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Date: Tue, 14 Jun 2022 16:16:53 +0800
From: 黄杰 <huangjie.albert@...edance.com>
To: Jason Wang <jasowang@...hat.com>
Cc: mst <mst@...hat.com>, yuanzhu@...edance.com,
virtualization <virtualization@...ts.linux-foundation.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio_ring : keep used_wrap_counter in vq->last_used_idx
Jason Wang <jasowang@...hat.com> 于2022年6月14日周二 15:45写道：
>
> On Tue, Jun 14, 2022 at 1:38 PM Albert Huang
> <huangjie.albert@...edance.com> 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.
> >
> > Signed-off-by: huangjie.albert <huangjie.albert@...edance.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.
Yes, the idx pass to the is_used_desc_packed can never >=
vq->packed.vring.num since we
tweak the math here :
>+ 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_DESC_F_USED));
>+ vq->last_used_idx = last_used;
>
> > +
> > 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?
Yes, this can avoid the out of bound result. because of We just
assign last_used_idx in the following code：
vq->last_used_idx = last_used;
and it is a 16bit/32bit operations,and are guaranteed to be atomic
>
> > + 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
Although the two values are the same, I see the meanings are different,
so I redefine one. Is it possible to rename
VRING_PACKED_EVENT_F_WRAP_CTR to VRING_PACKED_F_WRAP_CTR?
Jason, what is your take?
>
> > +
> > /* We support indirect buffer descriptors */
> > #define VIRTIO_RING_F_INDIRECT_DESC 28
> >
> > --
> > 2.31.1
> >
>
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