Re: [PATCH v4 1/6] lsm: Add LSM hook security_unix_find

["Mickaël Salaün" <mic@digikod.net>]

On Mon, Feb 09, 2026 at 12:10:11AM +0100, Günther Noack wrote:
> From: Justin Suess <utilityemal77@gmail.com>
> 
> Add a LSM hook security_unix_find.
> 
> This hook is called to check the path of a named unix socket before a
> connection is initiated. The peer socket may be inspected as well.
> 
> Why existing hooks are unsuitable:
> 
> Existing socket hooks, security_unix_stream_connect(),
> security_unix_may_send(), and security_socket_connect() don't provide
> TOCTOU-free / namespace independent access to the paths of sockets.
> 
> (1) We cannot resolve the path from the struct sockaddr in existing hooks.
> This requires another path lookup. A change in the path between the
> two lookups will cause a TOCTOU bug.
> 
> (2) We cannot use the struct path from the listening socket, because it
> may be bound to a path in a different namespace than the caller,
> resulting in a path that cannot be referenced at policy creation time.
> 
> Cc: Günther Noack <gnoack3000@gmail.com>
> Cc: Tingmao Wang <m@maowtm.org>
> Signed-off-by: Justin Suess <utilityemal77@gmail.com>
> ---
>  include/linux/lsm_hook_defs.h |  5 +++++
>  include/linux/security.h      | 11 +++++++++++
>  net/unix/af_unix.c            |  9 +++++++++
>  security/security.c           | 20 ++++++++++++++++++++
>  4 files changed, 45 insertions(+)
> 
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 8c42b4bde09c..7a0fd3dbfa29 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -317,6 +317,11 @@ LSM_HOOK(int, 0, post_notification, const struct cred *w_cred,
>  LSM_HOOK(int, 0, watch_key, struct key *key)
>  #endif /* CONFIG_SECURITY && CONFIG_KEY_NOTIFICATIONS */
>  
> +#if defined(CONFIG_SECURITY_NETWORK) && defined(CONFIG_SECURITY_PATH)
> +LSM_HOOK(int, 0, unix_find, const struct path *path, struct sock *other,
> +	 int flags)
> +#endif /* CONFIG_SECURITY_NETWORK && CONFIG_SECURITY_PATH */
> +
>  #ifdef CONFIG_SECURITY_NETWORK
>  LSM_HOOK(int, 0, unix_stream_connect, struct sock *sock, struct sock *other,
>  	 struct sock *newsk)
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 83a646d72f6f..99a33d8eb28d 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -1931,6 +1931,17 @@ static inline int security_mptcp_add_subflow(struct sock *sk, struct sock *ssk)
>  }
>  #endif	/* CONFIG_SECURITY_NETWORK */
>  
> +#if defined(CONFIG_SECURITY_NETWORK) && defined(CONFIG_SECURITY_PATH)
> +
> +int security_unix_find(const struct path *path, struct sock *other, int flags);
> +
> +#else /* CONFIG_SECURITY_NETWORK && CONFIG_SECURITY_PATH */
> +static inline int security_unix_find(const struct path *path, struct sock *other, int flags)
> +{
> +	return 0;
> +}
> +#endif /* CONFIG_SECURITY_NETWORK && CONFIG_SECURITY_PATH */
> +
>  #ifdef CONFIG_SECURITY_INFINIBAND
>  int security_ib_pkey_access(void *sec, u64 subnet_prefix, u16 pkey);
>  int security_ib_endport_manage_subnet(void *sec, const char *name, u8 port_num);
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index d0511225799b..db9d279b3883 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1226,10 +1226,19 @@ static struct sock *unix_find_bsd(struct sockaddr_un *sunaddr, int addr_len,
>  	if (!S_ISSOCK(inode->i_mode))
>  		goto path_put;
>  
> +	err = -ECONNREFUSED;

We don't see it in this patch but err is already set to -ECONNREFUSED.
This line might be confusing, and unrelated to the goal of this patch,
so we should remove it.


>  	sk = unix_find_socket_byinode(inode);
>  	if (!sk)
>  		goto path_put;
>  
> +	/*
> +	 * We call the hook because we know that the inode is a socket
> +	 * and we hold a valid reference to it via the path.

This comment can be alligned with 80 columns.

> +	 */
> +	err = security_unix_find(&path, sk, flags);

This hook makes sense and is quite generic.

> +	if (err)
> +		goto sock_put;
> +
>  	err = -EPROTOTYPE;
>  	if (sk->sk_type == type)
>  		touch_atime(&path);
> diff --git a/security/security.c b/security/security.c
> index 31a688650601..9e9515955098 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -4731,6 +4731,26 @@ int security_mptcp_add_subflow(struct sock *sk, struct sock *ssk)
>  
>  #endif	/* CONFIG_SECURITY_NETWORK */
>  
> +#if defined(CONFIG_SECURITY_NETWORK) && defined(CONFIG_SECURITY_PATH)
> +/*

This should be a docstring like other hooks: /**

> + * security_unix_find() - Check if a named AF_UNIX socket can connect
> + * @path: path of the socket being connected to
> + * @other: peer sock
> + * @flags: flags associated with the socket
> + *
> + * This hook is called to check permissions before connecting to a named
> + * AF_UNIX socket.
> + *
> + * Return: Returns 0 if permission is granted.
> + */
> +int security_unix_find(const struct path *path, struct sock *other, int flags)
> +{
> +	return call_int_hook(unix_find, path, other, flags);
> +}
> +EXPORT_SYMBOL(security_unix_find);
> +
> +#endif	/* CONFIG_SECURITY_NETWORK && CONFIG_SECURITY_PATH */
> +
>  #ifdef CONFIG_SECURITY_INFINIBAND
>  /**
>   * security_ib_pkey_access() - Check if access to an IB pkey is allowed
> -- 
> 2.52.0
> 
>