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

Michael S. Tsirkin
2021-10-13T10:02:57Z

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