linux-kernel - [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq

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Message-Id: <20190723075718.6275-7-jasowang@redhat.com>
Date:   Tue, 23 Jul 2019 03:57:18 -0400
From:   Jason Wang <jasowang@...hat.com>
To:     mst@...hat.com, jasowang@...hat.com
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()

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@...hat.com>
Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
Signed-off-by: Jason Wang <jasowang@...hat.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 5b8821d00fe4..a17df1f4069a 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -334,7 +334,9 @@ 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