Re: [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock

From: Jason Wang
Date: Wed Jul 03 2019 - 23:58:19 EST


On 2019/7/3 äå6:41, Stefano Garzarella wrote:
On Wed, Jul 03, 2019 at 05:53:58PM +0800, Jason Wang wrote:
On 2019/6/28 äå8:36, Stefano Garzarella wrote:
Some callbacks used by the upper layers can run while we are in the
.remove(). A potential use-after-free can happen, because we free
the_virtio_vsock without knowing if the callbacks are over or not.

To solve this issue we move the assignment of the_virtio_vsock at the
end of .probe(), when we finished all the initialization, and at the
beginning of .remove(), before to release resources.
For the same reason, we do the same also for the vdev->priv.

We use RCU to be sure that all callbacks that use the_virtio_vsock
ended before freeing it. This is not required for callbacks that
use vdev->priv, because after the vdev->config->del_vqs() we are sure
that they are ended and will no longer be invoked.

We also take the mutex during the .remove() to avoid that .probe() can
run while we are resetting the device.

Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
---
net/vmw_vsock/virtio_transport.c | 67 +++++++++++++++++++++-----------
1 file changed, 44 insertions(+), 23 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 9c287e3e393c..7ad510ec12e0 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -65,19 +65,22 @@ struct virtio_vsock {
u32 guest_cid;
};
-static struct virtio_vsock *virtio_vsock_get(void)
-{
- return the_virtio_vsock;
-}
-
static u32 virtio_transport_get_local_cid(void)
{
- struct virtio_vsock *vsock = virtio_vsock_get();
+ struct virtio_vsock *vsock;
+ u32 ret;
- if (!vsock)
- return VMADDR_CID_ANY;
+ rcu_read_lock();
+ vsock = rcu_dereference(the_virtio_vsock);
+ if (!vsock) {
+ ret = VMADDR_CID_ANY;
+ goto out_rcu;
+ }
- return vsock->guest_cid;
+ ret = vsock->guest_cid;
+out_rcu:
+ rcu_read_unlock();
+ return ret;
}
static void virtio_transport_loopback_work(struct work_struct *work)
@@ -197,14 +200,18 @@ virtio_transport_send_pkt(struct virtio_vsock_pkt *pkt)
struct virtio_vsock *vsock;
int len = pkt->len;
- vsock = virtio_vsock_get();
+ rcu_read_lock();
+ vsock = rcu_dereference(the_virtio_vsock);
if (!vsock) {
virtio_transport_free_pkt(pkt);
- return -ENODEV;
+ len = -ENODEV;
+ goto out_rcu;
}
- if (le64_to_cpu(pkt->hdr.dst_cid) == vsock->guest_cid)
- return virtio_transport_send_pkt_loopback(vsock, pkt);
+ if (le64_to_cpu(pkt->hdr.dst_cid) == vsock->guest_cid) {
+ len = virtio_transport_send_pkt_loopback(vsock, pkt);
+ goto out_rcu;
+ }
if (pkt->reply)
atomic_inc(&vsock->queued_replies);
@@ -214,6 +221,9 @@ virtio_transport_send_pkt(struct virtio_vsock_pkt *pkt)
spin_unlock_bh(&vsock->send_pkt_list_lock);
queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
+
+out_rcu:
+ rcu_read_unlock();
return len;
}
@@ -222,12 +232,14 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
{
struct virtio_vsock *vsock;
struct virtio_vsock_pkt *pkt, *n;
- int cnt = 0;
+ int cnt = 0, ret;
LIST_HEAD(freeme);
- vsock = virtio_vsock_get();
+ rcu_read_lock();
+ vsock = rcu_dereference(the_virtio_vsock);
if (!vsock) {
- return -ENODEV;
+ ret = -ENODEV;
+ goto out_rcu;
}
spin_lock_bh(&vsock->send_pkt_list_lock);
@@ -255,7 +267,11 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
queue_work(virtio_vsock_workqueue, &vsock->rx_work);
}
- return 0;
+ ret = 0;
+
+out_rcu:
+ rcu_read_unlock();
+ return ret;
}
static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
@@ -590,8 +606,6 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
vsock->rx_buf_max_nr = 0;
atomic_set(&vsock->queued_replies, 0);
- vdev->priv = vsock;
- the_virtio_vsock = vsock;
mutex_init(&vsock->tx_lock);
mutex_init(&vsock->rx_lock);
mutex_init(&vsock->event_lock);
@@ -613,6 +627,9 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
virtio_vsock_event_fill(vsock);
mutex_unlock(&vsock->event_lock);
+ vdev->priv = vsock;
+ rcu_assign_pointer(the_virtio_vsock, vsock);

You probably need to use rcu_dereference_protected() to access
the_virtio_vsock in the function in order to survive from sparse.

Ooo, thanks!

Do you mean when we check if the_virtio_vsock is not null at the beginning of
virtio_vsock_probe()?


I mean instead of:

ÂÂÂ /* Only one virtio-vsock device per guest is supported */
ÂÂÂ if (the_virtio_vsock) {
ÂÂÂ ÂÂÂ ret = -EBUSY;
ÂÂÂ ÂÂÂ goto out;
ÂÂÂ }

you should use:

if (rcu_dereference_protected(the_virtio_vosck, lock_dep_is_held(&the_virtio_vsock_mutex))

...



+
mutex_unlock(&the_virtio_vsock_mutex);
return 0;
@@ -627,6 +644,12 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
struct virtio_vsock *vsock = vdev->priv;
struct virtio_vsock_pkt *pkt;
+ mutex_lock(&the_virtio_vsock_mutex);
+
+ vdev->priv = NULL;
+ rcu_assign_pointer(the_virtio_vsock, NULL);

This is still suspicious, can we access the_virtio_vsock through vdev->priv?
If yes, we may still get use-after-free since it was not protected by RCU.
We will free the object only after calling the del_vqs(), so we are sure
that the vq_callbacks ended and will no longer be invoked.
So, IIUC it shouldn't happen.


Yes, but any dereference that is not done in vq_callbacks will be very dangerous in the future.

Thanks



Another more interesting question, I believe we will do singleton for
virtio_vsock structure. Then what's the point of using vdev->priv to access
the_virtio_vsock? It looks to me we can it brings extra troubles for doing
synchronization.
I thought about it when I tried to use RCU to stop the worker and I
think make sense. Maybe can be another series after this will be merged.

@Stefan, what do you think about that?

Thanks,
Stefano