[PATCH V5 6/9] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()

Jason Wang
2019-08-09T05:49:24Z

There's no need for RCU synchronization in vhost_uninit_vq_maps()
since we've already serialized with readers (memory accessors). This
also avoid the possible userspace DOS through ioctl() because of the
possible high latency caused by synchronize_rcu().

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
drivers/vhost/vhost.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index c12cdadb0855..cfc11f9ed9c9 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -333,7 +333,9 @@ static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
}
spin_unlock(&vq->mmu_lock);

- synchronize_rcu();
+ /* No need for synchronize_rcu() or kfree_rcu() since we are
+ * serialized with memory accessors (e.g vq mutex held).
+ */

for (i = 0; i < VHOST_NUM_ADDRS; i++)
if (map[i])
--
2.18.1