[Devel] [RH7 PATCH] ms/vhost: synchronize IOTLB message with dev cleanup

[Pavel Tikhomirov]

From: Jason Wang <jasowang at redhat.com>

Port ms commit 1b15ad683ab4 ("vhost: synchronize IOTLB message with dev cleanup")

DaeRyong Jeong reports a race between vhost_dev_cleanup() and
vhost_process_iotlb_msg():

Thread interleaving:
CPU0 (vhost_process_iotlb_msg)			CPU1 (vhost_dev_cleanup)
(In the case of both VHOST_IOTLB_UPDATE and
VHOST_IOTLB_INVALIDATE)

=====						=====
						vhost_umem_clean(dev->iotlb);
if (!dev->iotlb) {
	        ret = -EFAULT;
		        break;
}
						dev->iotlb = NULL;

The reason is we don't synchronize between them, fixing by protecting
vhost_process_iotlb_msg() with dev mutex.

Reported-by: DaeRyong Jeong <threeearcat at gmail.com>
Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
Signed-off-by: Jason Wang <jasowang at redhat.com>
Acked-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: David S. Miller <davem at davemloft.net>

Need some comment about locking here:

On these paths below we have no mutex held on other (non
vhost_process_iotlb_msg) side of critical section:

____fput
 __fput
  file->f_op->release
   vhost_net_release
   vhost_test_release
   vhost_scsi_release
   vhost_vsock_dev_release

    vhost_dev_cleanup

_But_ it is not a problem as vhost_process_iotlb_msg can be called only
from these code path:

file->f_op->aio_write
 vhost_net_aio_write
  vhost_chr_write_iter
   vhost_process_iotlb_msg

And locked variant of calling vhost_dev_cleanup is only called from:

filp->f_op->unlocked_ioctl
 vhost_net_ioctl
  vhost_net_reset_owner <- holds the mutex
 vhost_test_ioctl
  vhost_test_reset_owner <- holds the mutex

   vhost_dev_reset_owner
    vhost_dev_cleanup

When we call a write/ioctl helper the reference count on file struct
should be non-zero, and we can't get to ____fput for these file struct.
Thus ((struct vhost_net *)iocb->ki_filp->private_data)->dev is protected
from simultaneous unlocked cleanup from ____fput ... vhost_dev_cleanup
code path. Also these unlocked cleanup from ____fput can be called only
once for each vhost_dev so it can't race with itself.

https://jira.sw.ru/browse/PSBM-90318

Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 drivers/vhost/vhost.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 697cdca02567..7f54402ff674 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -974,6 +974,7 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
 {
 	int ret = 0;
 
+	mutex_lock(&dev->mutex);
 	vhost_dev_lock_vqs(dev);
 	switch (msg->type) {
 	case VHOST_IOTLB_UPDATE:
@@ -1009,6 +1010,8 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
 	}
 
 	vhost_dev_unlock_vqs(dev);
+	mutex_unlock(&dev->mutex);
+
 	return ret;
 }
 
-- 
2.20.1