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

Hi Stefano

-----Original Message-----
From: Stefano Garzarella <sgarzare@redhat.com>
Sent: Saturday, May 30, 2020 12:34 AM
To: Justin He <redacted>
Cc: Stefan Hajnoczi <stefanha@redhat.com>; David S. Miller
[off-list ref]; Jakub Kicinski [off-list ref];
kvm@vger.kernel.org; virtualization@lists.linux-foundation.org;
netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Kaly Xin
[off-list ref]; stable@vger.kernel.org
Subject: Re: [PATCH v2] virtio_vsock: Fix race condition in
virtio_transport_recv_pkt

On Fri, May 29, 2020 at 11:21:02PM +0800, Jia He wrote:

quoted

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

quoted

[  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

quoted

[  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

quoted

[  463.718980][ T5040] Hardware name: GIGABYTE R281-T91-00/MT91-FS1-00,

BIOS F06 09/25/2018

quoted

[  463.718982][ T5040] pstate: 60400009 (nZCv daif +PAN -UAO)
[  463.718995][ T5040] pc : virtio_transport_recv_pkt+0x4c8/0xd40

[vmw_vsock_virtio_transport_common]

quoted

[  463.718999][ T5040] lr : virtio_transport_recv_pkt+0x1fc/0xd40

[vmw_vsock_virtio_transport_common]

quoted

[  463.719000][ T5040] sp : ffff80002dbe3c40
[...]
[  463.719025][ T5040] Call trace:
[  463.719030][ T5040]  virtio_transport_recv_pkt+0x4c8/0xd40

[vmw_vsock_virtio_transport_common]

quoted

[  463.719034][ T5040]  vhost_vsock_handle_tx_kick+0x360/0x408

[vhost_vsock]

quoted

[  463.719041][ T5040]  vhost_worker+0x100/0x1a0 [vhost]
[  463.719048][ T5040]  kthread+0x128/0x130
[  463.719052][ T5040]  ret_from_fork+0x10/0x18

         ^         ^
Maybe we can remove these two columns from the commit message.

quoted

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)

Here we can add:
      sk->sk_shutdown = SHUTDOWN_MASK;

Indeed. This makes it more clearly

--
Cheers,
Justin (Jia He)

quoted

    ...
    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 <redacted>
Cc: stable@vger.kernel.org
Cc: Stefano Garzarella <sgarzare@redhat.com>
---
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

quoted

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,

quoted

 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

Anyway, the patch LGTM, let see what David and other say.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

Thanks,
Stefano

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