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Date: Thu, 16 Jun 2022 15:36:31 +0800
From: 黄杰 <huangjie.albert@...edance.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: jasowang@...hat.com, yuanzhu@...edance.com,
virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v2] virtio_ring : keep used_wrap_counter in vq->last_used_idx
Michael S. Tsirkin <mst@...hat.com> 于2022年6月16日周四 14:41写道：
>
> 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>
> > ---
> > 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);
> > + used_wrap_counter = !!((vq->last_used_idx) >> VRING_PACKED_EVENT_F_WRAP_CTR);
> > + return is_used_desc_packed(vq, last_used, used_wrap_counter);
>
> Hmm.
>
> If vq->last_used_idx is read twice like this the values can be inconsistent,
> no idea what the result will be if so.
>
> I think we need to read vq->last_used_idx with READ_ONCE.
>
> And I guess write it with WRITE_ONCE for symmetry.
>
if we read vq->last_used_idx with READ_ONCE. for example:
1)last_used = READ_ONCE (vq->last_used_idx) & ~(1 <<
VRING_PACKED_EVENT_F_WRAP_CTR);
2)used_wrap_counter = !!((READ_ONCE (vq->last_used_idx)) >>
VRING_PACKED_EVENT_F_WRAP_CTR);
1 and 2 may get different values, so think this will be better:
last_used_idx = vq->last_used_idx;
last_used = last_used_idx& ~(1 << VRING_PACKED_EVENT_F_WRAP_CTR);
used_wrap_counter = !!((last_used_idx) >> VRING_PACKED_EVENT_F_WRAP_CTR);
what do you think？
>
> > }
> >
> > 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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