Subject: Re: [RFC PATCH v1 2/9] landlock: Support TCP listen access-control

From: Mickaël Salaün <[email-masked]>
Date: Wed, 31 Jul 2024 20:30:01 +0200

On Sun, Jul 28, 2024 at 08:25:55AM +0800, Mikhail Ivanov wrote:
> LANDLOCK_ACCESS_NET_BIND_TCP is useful to limit the scope of "bindable"
> ports to forbid a malicious sandboxed process to impersonate a legitimate
> server process. However, bind(2) might be used by (TCP) clients to set the
> source port to a (legitimate) value. Controlling the ports that can be
> used for listening would allow (TCP) clients to explicitly bind to ports
> that are forbidden for listening.
> 
> Such control is implemented with a new LANDLOCK_ACCESS_NET_LISTEN_TCP
> access right that restricts listening on undesired ports with listen(2).
> 
> It's worth noticing that this access right doesn't affect changing
> backlog value using listen(2) on already listening socket.
> 
> * Create new LANDLOCK_ACCESS_NET_LISTEN_TCP flag.
> * Add hook to socket_listen(), which checks whether the socket is allowed
>   to listen on a binded local port.
> * Add check_tcp_socket_can_listen() helper, which validates socket
>   attributes before the actual access right check.
> * Update `struct landlock_net_port_attr` documentation with control of
>   binding to ephemeral port with listen(2) description.
> * Change ABI version to 6.
> 
> Closes: https://github.com/landlock-lsm/linux/issues/15
> Signed-off-by: Mikhail Ivanov <[email-masked]>

Thanks for this series!

I cannot apply this patch series though, could you please provide the
base commit?  BTW, this can be automatically put in the cover letter
with the git format-patch's --base argument.

> ---
>  include/uapi/linux/landlock.h                | 23 +++--
>  security/landlock/limits.h                   |  2 +-
>  security/landlock/net.c                      | 90 ++++++++++++++++++++
>  security/landlock/syscalls.c                 |  2 +-
>  tools/testing/selftests/landlock/base_test.c |  2 +-
>  5 files changed, 108 insertions(+), 11 deletions(-)
> 
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index 68625e728f43..6b8df3293eee 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -104,13 +104,16 @@ struct landlock_net_port_attr {
>  	/**
>  	 * @port: Network port in host endianness.
>  	 *
> -	 * It should be noted that port 0 passed to :manpage:`bind(2)` will
> -	 * bind to an available port from a specific port range. This can be
> -	 * configured thanks to the ``/proc/sys/net/ipv4/ip_local_port_range``
> -	 * sysctl (also used for IPv6). A Landlock rule with port 0 and the
> -	 * ``LANDLOCK_ACCESS_NET_BIND_TCP`` right means that requesting to bind
> -	 * on port 0 is allowed and it will automatically translate to binding
> -	 * on the related port range.
> +	 * It should be noted that some operations cause binding socket to a random
> +	 * available port from a specific port range. This can be configured thanks
> +	 * to the ``/proc/sys/net/ipv4/ip_local_port_range`` sysctl (also used for
> +	 * IPv6). Following operation requests are automatically translate to
> +	 * binding on the related port range:
> +	 *
> +	 * - A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_BIND_TCP``
> +	 *   right means that binding on port 0 is allowed.
> +	 * - A Landlock rule with port 0 and the ``LANDLOCK_ACCESS_NET_LISTEN_TCP``
> +	 *   right means listening without an explicit binding is allowed.
>  	 */
>  	__u64 port;
>  };
> @@ -251,7 +254,7 @@ struct landlock_net_port_attr {
>   * DOC: net_access
>   *
>   * Network flags
> - * ~~~~~~~~~~~~~~~~
> + * ~~~~~~~~~~~~~
>   *
>   * These flags enable to restrict a sandboxed process to a set of network
>   * actions. This is supported since the Landlock ABI version 4.
> @@ -261,9 +264,13 @@ struct landlock_net_port_attr {
>   * - %LANDLOCK_ACCESS_NET_BIND_TCP: Bind a TCP socket to a local port.
>   * - %LANDLOCK_ACCESS_NET_CONNECT_TCP: Connect an active TCP socket to
>   *   a remote port.
> + * - %LANDLOCK_ACCESS_NET_LISTEN_TCP: Listen for TCP socket connections on
> + *   a local port. This access right is available since the sixth version
> + *   of the Landlock ABI.
>   */
>  /* clang-format off */
>  #define LANDLOCK_ACCESS_NET_BIND_TCP			(1ULL << 0)
>  #define LANDLOCK_ACCESS_NET_CONNECT_TCP			(1ULL << 1)
> +#define LANDLOCK_ACCESS_NET_LISTEN_TCP			(1ULL << 2)
>  /* clang-format on */
>  #endif /* _UAPI_LINUX_LANDLOCK_H */
> diff --git a/security/landlock/limits.h b/security/landlock/limits.h
> index 4eb643077a2a..2ef147389474 100644
> --- a/security/landlock/limits.h
> +++ b/security/landlock/limits.h
> @@ -22,7 +22,7 @@
>  #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
>  #define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
>  
> -#define LANDLOCK_LAST_ACCESS_NET	LANDLOCK_ACCESS_NET_CONNECT_TCP
> +#define LANDLOCK_LAST_ACCESS_NET	LANDLOCK_ACCESS_NET_LISTEN_TCP
>  #define LANDLOCK_MASK_ACCESS_NET	((LANDLOCK_LAST_ACCESS_NET << 1) - 1)
>  #define LANDLOCK_NUM_ACCESS_NET		__const_hweight64(LANDLOCK_MASK_ACCESS_NET)
>  
> diff --git a/security/landlock/net.c b/security/landlock/net.c
> index 669ba260342f..a29cb27c3f14 100644
> --- a/security/landlock/net.c
> +++ b/security/landlock/net.c
> @@ -6,10 +6,12 @@
>   * Copyright © 2022-2023 Microsoft Corporation
>   */
>  
> +#include "net/sock.h"

These should not be quotes.

>  #include <linux/in.h>
>  #include <linux/net.h>
>  #include <linux/socket.h>
>  #include <net/ipv6.h>
> +#include <net/tcp.h>
>  
>  #include "common.h"
>  #include "cred.h"
> @@ -194,9 +196,97 @@ static int hook_socket_connect(struct socket *const sock,
>  					   LANDLOCK_ACCESS_NET_CONNECT_TCP);
>  }
>  
> +/*
> + * Checks that socket state and attributes are correct for listen.
> + * It is required to not wrongfully return -EACCES instead of -EINVAL.
> + *
> + * This checker requires sock->sk to be locked.
> + */
> +static int check_tcp_socket_can_listen(struct socket *const sock)

Is this function still useful with the listen LSM hook?

> +{
> +	struct sock *sk = sock->sk;
> +	unsigned char cur_sk_state = sk->sk_state;
> +	const struct tcp_ulp_ops *icsk_ulp_ops;
> +
> +	/* Allows only unconnected TCP socket to listen (cf. inet_listen). */
> +	if (sock->state != SS_UNCONNECTED)
> +		return -EINVAL;
> +
> +	/*
> +	 * Checks sock state. This is needed to ensure consistency with inet stack
> +	 * error handling (cf. __inet_listen_sk).
> +	 */
> +	if (WARN_ON_ONCE(!((1 << cur_sk_state) & (TCPF_CLOSE | TCPF_LISTEN))))
> +		return -EINVAL;
> +
> +	icsk_ulp_ops = inet_csk(sk)->icsk_ulp_ops;
> +
> +	/*
> +	 * ULP (Upper Layer Protocol) stands for protocols which are higher than
> +	 * transport protocol in OSI model. Linux has an infrastructure that
> +	 * allows TCP sockets to support logic of some ULP (e.g. TLS ULP).
> +	 *
> +	 * Sockets can listen only if ULP control hook has clone method.
> +	 */
> +	if (icsk_ulp_ops && !icsk_ulp_ops->clone)
> +		return -EINVAL;
> +	return 0;
> +}
> +
> +static int hook_socket_listen(struct socket *const sock, const int backlog)
> +{

Why can't we just call current_check_access_socket()?

> +	int err = 0;
> +	int family;
> +	__be16 port;
> +	struct sock *sk;
> +	const struct landlock_ruleset *const dom = get_current_net_domain();
> +
> +	if (!dom)
> +		return 0;
> +	if (WARN_ON_ONCE(dom->num_layers < 1))
> +		return -EACCES;
> +
> +	/* Checks if it's a (potential) TCP socket. */
> +	if (sock->type != SOCK_STREAM)
> +		return 0;
> +
> +	sk = sock->sk;
> +	family = sk->__sk_common.skc_family;
> +	/*
> +	 * Socket cannot be assigned AF_UNSPEC because this type is used only
> +	 * in the context of addresses.
> +	 *
> +	 * Doesn't restrict listening for non-TCP sockets.
> +	 */
> +	if (family != AF_INET && family != AF_INET6)
> +		return 0;
> +
> +	lock_sock(sk);
> +	/*
> +	 * Calling listen(2) for a listening socket does nothing with its state and
> +	 * only changes backlog value (cf. __inet_listen_sk). Checking of listen
> +	 * access right is not required.
> +	 */
> +	if (sk->sk_state == TCP_LISTEN)
> +		goto release_nocheck;
> +
> +	err = check_tcp_socket_can_listen(sock);
> +	if (unlikely(err))
> +		goto release_nocheck;
> +
> +	port = htons(inet_sk(sk)->inet_num);
> +	release_sock(sk);
> +	return check_access_socket(dom, port, LANDLOCK_ACCESS_NET_LISTEN_TCP);
> +
> +release_nocheck:
> +	release_sock(sk);
> +	return err;
> +}
> +
>  static struct security_hook_list landlock_hooks[] __ro_after_init = {
>  	LSM_HOOK_INIT(socket_bind, hook_socket_bind),
>  	LSM_HOOK_INIT(socket_connect, hook_socket_connect),
> +	LSM_HOOK_INIT(socket_listen, hook_socket_listen),
>  };
>  
>  __init void landlock_add_net_hooks(void)
> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> index 03b470f5a85a..3752bcc033d4 100644
> --- a/security/landlock/syscalls.c
> +++ b/security/landlock/syscalls.c
> @@ -149,7 +149,7 @@ static const struct file_operations ruleset_fops = {
>  	.write = fop_dummy_write,
>  };
>  
> -#define LANDLOCK_ABI_VERSION 5
> +#define LANDLOCK_ABI_VERSION 6
>  
>  /**
>   * sys_landlock_create_ruleset - Create a new ruleset
> diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
> index 3c1e9f35b531..52b00472a487 100644
> --- a/tools/testing/selftests/landlock/base_test.c
> +++ b/tools/testing/selftests/landlock/base_test.c
> @@ -75,7 +75,7 @@ TEST(abi_version)
>  	const struct landlock_ruleset_attr ruleset_attr = {
>  		.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
>  	};
> -	ASSERT_EQ(5, landlock_create_ruleset(NULL, 0,
> +	ASSERT_EQ(6, landlock_create_ruleset(NULL, 0,
>  					     LANDLOCK_CREATE_RULESET_VERSION));
>  
>  	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,
> -- 
> 2.34.1
> 
>