Re: [PATCH] virtio_ring : keep used_wrap_counter in vq->last_used_idx

[Jason Wang <jasowang@redhat.com>]

On Tue, Jun 14, 2022 at 1:38 PM Albert Huang
<huangjie.albert@bytedance.com> wrote:
>
> From: "huangjie.albert" <huangjie.albert@bytedance.com>
>
> the used_wrap_counter and the vq->last_used_idx may get
> out of sync if they are separate assignment，and interrupt
> might use an incorrect value to check for the used index.
>
> for example:OOB access
> ksoftirqd may consume the packet and it will call:
> virtnet_poll
>         -->virtnet_receive
>                 -->virtqueue_get_buf_ctx
>                         -->virtqueue_get_buf_ctx_packed
> and in virtqueue_get_buf_ctx_packed:
>
> vq->last_used_idx += vq->packed.desc_state[id].num;
> if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
>          vq->last_used_idx -= vq->packed.vring.num;
>          vq->packed.used_wrap_counter ^= 1;
> }
>
> if at the same time, there comes a vring interrupt，in vring_interrupt:
> we will call:
> vring_interrupt
>         -->more_used
>                 -->more_used_packed
>                         -->is_used_desc_packed
> in is_used_desc_packed, the last_used_idx maybe >= vq->packed.vring.num.
> so this could case a memory out of bounds bug.
>
> this patch is to keep the used_wrap_counter in vq->last_used_idx
> so we can get the correct value to check for used index in interrupt.
>
> Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
> ---
>  drivers/virtio/virtio_ring.c     | 60 ++++++++++++++++++--------------
>  include/uapi/linux/virtio_ring.h |  6 ++++
>  2 files changed, 40 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 13a7348cedff..35c3750e89e1 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -111,7 +111,12 @@ struct vring_virtqueue {
>         /* Number we've added since last sync. */
>         unsigned int num_added;
>
> -       /* Last used index we've seen. */
> +       /* Last used index  we've seen.
> +        * for split ring, it just contains last used index
> +        * for packed ring, it not only contains last used index, but also
> +        * used_wrap_counter, the VRING_PACKED_USED_INDEX_F_WRAP_CTR is
> +        * the bit shift in last_used_idx
> +        */
>         u16 last_used_idx;
>
>         /* Hint for event idx: already triggered no need to disable. */
> @@ -154,9 +159,6 @@ struct vring_virtqueue {
>                         /* Driver ring wrap counter. */
>                         bool avail_wrap_counter;
>
> -                       /* Device ring wrap counter. */
> -                       bool used_wrap_counter;
> -
>                         /* Avail used flags. */
>                         u16 avail_used_flags;
>
> @@ -1397,6 +1399,9 @@ static inline bool is_used_desc_packed(const struct vring_virtqueue *vq,
>         bool avail, used;
>         u16 flags;
>
> +       if (idx >= vq->packed.vring.num)
> +               return false;

I wonder if we can avoid this trick, more below.

> +
>         flags = le16_to_cpu(vq->packed.vring.desc[idx].flags);
>         avail = !!(flags & (1 << VRING_PACKED_DESC_F_AVAIL));
>         used = !!(flags & (1 << VRING_PACKED_DESC_F_USED));
> @@ -1406,8 +1411,12 @@ static inline bool is_used_desc_packed(const struct vring_virtqueue *vq,
>
>  static inline bool more_used_packed(const struct vring_virtqueue *vq)
>  {
> -       return is_used_desc_packed(vq, vq->last_used_idx,
> -                       vq->packed.used_wrap_counter);
> +       u16 last_used;
> +       bool used_wrap_counter;
> +
> +       last_used = vq->last_used_idx & ~(1 << VRING_PACKED_USED_INDEX_F_WRAP_CTR);
> +       used_wrap_counter = !!((vq->last_used_idx) >> VRING_PACKED_USED_INDEX_F_WRAP_CTR);
> +       return is_used_desc_packed(vq, last_used, used_wrap_counter);
>  }
>
>  static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> @@ -1416,6 +1425,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>  {
>         struct vring_virtqueue *vq = to_vvq(_vq);
>         u16 last_used, id;
> +       bool used_wrap_counter;
>         void *ret;
>
>         START_USE(vq);
> @@ -1434,7 +1444,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>         /* Only get used elements after they have been exposed by host. */
>         virtio_rmb(vq->weak_barriers);
>
> -       last_used = vq->last_used_idx;
> +       used_wrap_counter = !!((vq->last_used_idx >> VRING_PACKED_USED_INDEX_F_WRAP_CTR));
> +       last_used = (vq->last_used_idx) & (~(1 << VRING_PACKED_USED_INDEX_F_WRAP_CTR));
>         id = le16_to_cpu(vq->packed.vring.desc[last_used].id);
>         *len = le32_to_cpu(vq->packed.vring.desc[last_used].len);
>
> @@ -1451,12 +1462,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>         ret = vq->packed.desc_state[id].data;
>         detach_buf_packed(vq, id, ctx);
>
> -       vq->last_used_idx += vq->packed.desc_state[id].num;
> -       if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
> -               vq->last_used_idx -= vq->packed.vring.num;
> -               vq->packed.used_wrap_counter ^= 1;
> +       last_used += vq->packed.desc_state[id].num;
> +       if (unlikely(last_used >= vq->packed.vring.num)) {

Can we tweak the math here to avoid the out of bound result?

> +               last_used -= vq->packed.vring.num;
> +               used_wrap_counter ^= 1;
>         }
>
> +       last_used = (last_used | (used_wrap_counter << VRING_PACKED_DESC_F_USED));
> +       vq->last_used_idx = last_used;
> +
>         /*
>          * If we expect an interrupt for the next entry, tell host
>          * by writing event index and flush out the write before
> @@ -1465,9 +1479,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
>         if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
>                 virtio_store_mb(vq->weak_barriers,
>                                 &vq->packed.vring.driver->off_wrap,
> -                               cpu_to_le16(vq->last_used_idx |
> -                                       (vq->packed.used_wrap_counter <<
> -                                        VRING_PACKED_EVENT_F_WRAP_CTR)));
> +                               cpu_to_le16(vq->last_used_idx));
>
>         LAST_ADD_TIME_INVALID(vq);
>
> @@ -1499,9 +1511,7 @@ static unsigned int virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
>
>         if (vq->event) {
>                 vq->packed.vring.driver->off_wrap =
> -                       cpu_to_le16(vq->last_used_idx |
> -                               (vq->packed.used_wrap_counter <<
> -                                VRING_PACKED_EVENT_F_WRAP_CTR));
> +                       cpu_to_le16(vq->last_used_idx);
>                 /*
>                  * We need to update event offset and event wrap
>                  * counter first before updating event flags.
> @@ -1518,8 +1528,7 @@ static unsigned int virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
>         }
>
>         END_USE(vq);
> -       return vq->last_used_idx | ((u16)vq->packed.used_wrap_counter <<
> -                       VRING_PACKED_EVENT_F_WRAP_CTR);
> +       return vq->last_used_idx;
>  }
>
>  static bool virtqueue_poll_packed(struct virtqueue *_vq, u16 off_wrap)
> @@ -1550,9 +1559,9 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>         if (vq->event) {
>                 /* TODO: tune this threshold */
>                 bufs = (vq->packed.vring.num - vq->vq.num_free) * 3 / 4;
> -               wrap_counter = vq->packed.used_wrap_counter;
> +               wrap_counter = !!(vq->last_used_idx >> VRING_PACKED_USED_INDEX_F_WRAP_CTR);
>
> -               used_idx = vq->last_used_idx + bufs;
> +               used_idx = (vq->last_used_idx & ~(1 << VRING_PACKED_USED_INDEX_F_WRAP_CTR)) + bufs;
>                 if (used_idx >= vq->packed.vring.num) {
>                         used_idx -= vq->packed.vring.num;
>                         wrap_counter ^= 1;
> @@ -1582,9 +1591,9 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>          */
>         virtio_mb(vq->weak_barriers);
>
> -       if (is_used_desc_packed(vq,
> -                               vq->last_used_idx,
> -                               vq->packed.used_wrap_counter)) {
> +       wrap_counter = !!(vq->last_used_idx >> VRING_PACKED_USED_INDEX_F_WRAP_CTR);
> +       used_idx = (vq->last_used_idx & ~(1 << VRING_PACKED_USED_INDEX_F_WRAP_CTR));
> +       if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
>                 END_USE(vq);
>                 return false;
>         }
> @@ -1689,7 +1698,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
>         vq->notify = notify;
>         vq->weak_barriers = weak_barriers;
>         vq->broken = true;
> -       vq->last_used_idx = 0;
> +       vq->last_used_idx = 0 | (1 << VRING_PACKED_USED_INDEX_F_WRAP_CTR);
>         vq->event_triggered = false;
>         vq->num_added = 0;
>         vq->packed_ring = true;
> @@ -1720,7 +1729,6 @@ static struct virtqueue *vring_create_virtqueue_packed(
>
>         vq->packed.next_avail_idx = 0;
>         vq->packed.avail_wrap_counter = 1;
> -       vq->packed.used_wrap_counter = 1;
>         vq->packed.event_flags_shadow = 0;
>         vq->packed.avail_used_flags = 1 << VRING_PACKED_DESC_F_AVAIL;
>
> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
> index 476d3e5c0fe7..96bcc4d52fce 100644
> --- a/include/uapi/linux/virtio_ring.h
> +++ b/include/uapi/linux/virtio_ring.h
> @@ -77,6 +77,12 @@
>   */
>  #define VRING_PACKED_EVENT_F_WRAP_CTR  15
>
> +
> +/*
> + * used wrap Counter bit shift in vq->last_used_idx for packed ring
> + */
> +#define VRING_PACKED_USED_INDEX_F_WRAP_CTR  15

Let's simply reuse VRING_PACKED_EVENT_F_WRAP_CTR ?

Thanks

> +
>  /* We support indirect buffer descriptors */
>  #define VIRTIO_RING_F_INDIRECT_DESC    28
>
> --
> 2.31.1
>

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