af_unix: Add dead flag to struct scm_fp_list.

Commit 1af2dfa ("af_unix: Don't access successor in unix_del_edges()
during GC.") fixed use-after-free by avoid accessing edge->successor while
GC is in progress.

However, there could be a small race window where another process could
call unix_del_edges() while gc_in_progress is true and __skb_queue_purge()
is on the way.

So, we need another marker for struct scm_fp_list which indicates if the
skb is garbage-collected.

This patch adds dead flag in struct scm_fp_list and set it true before
calling __skb_queue_purge().

Fixes: 1af2dfa ("af_unix: Don't access successor in unix_del_edges() during GC.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240508171150.50601-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/scm.h

@@ -33,6 +33,7 @@ struct scm_fp_list {
 	short			max;
 #ifdef CONFIG_UNIX
 	bool			inflight;
+	bool			dead;
 	struct list_head	vertices;
 	struct unix_edge	*edges;
 #endif

net/core/scm.c

@@ -91,6 +91,7 @@ static int scm_fp_copy(struct cmsghdr *cmsg, struct scm_fp_list **fplp)
 		fpl->user = NULL;
 #if IS_ENABLED(CONFIG_UNIX)
 		fpl->inflight = false;
+		fpl->dead = false;
 		fpl->edges = NULL;
 		INIT_LIST_HEAD(&fpl->vertices);
 #endif

net/unix/garbage.c

@@ -158,13 +158,11 @@ static void unix_add_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
 	unix_update_graph(unix_edge_successor(edge));
 }
 
-static bool gc_in_progress;
-
 static void unix_del_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
 {
 	struct unix_vertex *vertex = edge->predecessor->vertex;
 
-	if (!gc_in_progress)
+	if (!fpl->dead)
 		unix_update_graph(unix_edge_successor(edge));
 
 	list_del(&edge->vertex_entry);
@@ -240,7 +238,7 @@ void unix_del_edges(struct scm_fp_list *fpl)
 		unix_del_edge(fpl, edge);
 	} while (i < fpl->count_unix);
 
-	if (!gc_in_progress) {
+	if (!fpl->dead) {
 		receiver = fpl->edges[0].successor;
 		receiver->scm_stat.nr_unix_fds -= fpl->count_unix;
 	}
@@ -559,9 +557,12 @@ static void unix_walk_scc_fast(struct sk_buff_head *hitlist)
 	list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
 }
 
+static bool gc_in_progress;
+
 static void __unix_gc(struct work_struct *work)
 {
 	struct sk_buff_head hitlist;
+	struct sk_buff *skb;
 
 	spin_lock(&unix_gc_lock);
 
@@ -579,6 +580,11 @@ static void __unix_gc(struct work_struct *work)
 
 	spin_unlock(&unix_gc_lock);
 
+	skb_queue_walk(&hitlist, skb) {
+		if (UNIXCB(skb).fp)
+			UNIXCB(skb).fp->dead = true;
+	}
+
 	__skb_queue_purge(&hitlist);
 skip_gc:
 	WRITE_ONCE(gc_in_progress, false);