Re: [PATCH 1/2] virtio-rng: implement entropy leak feature

[Michael S. Tsirkin]

On Thu, Jan 19, 2023 at 07:43:47PM +0100, Babis Chalios wrote:
> Implement the virtio-rng feature that allows a guest driver to request
> from the device to perform certain operations in the event of an
> "entropy leak", such as when taking a VM snapshot or restoring a VM from
> a snapshot. The guest can request one of two operations: (i) fill a
> buffer with random bytes, or (ii) perform a memory copy between two
> bytes.
> 
> The feature is similar to Microsoft's Virtual Machine Generation ID and
> it can be used to (1) avoid the race-condition that exists in our
> current VMGENID implementation, between the time vcpus are resumed and
> the ACPI notification is being handled and (2) provide mechanisms for
> notifying user-space about snapshot-related events.
> 
> This commit implements the protocol between guest and device.
> Additionally, it makes sure there is always a request for random bytes
> in the event of entropy leak in-flight. Once such an event is observed,
> the driver feeds these bytes to as entropy using
> `add_device_randomness`.
> 
> Keep in mind that this commit does not solve the race-condition issue,
> it adds fresh entropy whenever the driver handles the used buffer from
> the fill-on-leak request. In order to close the race window, we need to
> expose some API so that other kernel subsystems can request directly
> notifications from the device.
> 
> Signed-off-by: Babis Chalios <bchalios@xxxxxxxxx>
> ---
>  drivers/char/hw_random/virtio-rng.c | 200 +++++++++++++++++++++++++++-
>  include/uapi/linux/virtio_rng.h     |   3 +
>  2 files changed, 197 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
> index a6f3a8a2aca6..389a091a8801 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -4,12 +4,14 @@
>   *  Copyright (C) 2007, 2008 Rusty Russell IBM Corporation
>   */
>  
> +#include "asm-generic/errno.h"

Why are you pulling this in?
If you really need errno the thing to include
is linux/errno.h

checkpatch has a rule to catch such cases:

# warn if <asm/foo.h> is #included and <linux/foo.h> is available and includes
# itself <asm/foo.h> (uses RAW line)    

did you run checkpatch?


>  #include <linux/err.h>
>  #include <linux/hw_random.h>
>  #include <linux/scatterlist.h>
>  #include <linux/spinlock.h>
>  #include <linux/virtio.h>
>  #include <linux/virtio_rng.h>
> +#include <linux/random.h>
>  #include <linux/module.h>
>  #include <linux/slab.h>
>  
> @@ -18,6 +20,12 @@ static DEFINE_IDA(rng_index_ida);
>  struct virtrng_info {
>  	struct hwrng hwrng;
>  	struct virtqueue *vq;
> +	/* Leak queues */
> +	bool has_leakqs;
> +	struct virtqueue *leakq[2];
> +	spinlock_t lock;
> +	int active_leakq;
> +
>  	char name[25];
>  	int index;
>  	bool hwrng_register_done;
> @@ -29,27 +37,159 @@ struct virtrng_info {
>  	/* minimal size returned by rng_buffer_size() */
>  #if SMP_CACHE_BYTES < 32
>  	u8 data[32];
> +	u8 leak_data[32];
>  #else
>  	u8 data[SMP_CACHE_BYTES];
> +	u8 leak_data[SMP_CACHE_BYTES];
>  #endif
>  };
>  
> +/* Swaps the queues and returns the new active leak queue. */
> +static struct virtqueue *swap_leakqs(struct virtrng_info *vi)
> +{
> +	vi->active_leakq = 1 - vi->active_leakq;
> +	return vi->leakq[vi->active_leakq];
> +}
> +
> +static struct virtqueue *get_active_leakq(struct virtrng_info *vi)
> +{
> +	return vi->leakq[vi->active_leakq];
> +}
> +
> +int add_fill_on_leak_request(struct virtrng_info *vi, struct virtqueue *vq, void *data, size_t len)
> +{
> +	struct scatterlist sg;
> +	int ret;
> +
> +	sg_init_one(&sg, data, len);
> +	ret = virtqueue_add_inbuf(vq, &sg, 1, data, GFP_KERNEL);
> +	if (ret)
> +		goto err;
> +
> +err:
> +	return ret;
> +}
> +
> +int virtrng_fill_on_leak(struct virtrng_info *vi, void *data, size_t len)
> +{
> +	struct virtqueue *vq;
> +	unsigned long flags;
> +	int ret;
> +
> +	if (!vi->has_leakqs)
> +		return -EOPNOTSUPP;
> +
> +	spin_lock_irqsave(&vi->lock, flags);
> +
> +	vq = get_active_leakq(vi);
> +	ret = add_fill_on_leak_request(vi, vq, data, len);
> +	if (ret)
> +		virtqueue_kick(vq);
> +
> +	spin_unlock_irqrestore(&vi->lock, flags);
> +
> +	return ret;
> +}
> +
> +int add_copy_on_leak_request(struct virtrng_info *vi, struct virtqueue *vq,
> +		void *to, void *from, size_t len)
> +{
> +	int ret;
> +	struct scatterlist out, in, *sgs[2];
> +
> +	sg_init_one(&out, from, len);
> +	sgs[0] = &out;
> +	sg_init_one(&in, to, len);
> +	sgs[1] = &in;
> +
> +	ret = virtqueue_add_sgs(vq, sgs, 1, 1, to, GFP_KERNEL);
> +	if (ret)
> +		goto err;
> +
> +err:
> +	return ret;
> +}
> +
> +int virtrng_copy_on_leak(struct virtrng_info *vi, void *to, void *from, size_t len)
> +{
> +	struct virtqueue *vq;
> +	unsigned long flags;
> +	int ret;
> +
> +	if (!vi->has_leakqs)
> +		return -EOPNOTSUPP;
> +
> +	spin_lock_irqsave(&vi->lock, flags);
> +
> +	vq = get_active_leakq(vi);
> +	ret = add_copy_on_leak_request(vi, vq, to, from, len);
> +	if (ret)
> +		virtqueue_kick(vq);
> +
> +	spin_unlock_irqrestore(&vi->lock, flags);
> +
> +	return ret;
> +}
> +
> +static void entropy_leak_detected(struct virtqueue *vq)
> +{
> +	struct virtrng_info *vi = vq->vdev->priv;
> +	struct virtqueue *activeq;
> +	unsigned int len;
> +	unsigned long flags;
> +	void *buffer;
> +	bool kick_activeq = false;
> +
> +	spin_lock_irqsave(&vi->lock, flags);
> +
> +	activeq = get_active_leakq(vi);
> +	/* Drain all the used buffers from the queue */
> +	while ((buffer = virtqueue_get_buf(vq, &len)) != NULL) {
> +		if (vq == activeq) {
> +			pr_debug("%s: entropy leak detected!", vi->name);
> +			activeq = swap_leakqs(vi);
> +		}
> +
> +		if (buffer == vi->leak_data) {
> +			add_device_randomness(vi->leak_data, sizeof(vi->leak_data));
> +
> +			/* Ensure we always have a pending request for random bytes on entropy
> +			 * leak. Do it here, after we have swapped leak queues, so it gets handled
> +			 * with the next entropy leak event.
> +			 */
> +			add_fill_on_leak_request(vi, activeq, vi->leak_data, sizeof(vi->leak_data));
> +			kick_activeq = true;
> +		}
> +	}
> +
> +	if (kick_activeq)
> +		virtqueue_kick(activeq);
> +
> +	spin_unlock_irqrestore(&vi->lock, flags);
> +}
> +
>  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))
> -		return;
> +		goto unlock;
>  
>  	vi->data_idx = 0;
>  
>  	complete(&vi->have_data);
> +
> +unlock:
> +	spin_unlock_irqrestore(&vi->lock, flags);
>  }
>  
>  static void request_entropy(struct virtrng_info *vi)
>  {
>  	struct scatterlist sg;
> +	unsigned long flags;
>  
>  	reinit_completion(&vi->have_data);
>  	vi->data_avail = 0;
> @@ -57,10 +197,12 @@ static void request_entropy(struct virtrng_info *vi)
>  
>  	sg_init_one(&sg, vi->data, sizeof(vi->data));
>  
> +	spin_lock_irqsave(&vi->lock, flags);
>  	/* There should always be room for one buffer. */
>  	virtqueue_add_inbuf(vi->vq, &sg, 1, vi->data, GFP_KERNEL);
>  
>  	virtqueue_kick(vi->vq);
> +	spin_unlock_irqrestore(&vi->lock, flags);
>  }
>  
>  static unsigned int copy_data(struct virtrng_info *vi, void *buf,
> @@ -126,6 +268,40 @@ static void virtio_cleanup(struct hwrng *rng)
>  	complete(&vi->have_data);
>  }
>  
> +static int init_virtqueues(struct virtrng_info *vi, struct virtio_device *vdev)
> +{
> +	int ret = -ENOMEM, total_vqs = 1;
> +	struct virtqueue *vqs[3];
> +	const char *names[3];
> +	vq_callback_t *callbacks[3];
> +
> +	if (vi->has_leakqs)
> +		total_vqs = 3;
> +
> +	callbacks[0] = random_recv_done;
> +	names[0] = "input";
> +	if (vi->has_leakqs) {
> +		callbacks[1] = entropy_leak_detected;
> +		names[1] = "leakq.1";
> +		callbacks[2] = entropy_leak_detected;
> +		names[2] = "leakq.2";
> +	}
> +
> +	ret = virtio_find_vqs(vdev, total_vqs, vqs, callbacks, names, NULL);
> +	if (ret)
> +		goto err;
> +
> +	vi->vq = vqs[0];
> +
> +	if (vi->has_leakqs) {
> +		vi->leakq[0] = vqs[1];
> +		vi->leakq[1] = vqs[2];
> +	}
> +
> +err:
> +	return ret;
> +}
> +
>  static int probe_common(struct virtio_device *vdev)
>  {
>  	int err, index;
> @@ -152,18 +328,24 @@ static int probe_common(struct virtio_device *vdev)
>  	};
>  	vdev->priv = vi;
>  
> -	/* We expect a single virtqueue. */
> -	vi->vq = virtio_find_single_vq(vdev, random_recv_done, "input");
> -	if (IS_ERR(vi->vq)) {
> -		err = PTR_ERR(vi->vq);
> -		goto err_find;
> +	vi->has_leakqs = virtio_has_feature(vdev, VIRTIO_RNG_F_LEAK);
> +	if (vi->has_leakqs) {
> +		spin_lock_init(&vi->lock);
> +		vi->active_leakq = 0;
>  	}
>  
> +	err = init_virtqueues(vi, vdev);
> +	if (err)
> +		goto err_find;
> +
>  	virtio_device_ready(vdev);
>  
>  	/* we always have a pending entropy request */
>  	request_entropy(vi);
>  
> +	/* we always have a fill_on_leak request pending */
> +	virtrng_fill_on_leak(vi, vi->leak_data, sizeof(vi->leak_data));
> +
>  	return 0;
>  
>  err_find:
> @@ -246,7 +428,13 @@ static const struct virtio_device_id id_table[] = {
>  	{ 0 },
>  };
>  
> +static unsigned int features[] = {
> +	VIRTIO_RNG_F_LEAK,
> +};
> +
>  static struct virtio_driver virtio_rng_driver = {
> +	.feature_table = features,
> +	.feature_table_size = ARRAY_SIZE(features),
>  	.driver.name =	KBUILD_MODNAME,
>  	.driver.owner =	THIS_MODULE,
>  	.id_table =	id_table,
> diff --git a/include/uapi/linux/virtio_rng.h b/include/uapi/linux/virtio_rng.h
> index c4d5de896f0c..d9774951547e 100644
> --- a/include/uapi/linux/virtio_rng.h
> +++ b/include/uapi/linux/virtio_rng.h
> @@ -5,4 +5,7 @@
>  #include <linux/virtio_ids.h>
>  #include <linux/virtio_config.h>
>  
> +/* The feature bitmap for virtio entropy device */
> +#define VIRTIO_RNG_F_LEAK 0
> +
>  #endif /* _LINUX_VIRTIO_RNG_H */
> -- 
> 2.38.1
> 
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