[PATCH 5.4 44/47] ax25: NPD bug when detaching AX25 device

Greg Kroah-Hartman
2021-12-27T15:36:39Z

From: Lin Ma <linma@zju.edu.cn>

commit 1ade48d0c27d5da1ccf4b583d8c5fc8b534a3ac8 upstream.

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@zju.edu.cn>
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ax25/af_ax25.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -85,8 +85,10 @@ static void ax25_kill_by_device(struct n
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);