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

[Justin Suess <utilityemal77@gmail.com>]

> 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.

Done. I debated keeping it, but it seems more appropriate to follow the
convention. Thanks for the catch.

> 
> 
> >  	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.

Done.

> > +	 */
> > +	err = security_unix_find(&path, sk, flags);
> 
> This hook makes sense and is quite generic.

Indeed, I suspect it will be useful for other path-based LSM.

> > +	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: /**

Done.

> 
> > + * 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
> > 
> > 
>

Below follows revised lsm hook patch based on feedback.

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..369812b79dd8 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1230,6 +1230,14 @@ static struct sock *unix_find_bsd(struct sockaddr_un *sunaddr, int addr_len,
 	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.
+	 */
+	err = security_unix_find(&path, sk, flags);
+	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..eaf8f8fdf0c2 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 */
+
 #ifdef CONFIG_SECURITY_INFINIBAND
 /**
  * security_ib_pkey_access() - Check if access to an IB pkey is allowed