Subject: [PATCH v2] virtio_vsock: Fix race condition in virtio_transport_recv_pkt

From: Jia He <[email-masked]>
Date: Fri, 29 May 2020 23:21:02 +0800

When client tries to connect(SOCK_STREAM) the server in the guest with
NONBLOCK mode, there will be a panic on a ThunderX2 (armv8a server):
[  463.718844][ T5040] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[  463.718848][ T5040] Mem abort info:
[  463.718849][ T5040]   ESR = 0x96000044
[  463.718852][ T5040]   EC = 0x25: DABT (current EL), IL = 32 bits
[  463.718853][ T5040]   SET = 0, FnV = 0
[  463.718854][ T5040]   EA = 0, S1PTW = 0
[  463.718855][ T5040] Data abort info:
[  463.718856][ T5040]   ISV = 0, ISS = 0x00000044
[  463.718857][ T5040]   CM = 0, WnR = 1
[  463.718859][ T5040] user pgtable: 4k pages, 48-bit VAs, pgdp=0000008f6f6e9000
[  463.718861][ T5040] [0000000000000000] pgd=0000000000000000
[  463.718866][ T5040] Internal error: Oops: 96000044 [#1] SMP
[...]
[  463.718977][ T5040] CPU: 213 PID: 5040 Comm: vhost-5032 Tainted: G           O      5.7.0-rc7+ #139
[  463.718980][ T5040] Hardware name: GIGABYTE R281-T91-00/MT91-FS1-00, BIOS F06 09/25/2018
[  463.718982][ T5040] pstate: 60400009 (nZCv daif +PAN -UAO)
[  463.718995][ T5040] pc : virtio_transport_recv_pkt+0x4c8/0xd40 [vmw_vsock_virtio_transport_common]
[  463.718999][ T5040] lr : virtio_transport_recv_pkt+0x1fc/0xd40 [vmw_vsock_virtio_transport_common]
[  463.719000][ T5040] sp : ffff80002dbe3c40
[...]
[  463.719025][ T5040] Call trace:
[  463.719030][ T5040]  virtio_transport_recv_pkt+0x4c8/0xd40 [vmw_vsock_virtio_transport_common]
[  463.719034][ T5040]  vhost_vsock_handle_tx_kick+0x360/0x408 [vhost_vsock]
[  463.719041][ T5040]  vhost_worker+0x100/0x1a0 [vhost]
[  463.719048][ T5040]  kthread+0x128/0x130
[  463.719052][ T5040]  ret_from_fork+0x10/0x18

The race condition as follows:
Task1                            Task2
=====                            =====
__sock_release                   virtio_transport_recv_pkt
  __vsock_release                  vsock_find_bound_socket (found)
    lock_sock_nested
    vsock_remove_sock
    sock_orphan
      sk_set_socket(sk, NULL)
    ...
    release_sock
                                lock_sock
                                   virtio_transport_recv_connecting
                                     sk->sk_socket->state (panic)

The root cause is that vsock_find_bound_socket can't hold the lock_sock,
so there is a small race window between vsock_find_bound_socket() and
lock_sock(). If there is __vsock_release() in another task, sk->sk_socket
will be set to NULL inadvertently.

This fixes it by checking sk->sk_shutdown.

Signed-off-by: Jia He <[email-masked]>
Cc: [email-masked]
Cc: Stefano Garzarella <[email-masked]>
---
v2: use lightweight checking suggested by Stefano Garzarella

 net/vmw_vsock/virtio_transport_common.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 69efc891885f..0edda1edf988 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1132,6 +1132,14 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 
 	lock_sock(sk);
 
+	/* Check if sk has been released before lock_sock */
+	if (sk->sk_shutdown == SHUTDOWN_MASK) {
+		(void)virtio_transport_reset_no_sock(t, pkt);
+		release_sock(sk);
+		sock_put(sk);
+		goto free_pkt;
+	}
+
 	/* Update CID in case it has changed after a transport reset event */
 	vsk->local_addr.svm_cid = dst.svm_cid;
 
-- 
2.17.1