[PATCH v1] landlock: Harden sock_is_scoped() against file-less sockets

sock_is_scoped() dereferences other->sk_socket->file->f_cred to read the
peer's Landlock domain when evaluating
LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET, without first checking that the
peer has a backing socket and file.  hook_unix_find() performs the same
dereference for LANDLOCK_ACCESS_FS_RESOLVE_UNIX and does guard it.
Guard it here too and treat a peer with no backing file, such as a
kernel socket created by sock_create_kern(), as unscoped.

This is defensive hardening, not a fix for a reachable bug.  The
unix_stream_connect() and unix_may_send() hooks run with the peer held
under unix_state_lock() and only after the AF_UNIX core has excluded
SOCK_DEAD, and no in-tree code binds a file-less AF_UNIX socket to an
abstract address, so other->sk_socket->file is always valid at these
call sites today.

Cc: Günther Noack <[email protected]>
Signed-off-by: Mickaël Salaün <[email protected]>
---
 security/landlock/task.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/security/landlock/task.c b/security/landlock/task.c
index 7ddf211f75c3..474642edbd59 100644
--- a/security/landlock/task.c
+++ b/security/landlock/task.c
@@ -243,6 +243,17 @@ static bool sock_is_scoped(struct sock *const other,
 
 	/* The credentials will not change. */
 	lockdep_assert_held(&unix_sk(other)->lock);
+
+	/*
+	 * A live kernel socket (e.g. from sock_create_kern()) has no backing
+	 * file, hence no Landlock domain, so treat it as unscoped.  The
+	 * sk_socket check only guards that dereference; sk_socket is NULL
+	 * solely for a dead peer, which the caller already excludes under the
+	 * held lock, so no separate SOCK_DEAD check is needed.
+	 */
+	if (unlikely(!other->sk_socket || !other->sk_socket->file))
+		return false;
+
 	dom_other = landlock_cred(other->sk_socket->file->f_cred)->domain;
 	return domain_is_scoped(domain, dom_other,
 				LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET);

base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
-- 
2.54.0