On Wed, May 29, 2024 at 12:01:45AM +0800, Heng Qi wrote:

On Tue, 28 May 2024 11:46:28 -0400, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

On Tue, May 28, 2024 at 03:52:26PM +0800, Heng Qi wrote:

quoted

Refactored the handling of control_buf to be within the cvq_lock
critical section, mitigating race conditions between reading device
responses and new command submissions.

Fixes: 6f45ab3e0409 ("virtio_net: Add a lock for the command VQ.")
Signed-off-by: Heng Qi <redacted>

I don't get what does this change. status can change immediately
after you drop the mutex, can it not? what exactly is the
race conditions you are worried about?

See the following case:

1. Command A is acknowledged and successfully executed by the device.
2. After releasing the mutex (mutex_unlock), process P1 gets preempted before
   it can read vi->ctrl->status, *which should be VIRTIO_NET_OK*.
3. A new command B (like the DIM command) is issued.
4. Post vi->ctrl->status being set to VIRTIO_NET_ERR by
   virtnet_send_command_reply(), process P2 gets preempted.
5. Process P1 resumes, reads *vi->ctrl->status as VIRTIO_NET_ERR*, and reports
   this error back for Command A. <-- Race causes incorrect results to be read.

Thanks.

Why is it important that P1 gets VIRTIO_NET_OK?
After all it is no longer the state.

quoted

quoted

---
 drivers/net/virtio_net.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6b0512a628e0..3d8407d9e3d2 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -2686,6 +2686,7 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
 {
 	struct scatterlist *sgs[5], hdr, stat;
 	u32 out_num = 0, tmp, in_num = 0;
+	bool ret;
 	int err;
 
 	/* Caller should know better */

@@ -2731,8 +2732,9 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
 	}
 
 unlock:
+	ret = vi->ctrl->status == VIRTIO_NET_OK;
 	mutex_unlock(&vi->cvq_lock);
-	return vi->ctrl->status == VIRTIO_NET_OK;
+	return ret;
 }
 
 static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,

-- 
2.32.0.3.g01195cf9f