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Date: Thu, 16 Jun 2022 02:07:15 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Albert Huang <huangjie.albert@...edance.com>
Cc: jasowang@...hat.com, yuanzhu@...edance.com,
virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] virtio_ring : keep used_wrap_counter in
vq->last_used_idx
On Thu, Jun 16, 2022 at 01:12:21PM +0800, Albert Huang wrote:
> From: "huangjie.albert" <huangjie.albert@...edance.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.
>
> v1->v2:
> - reuse the VRING_PACKED_EVENT_F_WRAP_CTR
> - Remove parameter judgment in is_used_desc_packed,
> because it can't be illegal
>
> Signed-off-by: huangjie.albert <huangjie.albert@...edance.com>
This looks good, just a small suggestion below:
> ---
> drivers/virtio/virtio_ring.c | 57 ++++++++++++++++++++----------------
> 1 file changed, 31 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 13a7348cedff..b22d97c9a755 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_EVENT_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;
>
> @@ -1406,8 +1408,12 @@ 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_EVENT_F_WRAP_CTR);
This only works if last_used_idx is 16 bit and
VRING_PACKED_EVENT_F_WRAP_CTR is 15.
I think you want
/* all bits below VRING_PACKED_EVENT_F_WRAP_CTR */
vq->last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> + used_wrap_counter = !!((vq->last_used_idx) >> VRING_PACKED_EVENT_F_WRAP_CTR);
A bit more efficient and clear:
!!(q->last_used_idx & (1 << VRING_PACKED_EVENT_F_WRAP_CTR))
Also this logic is repeated in multiple places. Let's add a couple of inline
functions:
static inline bool packed_used_wrap_counter(vq)
static inline u16 packed_last_used(vq)
then use these everywhere.
> + return is_used_desc_packed(vq, last_used, used_wrap_counter);
> }
>
> static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> @@ -1416,6 +1422,7 @@ 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 +1441,8 @@ 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_EVENT_F_WRAP_CTR));
> + last_used = (vq->last_used_idx) & (~(1 << VRING_PACKED_EVENT_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 +1459,15 @@ 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)) {
> + last_used -= vq->packed.vring.num;
> + used_wrap_counter ^= 1;
> }
>
> + last_used = (last_used | (used_wrap_counter << VRING_PACKED_EVENT_F_WRAP_CTR));
> + 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 +1476,7 @@ 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 +1508,7 @@ 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 +1525,7 @@ 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 +1556,9 @@ 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_EVENT_F_WRAP_CTR);
>
> - used_idx = vq->last_used_idx + bufs;
> + used_idx = (vq->last_used_idx & ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR)) + bufs;
> if (used_idx >= vq->packed.vring.num) {
> used_idx -= vq->packed.vring.num;
> wrap_counter ^= 1;
> @@ -1582,9 +1588,9 @@ 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_EVENT_F_WRAP_CTR);
> + used_idx = (vq->last_used_idx & ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> + if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
> END_USE(vq);
> return false;
> }
> @@ -1689,7 +1695,7 @@ 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_EVENT_F_WRAP_CTR);
> vq->event_triggered = false;
> vq->num_added = 0;
> vq->packed_ring = true;
> @@ -1720,7 +1726,6 @@ 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;
>
> --
> 2.31.1
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