Re: [PATCH V2 09/12] virtio_ring: validate used buffer length

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 12, 2021 at 02:52:24PM +0800, Jason Wang wrote:
> This patch validate the used buffer length provided by the device
> before trying to use it. This is done by record the in buffer length
> in a new field in desc_state structure during virtqueue_add(), then we
> can fail the virtqueue_get_buf() when we find the device is trying to
> give us a used buffer length which is greater than the in buffer
> length.
> 
> Since some drivers have already done the validation by itself, this

by themselves

> patch tries to makes the core validation optional. For the driver that
> doesn't want the validation, it can set the validate_used to be
> true (which could be overridden by force_used_validation). To be more
> efficient, a dedicate array is used for storing the validate used
> length, this helps to eliminate the cache stress if validation is done
> by the driver.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_ring.c | 56 ++++++++++++++++++++++++++++++++++++
>  include/linux/virtio.h       |  2 ++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index d2ca0a7365f8..dee962bd8b04 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -14,6 +14,9 @@
>  #include <linux/spinlock.h>
>  #include <xen/xen.h>
>  
> +static bool force_used_validation = false;
> +module_param(force_used_validation, bool, 0444);
> +
>  #ifdef DEBUG
>  /* For development, we want to crash whenever the ring is screwed. */
>  #define BAD_RING(_vq, fmt, args...)				\
> @@ -182,6 +185,9 @@ struct vring_virtqueue {
>  		} packed;
>  	};
>  
> +	/* Per-descriptor in buffer length */
> +	u32 *buflen;
> +
>  	/* How to notify other side. FIXME: commonalize hcalls! */
>  	bool (*notify)(struct virtqueue *vq);
>  
> @@ -490,6 +496,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>  	unsigned int i, n, avail, descs_used, prev, err_idx;
>  	int head;
>  	bool indirect;
> +	u32 buflen = 0;
>  
>  	START_USE(vq);
>  
> @@ -571,6 +578,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>  						     VRING_DESC_F_NEXT |
>  						     VRING_DESC_F_WRITE,
>  						     indirect);
> +			buflen += sg->length;
>  		}
>  	}
>  	/* Last one doesn't continue. */
> @@ -610,6 +618,10 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>  	else
>  		vq->split.desc_state[head].indir_desc = ctx;
>  
> +	/* Store in buffer length if necessary */
> +	if (vq->buflen)
> +		vq->buflen[head] = buflen;
> +
>  	/* Put entry in available array (but don't update avail->idx until they
>  	 * do sync). */
>  	avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
> @@ -784,6 +796,11 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
>  		BAD_RING(vq, "id %u is not a head!\n", i);
>  		return NULL;
>  	}
> +	if (vq->buflen && unlikely(*len > vq->buflen[i])) {
> +		BAD_RING(vq, "used len %d is larger than in buflen %u\n",
> +			*len, vq->buflen[i]);
> +		return NULL;
> +	}
>  
>  	/* detach_buf_split clears data, so grab it now. */
>  	ret = vq->split.desc_state[i].data;
> @@ -1062,6 +1079,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>  	unsigned int i, n, err_idx;
>  	u16 head, id;
>  	dma_addr_t addr;
> +	u32 buflen = 0;
>  
>  	head = vq->packed.next_avail_idx;
>  	desc = alloc_indirect_packed(total_sg, gfp);
> @@ -1089,6 +1107,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>  			desc[i].addr = cpu_to_le64(addr);
>  			desc[i].len = cpu_to_le32(sg->length);
>  			i++;
> +			if (n >= out_sgs)
> +				buflen += sg->length;
>  		}
>  	}
>  
> @@ -1142,6 +1162,10 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
>  	vq->packed.desc_state[id].indir_desc = desc;
>  	vq->packed.desc_state[id].last = id;
>  
> +	/* Store in buffer length if necessary */
> +	if (vq->buflen)
> +		vq->buflen[id] = buflen;
> +
>  	vq->num_added += 1;
>  
>  	pr_debug("Added buffer head %i to %p\n", head, vq);
> @@ -1176,6 +1200,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>  	unsigned int i, n, c, descs_used, err_idx;
>  	__le16 head_flags, flags;
>  	u16 head, id, prev, curr, avail_used_flags;
> +	u32 buflen = 0;
>  
>  	START_USE(vq);
>  
> @@ -1250,6 +1275,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>  					1 << VRING_PACKED_DESC_F_AVAIL |
>  					1 << VRING_PACKED_DESC_F_USED;
>  			}
> +			if (n >= out_sgs)
> +				buflen += sg->length;
>  		}
>  	}
>  
> @@ -1269,6 +1296,10 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
>  	vq->packed.desc_state[id].indir_desc = ctx;
>  	vq->packed.desc_state[id].last = prev;
>  
> +	/* Store in buffer length if necessary */
> +	if (vq->buflen)
> +		vq->buflen[id] = buflen;
> +
>  	/*
>  	 * A driver MUST NOT make the first descriptor in the list
>  	 * available before all subsequent descriptors comprising
> @@ -1455,6 +1486,11 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>  		BAD_RING(vq, "id %u is not a head!\n", id);
>  		return NULL;
>  	}
> +	if (vq->buflen && unlikely(*len > vq->buflen[id])) {
> +		BAD_RING(vq, "used len %d is larger than in buflen %u\n",
> +			*len, vq->buflen[id]);
> +		return NULL;
> +	}
>  
>  	/* detach_buf_packed clears data, so grab it now. */
>  	ret = vq->packed.desc_state[id].data;
> @@ -1660,6 +1696,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
>  	struct vring_virtqueue *vq;
>  	struct vring_packed_desc *ring;
>  	struct vring_packed_desc_event *driver, *device;
> +	struct virtio_driver *drv = drv_to_virtio(vdev->dev.driver);
>  	dma_addr_t ring_dma_addr, driver_event_dma_addr, device_event_dma_addr;
>  	size_t ring_size_in_bytes, event_size_in_bytes;
>  
> @@ -1749,6 +1786,13 @@ static struct virtqueue *vring_create_virtqueue_packed(
>  	if (!vq->packed.desc_extra)
>  		goto err_desc_extra;
>  
> +	if (!drv->validate_used || force_used_validation) {
> +		vq->buflen = kmalloc_array(num, sizeof(*vq->buflen),
> +					   GFP_KERNEL);
> +		if (!vq->buflen)
> +			goto err_buflen;
> +	}
> +
>  	/* No callback?  Tell other side not to bother us. */
>  	if (!callback) {
>  		vq->packed.event_flags_shadow = VRING_PACKED_EVENT_FLAG_DISABLE;
> @@ -1761,6 +1805,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
>  	spin_unlock(&vdev->vqs_list_lock);
>  	return &vq->vq;
>  
> +err_buflen:
> +	kfree(vq->packed.desc_extra);
>  err_desc_extra:
>  	kfree(vq->packed.desc_state);
>  err_desc_state:
> @@ -2168,6 +2214,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>  					void (*callback)(struct virtqueue *),
>  					const char *name)
>  {
> +	struct virtio_driver *drv = drv_to_virtio(vdev->dev.driver);
>  	struct vring_virtqueue *vq;
>  
>  	if (virtio_has_feature(vdev, VIRTIO_F_RING_PACKED))
> @@ -2227,6 +2274,13 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>  	if (!vq->split.desc_extra)
>  		goto err_extra;
>  
> +	if (!drv->validate_used || force_used_validation) {
> +		vq->buflen = kmalloc_array(vring.num, sizeof(*vq->buflen),
> +					   GFP_KERNEL);
> +		if (!vq->buflen)
> +			goto err_buflen;
> +	}
> +
>  	/* Put everything in free lists. */
>  	vq->free_head = 0;
>  	memset(vq->split.desc_state, 0, vring.num *
> @@ -2237,6 +2291,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
>  	spin_unlock(&vdev->vqs_list_lock);
>  	return &vq->vq;
>  
> +err_buflen:
> +	kfree(vq->split.desc_extra);
>  err_extra:
>  	kfree(vq->split.desc_state);
>  err_state:
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 0cd8685aeba4..79e4f6765e3a 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -153,6 +153,7 @@ size_t virtio_max_dma_size(struct virtio_device *vdev);
>   * @feature_table_legacy: same as feature_table but when working in legacy mode.
>   * @feature_table_size_legacy: number of entries in feature table legacy array.
>   * @validate: optional function to do early checks
> + * @validate_used: whether the driver can validate used buffer length

So you set it to true to *not* have core validate used. Ugh.
I understand completely why it's like this: to have the default false
be safe. But let's call this out.

	@suppress_used_validation: set to not have core validate used length


>   * @probe: the function to call when a device is found.  Returns 0 or -errno.
>   * @scan: optional function to call after successful probe; intended
>   *    for virtio-scsi to invoke a scan.
> @@ -169,6 +170,7 @@ struct virtio_driver {
>  	unsigned int feature_table_size;
>  	const unsigned int *feature_table_legacy;
>  	unsigned int feature_table_size_legacy;
> +	bool validate_used;
>  	int (*validate)(struct virtio_device *dev);
>  	int (*probe)(struct virtio_device *dev);
>  	void (*scan)(struct virtio_device *dev);
> -- 
> 2.25.1