netdev - [PATCH v0] ax25: NPD bug when detaching AX25 device

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Date:   Fri, 17 Dec 2021 10:29:41 +0800
From:   Lin Ma <linma@....edu.cn>
To:     linux-hams@...r.kernel.org
Cc:     jreuter@...na.de, ralf@...ux-mips.org, davem@...emloft.net,
        kuba@...nel.org, netdev@...r.kernel.org, nagi@....edu.cn,
        Lin Ma <linma@....edu.cn>
Subject: [PATCH v0] ax25: NPD bug when detaching AX25 device

The existing cleanup routine implementation is not well synchronized
with the syscall routine. When a device is detaching, below race could
occur.

static int ax25_sendmsg(...) {
  ...
  lock_sock()
  ax25 = sk_to_ax25(sk);
  if (ax25->ax25_dev == NULL) // CHECK
  ...
  ax25_queue_xmit(skb, ax25->ax25_dev->dev); // USE
  ...
}

static void ax25_kill_by_device(...) {
  ...
  if (s->ax25_dev == ax25_dev) {
    s->ax25_dev = NULL;
    ...
}

Other syscall functions like ax25_getsockopt, ax25_getname,
ax25_info_show also suffer from similar races. To fix them, this patch
introduce lock_sock() into ax25_kill_by_device in order to guarantee
that the nullify action in cleanup routine cannot proceed when another
socket request is pending.

Signed-off-by: Hanjie Wu <nagi@....edu.cn>
Signed-off-by: Lin Ma <linma@....edu.cn>
---
 net/ax25/af_ax25.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 2f34bbdde0e8..cfca99e295b8 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -85,8 +85,10 @@ static void ax25_kill_by_device(struct net_device *dev)
 again:
 	ax25_for_each(s, &ax25_list) {
 		if (s->ax25_dev == ax25_dev) {
-			s->ax25_dev = NULL;
 			spin_unlock_bh(&ax25_list_lock);
+			lock_sock(s->sk);
+			s->ax25_dev = NULL;
+			release_sock(s->sk);
 			ax25_disconnect(s, ENETUNREACH);
 			spin_lock_bh(&ax25_list_lock);
 
-- 
2.33.1