Re: [PATCH v4] virtio_ring : keep used_wrap_counter in vq->last_used_idx

From: Michael S. Tsirkin
Date: Wed Jun 22 2022 - 08:16:56 EST

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On Wed, Jun 22, 2022 at 04:51:22PM +0800, Jason Wang wrote:
> On Fri, Jun 17, 2022 at 10:04 AM Albert Huang
> <huangjie.albert@xxxxxxxxxxxxx> wrote:
> >
> > From: "huangjie.albert" <huangjie.albert@xxxxxxxxxxxxx>
> >
> > 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.
> >
> > v3->v4:
> > - use READ_ONCE/WRITE_ONCE to get/set vq->last_used_idx
> >
> > v2->v3:
> > - add inline function to get used_wrap_counter and last_used
> > - when use vq->last_used_idx, only read once
> > if vq->last_used_idx is read twice, the values can be inconsistent.
> > - use last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR))
> > to get the all bits below VRING_PACKED_EVENT_F_WRAP_CTR
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> > drivers/virtio/virtio_ring.c | 75 ++++++++++++++++++++++--------------
> > 1 file changed, 47 insertions(+), 28 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 13a7348cedff..719fbbe716d6 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:
> > + * bits up to VRING_PACKED_EVENT_F_WRAP_CTR include the last used index.
> > + * bits from VRING_PACKED_EVENT_F_WRAP_CTR include the used wrap counter.
> > + */
> > 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;
> >
> > @@ -973,6 +975,15 @@ static struct virtqueue *vring_create_virtqueue_split(
> > /*
> > * Packed ring specific functions - *_packed().
> > */
> > +static inline bool packed_used_wrap_counter(u16 last_used_idx)
> > +{
> > + return !!(last_used_idx & (1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> > +}
> > +
> > +static inline u16 packed_last_used(u16 last_used_idx)
> > +{
> > + return last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> > +}
>
> Any reason we need a minus after the shift?

The point is to say "all bits above VRING_PACKED_EVENT_F_WRAP_CTR".
Has no effect currently but will if last_used_idx is extended to 32 bit.

> Others look good.
>
> Thanks
>
> >
> > static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > struct vring_desc_extra *extra)
> > @@ -1406,8 +1417,14 @@ 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;
> > + u16 last_used_idx;
> > + bool used_wrap_counter;
> > +
> > + last_used_idx = READ_ONCE(vq->last_used_idx);
> > + last_used = packed_last_used(last_used_idx);
> > + used_wrap_counter = packed_used_wrap_counter(last_used_idx);
> > + return is_used_desc_packed(vq, last_used, used_wrap_counter);
> > }
> >
> > static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > @@ -1415,7 +1432,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > void **ctx)
> > {
> > struct vring_virtqueue *vq = to_vvq(_vq);
> > - u16 last_used, id;
> > + u16 last_used, id, last_used_idx;
> > + bool used_wrap_counter;
> > void *ret;
> >
> > START_USE(vq);
> > @@ -1434,7 +1452,9 @@ 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;
> > + last_used_idx = READ_ONCE(vq->last_used_idx);
> > + used_wrap_counter = packed_used_wrap_counter(last_used_idx);
> > + last_used = packed_last_used(last_used_idx);
> > id = le16_to_cpu(vq->packed.vring.desc[last_used].id);
> > *len = le32_to_cpu(vq->packed.vring.desc[last_used].len);
> >
> > @@ -1451,12 +1471,15 @@ 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));
> > + WRITE_ONCE(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 +1488,7 @@ 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 +1520,7 @@ 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 +1537,7 @@ 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)
> > @@ -1537,7 +1555,7 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, u16 off_wrap)
> > static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> > {
> > struct vring_virtqueue *vq = to_vvq(_vq);
> > - u16 used_idx, wrap_counter;
> > + u16 used_idx, wrap_counter, last_used_idx;
> > u16 bufs;
> >
> > START_USE(vq);
> > @@ -1550,9 +1568,10 @@ 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;
> > + last_used_idx = READ_ONCE(vq->last_used_idx);
> > + wrap_counter = packed_used_wrap_counter(last_used_idx);
> >
> > - used_idx = vq->last_used_idx + bufs;
> > + used_idx = packed_last_used(last_used_idx) + bufs;
> > if (used_idx >= vq->packed.vring.num) {
> > used_idx -= vq->packed.vring.num;
> > wrap_counter ^= 1;
> > @@ -1582,9 +1601,10 @@ 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)) {
> > + last_used_idx = READ_ONCE(vq->last_used_idx);
> > + wrap_counter = packed_used_wrap_counter(last_used_idx);
> > + used_idx = packed_last_used(last_used_idx);
> > + if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
> > END_USE(vq);
> > return false;
> > }
> > @@ -1689,7 +1709,7 @@ 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 +1740,6 @@ 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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In reply to: Jason Wang: "Re: [PATCH v4] virtio_ring : keep used_wrap_counter in vq->last_used_idx"
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