[PATCH v6 1/9] lsm: Add LSM hook security_unix_find

[Paul Moore]

On Mar 15, 2026 =?UTF-8?q?G=C3=BCnther=20Noack?= <gnoack3000 at gmail.com> wrote:
> 
> 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 at gmail.com>
> Cc: Tingmao Wang <m at maowtm.org>
> Signed-off-by: Justin Suess <utilityemal77 at gmail.com>
> ---
>  include/linux/lsm_hook_defs.h |  5 +++++
>  include/linux/security.h      | 11 +++++++++++
>  net/unix/af_unix.c            | 13 ++++++++++---
>  security/security.c           | 20 ++++++++++++++++++++
>  4 files changed, 46 insertions(+), 3 deletions(-)

Some really minor nitpicky things (below), but nothing critical.
However, as we discussed, I would like to see the AppArmor folks comment
on the new hook before we merge anything as I know they have an interest
here.

> 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 */

I'd suggest moving this into the CONFIG_SECURITY_NETWORK that is directly
below this block so you only have to check the CONFIG_SECURITY_PATH
state.  You can place it directly after the existing security_unix*()
hooks.

>  #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 */

Similar to above, I would suggest moving this into the
CONFIG_SECURITY_NETWORK block directly above this so you only need to
check for CONFIG_SECURITY_PATH when declaring the security_unix_find()
hook.

Extra bonus points if you locate it next to the existing security_unix*()
hooks.

>  #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 3756a93dc63a..aced28179bac 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1231,11 +1231,18 @@ static struct sock *unix_find_bsd(struct sockaddr_un *sunaddr, int addr_len,
>  		goto path_put;
>  
>  	err = -EPROTOTYPE;
> -	if (sk->sk_type == type)
> -		touch_atime(&path);
> -	else
> +	if (sk->sk_type != type)
>  		goto sock_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.
> +	 */

I'm not entirely sure that this comment is necessary as it doesn't tell
us anything we don't already know from a quick glance at the code.  Is
there something sneaky, or hard to see, that we should know about?

> +	err = security_unix_find(&path, sk, flags);
> +	if (err)
> +		goto sock_put;
> +	touch_atime(&path);
> +

This is hyper nitpicky, but I'd probably put one line of vertical
whitespace before the touch_atime() call as it has nothing to do with
the LSM hook being called.

>  	path_put(&path);
>  
>  	return sk;
> diff --git a/security/security.c b/security/security.c
> index 67af9228c4e9..c73196b8db4b 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)
> +/**
> + * 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 */

You can probably guess that I'm going to suggest placing this inside the
existing CONFIG_SECURITY_NETWORK block, right after the existing UNIX
hooks :)

--
paul-moore.com