 
> 2.24.0.432.g9d3f5f5b63-goog
> 
> 

Just out of curiosity, why are the portreserve and portrelease utilities not a
solution to this use case?

Neil

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