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UPSTREAM: af_unix: Fix up unix_edge.successor for embryo socket.

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commit dcf70df2048d27c5d186f013f101a4aefd63aa41 upstream.

To garbage collect inflight AF_UNIX sockets, we must define the
cyclic reference appropriately.  This is a bit tricky if the loop
consists of embryo sockets.

Suppose that the fd of AF_UNIX socket A is passed to D and the fd B
to C and that C and D are embryo sockets of A and B, respectively.
It may appear that there are two separate graphs, A (-> D) and
B (-> C), but this is not correct.

     A --. .-- B
          X
     C <-' `-> D

Now, D holds A's refcount, and C has B's refcount, so unix_release()
will never be called for A and B when we close() them.  However, no
one can call close() for D and C to free skbs holding refcounts of A
and B because C/D is in A/B's receive queue, which should have been
purged by unix_release() for A and B.

So, here's another type of cyclic reference.  When a fd of an AF_UNIX
socket is passed to an embryo socket, the reference is indirectly held
by its parent listening socket.

  .-> A                            .-> B
  |   `- sk_receive_queue          |   `- sk_receive_queue
  |      `- skb                    |      `- skb
  |         `- sk == C             |         `- sk == D
  |            `- sk_receive_queue |           `- sk_receive_queue
  |               `- skb +---------'               `- skb +-.
  |                                                         |
  `---------------------------------------------------------'

Technically, the graph must be denoted as A <-> B instead of A (-> D)
and B (-> C) to find such a cyclic reference without touching each
socket's receive queue.

  .-> A --. .-- B <-.
  |        X        |  ==  A <-> B
  `-- C <-' `-> D --'

We apply this fixup during GC by fetching the real successor by
unix_edge_successor().

When we call accept(), we clear unix_sock.listener under unix_gc_lock
not to confuse GC.

Bug: 404256079
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-9-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 27a07364ce)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I6aff43d0002d1417db58f50baff52189e19b96e7
This commit is contained in:
Kuniyuki Iwashima
2025-05-21 14:45:22 +00:00
committed by Lee Jones
parent f5ea8b439d
commit 0c40a05117
3 changed files with 21 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

1
include/net/af_unix.h
View File

@@ -24,6 +24,7 @@ void unix_inflight(struct user_struct *user, struct file *fp);
void unix_notinflight(struct user_struct *user, struct file *fp);
void unix_add_edges(struct scm_fp_list *fpl, struct unix_sock *receiver);
void unix_del_edges(struct scm_fp_list *fpl);
void unix_update_edges(struct unix_sock *receiver);
int unix_prepare_fpl(struct scm_fp_list *fpl);
void unix_destroy_fpl(struct scm_fp_list *fpl);
void unix_gc(void);

2
net/unix/af_unix.c
View File

@@ -1705,7 +1705,7 @@ static int unix_accept(struct socket *sock, struct socket *newsock, int flags,
}
tsk = skb->sk;
unix_sk(tsk)->listener = NULL;
unix_update_edges(unix_sk(tsk));
skb_free_datagram(sk, skb);
wake_up_interruptible(&unix_sk(sk)->peer_wait);

20
net/unix/garbage.c
View File

@@ -101,6 +101,17 @@ struct unix_sock *unix_get_socket(struct file *filp)
return NULL;
}
static struct unix_vertex *unix_edge_successor(struct unix_edge *edge)
{
/* If an embryo socket has a fd,
* the listener indirectly holds the fd's refcnt.
*/
if (edge->successor->listener)
return unix_sk(edge->successor->listener)->vertex;
return edge->successor->vertex;
}
static LIST_HEAD(unix_unvisited_vertices);
enum unix_vertex_index {
@@ -209,6 +220,13 @@ out:
fpl->inflight = false;
}
void unix_update_edges(struct unix_sock *receiver)
{
spin_lock(&unix_gc_lock);
receiver->listener = NULL;
spin_unlock(&unix_gc_lock);
}
int unix_prepare_fpl(struct scm_fp_list *fpl)
{
struct unix_vertex *vertex;
@@ -268,7 +286,7 @@ next_vertex:
/* Explore neighbour vertices (receivers of the current vertex's fd). */
list_for_each_entry(edge, &vertex->edges, vertex_entry) {
struct unix_vertex *next_vertex = edge->successor->vertex;
struct unix_vertex *next_vertex = unix_edge_successor(edge);
if (!next_vertex)
continue;