Re: [syzbot] [net?] KASAN: slab-use-after-free Read in unix_del_edges

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: syzbot <syzbot+f3f3eef1d2100200e593@syzkaller.appspotmail.com>
Date: Fri, 19 Apr 2024 02:39:21 -0700
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    7b4f2bc91c15 Add linux-next specific files for 20240418
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=14a54a53180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=ae644165a243bf62
> dashboard link: https://syzkaller.appspot.com/bug?extid=f3f3eef1d2100200e593
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=155e53af180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128b1d53180000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/524a18e6c5be/disk-7b4f2bc9.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/029f1b84d653/vmlinux-7b4f2bc9.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/c02d1542e886/bzImage-7b4f2bc9.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+f3f3eef1d2100200e593@syzkaller.appspotmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git main

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 95240a59808f..039c1c8c73f4 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -158,11 +158,14 @@ 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;
 
-	unix_update_graph(unix_edge_successor(edge));
+	if (!gc_in_progress)
+		unix_update_graph(unix_edge_successor(edge));
 
 	list_del(&edge->vertex_entry);
 	vertex->out_degree--;
@@ -237,8 +240,10 @@ void unix_del_edges(struct scm_fp_list *fpl)
 		unix_del_edge(fpl, edge);
 	} while (i < fpl->count_unix);
 
-	receiver = fpl->edges[0].successor;
-	receiver->scm_stat.nr_unix_fds -= fpl->count_unix;
+	if (!gc_in_progress) {
+		receiver = fpl->edges[0].successor;
+		receiver->scm_stat.nr_unix_fds -= fpl->count_unix;
+	}
 	WRITE_ONCE(unix_tot_inflight, unix_tot_inflight - fpl->count_unix);
 out:
 	WRITE_ONCE(fpl->user->unix_inflight, fpl->user->unix_inflight - fpl->count);
@@ -550,8 +555,6 @@ 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;