Date:   Wed, 3 Aug 2022 08:55:49 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Laurent Vivier <lvivier@...hat.com>, linux-kernel@...r.kernel.org,
        amit@...nel.org, Herbert Xu <herbert@...dor.apana.org.au>,
        Matt Mackall <mpm@...enic.com>,
        virtualization@...ts.linux-foundation.org,
        Dmitriy Vyukov <dvyukov@...gle.com>, rusty@...tcorp.com.au,
        akong@...hat.com, Alexander Potapenko <glider@...gle.com>,
        linux-crypto@...r.kernel.org,
        Mauricio De Carvalho <Mauricio.DeCarvalho@....com>,
        Kevin Brodsky <Kevin.Brodsky@....com>
Subject: Re: [PATCH v2 4/4] hwrng: virtio - always add a pending request

On Wed, Aug 03, 2022 at 01:25:15PM +0100, Vladimir Murzin wrote:
> On 8/3/22 12:39, Michael S. Tsirkin wrote:
> > On Wed, Aug 03, 2022 at 09:57:35AM +0100, Vladimir Murzin wrote:
> >> On 8/2/22 13:49, Vladimir Murzin wrote:
> >>> Hi Laurent,
> >>>
> >>> On 10/28/21 11:11, Laurent Vivier wrote:
> >>>> If we ensure we have already some data available by enqueuing
> >>>> again the buffer once data are exhausted, we can return what we
> >>>> have without waiting for the device answer.
> >>>>
> >>>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
> >>>> ---
> >>>>  drivers/char/hw_random/virtio-rng.c | 26 ++++++++++++--------------
> >>>>  1 file changed, 12 insertions(+), 14 deletions(-)
> >>>>
> >>>> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> >>>> index 8ba97cf4ca8f..0a7dde135db1 100644
> >>>> --- a/drivers/char/hw_random/virtio-rng.c
> >>>> +++ b/drivers/char/hw_random/virtio-rng.c
> >>>> @@ -20,7 +20,6 @@ struct virtrng_info {
> >>>>  	struct virtqueue *vq;
> >>>>  	char name[25];
> >>>>  	int index;
> >>>> -	bool busy;
> >>>>  	bool hwrng_register_done;
> >>>>  	bool hwrng_removed;
> >>>>  	/* data transfer */
> >>>> @@ -44,16 +43,18 @@ static void random_recv_done(struct virtqueue *vq)
> >>>>  		return;
> >>>>  
> >>>>  	vi->data_idx = 0;
> >>>> -	vi->busy = false;
> >>>>  
> >>>>  	complete(&vi->have_data);
> >>>>  }
> >>>>  
> >>>> -/* The host will fill any buffer we give it with sweet, sweet randomness. */
> >>>> -static void register_buffer(struct virtrng_info *vi)
> >>>> +static void request_entropy(struct virtrng_info *vi)
> >>>>  {
> >>>>  	struct scatterlist sg;
> >>>>  
> >>>> +	reinit_completion(&vi->have_data);
> >>>> +	vi->data_avail = 0;
> >>>> +	vi->data_idx = 0;
> >>>> +
> >>>>  	sg_init_one(&sg, vi->data, sizeof(vi->data));
> >>>>  
> >>>>  	/* There should always be room for one buffer. */
> >>>> @@ -69,6 +70,8 @@ static unsigned int copy_data(struct virtrng_info *vi, void *buf,
> >>>>  	memcpy(buf, vi->data + vi->data_idx, size);
> >>>>  	vi->data_idx += size;
> >>>>  	vi->data_avail -= size;
> >>>> +	if (vi->data_avail == 0)
> >>>> +		request_entropy(vi);
> >>>>  	return size;
> >>>>  }
> >>>>  
> >>>> @@ -98,13 +101,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
> >>>>  	 * so either size is 0 or data_avail is 0
> >>>>  	 */
> >>>>  	while (size != 0) {
> >>>> -		/* data_avail is 0 */
> >>>> -		if (!vi->busy) {
> >>>> -			/* no pending request, ask for more */
> >>>> -			vi->busy = true;
> >>>> -			reinit_completion(&vi->have_data);
> >>>> -			register_buffer(vi);
> >>>> -		}
> >>>> +		/* data_avail is 0 but a request is pending */
> >>>>  		ret = wait_for_completion_killable(&vi->have_data);
> >>>>  		if (ret < 0)
> >>>>  			return ret;
> >>>> @@ -126,8 +123,7 @@ static void virtio_cleanup(struct hwrng *rng)
> >>>>  {
> >>>>  	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
> >>>>  
> >>>> -	if (vi->busy)
> >>>> -		complete(&vi->have_data);
> >>>> +	complete(&vi->have_data);
> >>>>  }
> >>>>  
> >>>>  static int probe_common(struct virtio_device *vdev)
> >>>> @@ -163,6 +159,9 @@ static int probe_common(struct virtio_device *vdev)
> >>>>  		goto err_find;
> >>>>  	}
> >>>>  
> >>>> +	/* we always have a pending entropy request */
> >>>> +	request_entropy(vi);
> >>>> +
> >>>>  	return 0;
> >>>>  
> >>>>  err_find:
> >>>> @@ -181,7 +180,6 @@ static void remove_common(struct virtio_device *vdev)
> >>>>  	vi->data_idx = 0;
> >>>>  	complete(&vi->have_data);
> >>>>  	vdev->config->reset(vdev);
> >>>> -	vi->busy = false;
> >>>>  	if (vi->hwrng_register_done)
> >>>>  		hwrng_unregister(&vi->hwrng);
> >>>>  	vdev->config->del_vqs(vdev);
> >>>
> >>> We observed that after this commit virtio-rng implementation in FVP doesn't
> >>> work
> >>>
> >>> INFO: bp.virtio_rng: Selected Random Generator Device: XORSHIFT DEVICE
> >>> INFO: bp.virtio_rng: Using seed value: 0x5674bba8
> >>> Error: FVP_Base_AEMvA: bp.virtio_rng: <vq0-requestq> Found invalid descriptor index
> >>> In file: (unknown):0
> >>> In process: FVP_Base_AEMvA.thread_p_12 @ 935500020 ns
> >>> Info: FVP_Base_AEMvA: bp.virtio_rng: Could not extract buffer
> >>>
> >>> while basic baremetal test works as expected
> >>>
> >>> INFO: bp.virtio_rng: Selected Random Generator Device: XORSHIFT DEVICE
> >>> INFO: bp.virtio_rng: Using seed value: 0x541c142e
> >>> Info: FVP_Base_AEMv8A: bp.virtio_rng: Generated Number: 0x4b098991ceb377e6
> >>> Info: FVP_Base_AEMv8A: bp.virtio_rng: Generated Number: 0xbdcbe3f765ba62f7
> >>>
> >>> We are trying to get an idea what is missing and where, yet none of us familiar
> >>> with the driver :(
> >>>
> >>> I'm looping Kevin who originally reported that and Mauricio who is looking form
> >>> the FVP side. 
> >>
> >> With the following diff FVP works agin
> >>
> >> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> >> index a6f3a8a2ac..042503ad6c 100644
> >> --- a/drivers/char/hw_random/virtio-rng.c
> >> +++ b/drivers/char/hw_random/virtio-rng.c
> >> @@ -54,6 +54,7 @@ static void request_entropy(struct virtrng_info *vi)
> >>         reinit_completion(&vi->have_data);
> >>         vi->data_avail = 0;
> >>         vi->data_idx = 0;
> >> +       smp_mb();
> >>  
> >>         sg_init_one(&sg, vi->data, sizeof(vi->data));
> >>  
> >>
> >> What do you reckon?
> >>
> >> Cheers
> >> Vladimir
> > 
> > Thanks for debugging this!
> > 
> > OK, interesting.
> > 
> > data_idx and data_avail are accessed from virtio_read.
> > 
> > Which as far as I can tell is invoked just with reading_mutex.
> > 
> > 
> > But, request_entropy is called from probe when device is registered
> > this time without locks
> > so it can trigger while another thread is calling virtio_read.
> > 
> > Second request_entropy is called from a callback random_recv_done
> > also without locks.
> > 
> > So it's great that smp_mb helped here but I suspect in fact we
> > need locking. Laurent?
> > 
> 
> I'm sorry for the noise, but it looks like I'm seeing issue for some different reasons.
> I manage to reproduce issue even with smb_mb() in place. The reason I though it helped
> is because I changed both environment and added smb_mb().
> 
> Anyway, thank you for your time!
> 
> Cheers
> Vladimir

Well we at least have a race condition found by code review here. Here's
a quick hack attempting to fix it. I don't like it much since
it adds buffers with GFP_ATOMIC and kicks under a spinlock, but
for now we can at least test it. I did a quick build but that's
all, a bit rushed now sorry. Would appreciate knowing whether
this addresses the issue for you.


diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index a6f3a8a2aca6..36121c8d0315 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -23,6 +23,7 @@ struct virtrng_info {
 	bool hwrng_register_done;
 	bool hwrng_removed;
 	/* data transfer */
+	spinlock_t lock;
 	struct completion have_data;
 	unsigned int data_avail;
 	unsigned int data_idx;
@@ -37,6 +38,9 @@ struct virtrng_info {
 static void random_recv_done(struct virtqueue *vq)
 {
 	struct virtrng_info *vi = vq->vdev->priv;
+	unsigned long flags;
+
+	spin_lock_irqsave(&vi->lock, flags);
 
 	/* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */
 	if (!virtqueue_get_buf(vi->vq, &vi->data_avail))
@@ -45,20 +49,20 @@ static void random_recv_done(struct virtqueue *vq)
 	vi->data_idx = 0;
 
 	complete(&vi->have_data);
+	spin_unlock_irqrestore(&vi->lock, flags);
 }
 
 static void request_entropy(struct virtrng_info *vi)
 {
 	struct scatterlist sg;
 
-	reinit_completion(&vi->have_data);
-	vi->data_avail = 0;
+	BUG_ON(vi->data_avail != 0);
 	vi->data_idx = 0;
 
 	sg_init_one(&sg, vi->data, sizeof(vi->data));
 
 	/* There should always be room for one buffer. */
-	virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_KERNEL);
+	virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_ATOMIC);
 
 	virtqueue_kick(vi->vq);
 }
@@ -70,8 +74,10 @@ static unsigned int copy_data(struct virtrng_info *vi, void *buf,
 	memcpy(buf, vi->data + vi->data_idx, size);
 	vi->data_idx += size;
 	vi->data_avail -= size;
-	if (vi->data_avail == 0)
+	if (vi->data_avail == 0) {
+		reinit_completion(&vi->have_data);
 		request_entropy(vi);
+	}
 	return size;
 }
 
@@ -81,18 +87,21 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
 	unsigned int chunk;
 	size_t read;
+	unsigned long flags;
 
 	if (vi->hwrng_removed)
 		return -ENODEV;
 
 	read = 0;
 
+	spin_lock_irqsave(&vi->lock, flags);
 	/* copy available data */
 	if (vi->data_avail) {
 		chunk = copy_data(vi, buf, size);
 		size -= chunk;
 		read += chunk;
 	}
+	spin_unlock_irqrestore(&vi->lock, flags);
 
 	if (!wait)
 		return read;
@@ -108,12 +117,14 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 		/* if vi->data_avail is 0, we have been interrupted
 		 * by a cleanup, but buffer stays in the queue
 		 */
+		spin_lock_irqsave(&vi->lock, flags);
 		if (vi->data_avail == 0)
 			return read;
 
 		chunk = copy_data(vi, buf + read, size);
 		size -= chunk;
 		read += chunk;
+		spin_unlock_irqrestore(&vi->lock, flags);
 	}
 
 	return read;
@@ -122,19 +133,25 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 static void virtio_cleanup(struct hwrng *rng)
 {
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
+	unsigned long flags;
 
+	spin_lock_irqsave(&vi->lock, flags);
 	complete(&vi->have_data);
+	spin_unlock_irqrestore(&vi->lock, flags);
 }
 
 static int probe_common(struct virtio_device *vdev)
 {
 	int err, index;
 	struct virtrng_info *vi = NULL;
+	unsigned long flags;
 
 	vi = kzalloc(sizeof(struct virtrng_info), GFP_KERNEL);
 	if (!vi)
 		return -ENOMEM;
 
+	spin_lock_init(&vi->lock);
+
 	vi->index = index = ida_simple_get(&rng_index_ida, 0, 0, GFP_KERNEL);
 	if (index < 0) {
 		err = index;
@@ -162,7 +179,10 @@ static int probe_common(struct virtio_device *vdev)
 	virtio_device_ready(vdev);
 
 	/* we always have a pending entropy request */
-	request_entropy(vi);
+	spin_lock_irqsave(&vi->lock, flags);
+	if (vi->data_avail == 0)
+		request_entropy(vi);
+	spin_unlock_irqrestore(&vi->lock, flags);
 
 	return 0;
 

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