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vhost: move smp_rmb() into vhost_get_avail_idx()

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All callers of vhost_get_avail_idx() use smp_rmb() to
order the available ring entry read and avail_idx read.

Make vhost_get_avail_idx() call smp_rmb() itself whenever the avail_idx
is accessed. This way, the callers don't need to worry about the memory
barrier. As a side benefit, we also validate the index on all paths now,
which will hopefully help prevent/catch earlier future bugs.

Note that current code is inconsistent in how the errors are handled.
They are treated as an empty ring in some places, but as non-empty
ring in other places. This patch doesn't attempt to change the existing
behaviour.

No functional change intended.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Message-Id: <20240429232748.642356-1-gshan@redhat.com>
This commit is contained in:
Michael S. Tsirkin 2024-04-30 09:27:48 +10:00
parent fdba68d2ad
commit 7ad4723976
1 changed files with 42 additions and 63 deletions
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105
drivers/vhost/vhost.c
View File

@ -1346,10 +1346,36 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
mutex_unlock(&d->vqs[i]->mutex);
}
static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
__virtio16 *idx)
static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
{
return vhost_get_avail(vq, *idx, &vq->avail->idx);
__virtio16 idx;
int r;
r = vhost_get_avail(vq, idx, &vq->avail->idx);
if (unlikely(r < 0)) {
vq_err(vq, "Failed to access available index at %p (%d)\n",
&vq->avail->idx, r);
return r;
}
/* Check it isn't doing very strange thing with available indexes */
vq->avail_idx = vhost16_to_cpu(vq, idx);
if (unlikely((u16)(vq->avail_idx - vq->last_avail_idx) > vq->num)) {
vq_err(vq, "Invalid available index change from %u to %u",
vq->last_avail_idx, vq->avail_idx);
return -EINVAL;
}
/* We're done if there is nothing new */
if (vq->avail_idx == vq->last_avail_idx)
return 0;
/*
* We updated vq->avail_idx so we need a memory barrier between
* the index read above and the caller reading avail ring entries.
*/
smp_rmb();
return 1;
}
static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
@ -2554,38 +2580,17 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
{
struct vring_desc desc;
unsigned int i, head, found = 0;
u16 last_avail_idx;
__virtio16 avail_idx;
u16 last_avail_idx = vq->last_avail_idx;
__virtio16 ring_head;
int ret, access;
/* Check it isn't doing very strange things with descriptor numbers. */
last_avail_idx = vq->last_avail_idx;
if (vq->avail_idx == vq->last_avail_idx) {
if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
vq_err(vq, "Failed to access avail idx at %p\n",
&vq->avail->idx);
return -EFAULT;
}
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
ret = vhost_get_avail_idx(vq);
if (unlikely(ret < 0))
return ret;
if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
vq_err(vq, "Guest moved avail index from %u to %u",
last_avail_idx, vq->avail_idx);
return -EFAULT;
}
/* If there's nothing new since last we looked, return
* invalid.
*/
if (vq->avail_idx == last_avail_idx)
if (!ret)
return vq->num;
/* Only get avail ring entries after they have been
* exposed by guest.
*/
smp_rmb();
}
/* Grab the next descriptor number they're advertising, and increment
@ -2846,35 +2851,21 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
/* return true if we're sure that avaiable ring is empty */
bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
__virtio16 avail_idx;
int r;
if (vq->avail_idx != vq->last_avail_idx)
return false;
r = vhost_get_avail_idx(vq, &avail_idx);
if (unlikely(r))
return false;
r = vhost_get_avail_idx(vq);
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
if (vq->avail_idx != vq->last_avail_idx) {
/* Since we have updated avail_idx, the following
* call to vhost_get_vq_desc() will read available
* ring entries. Make sure that read happens after
* the avail_idx read.
*/
smp_rmb();
return false;
}
return true;
/* Note: we treat error as non-empty here */
return r == 0;
}
EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
/* OK, now we need to know about added descriptors. */
bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
{
__virtio16 avail_idx;
int r;
if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
@ -2898,25 +2889,13 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
/* They could have slipped one in as we were doing that: make
* sure it's written, then check again. */
smp_mb();
r = vhost_get_avail_idx(vq, &avail_idx);
if (r) {
vq_err(vq, "Failed to check avail idx at %p: %d\n",
&vq->avail->idx, r);
r = vhost_get_avail_idx(vq);
/* Note: we treat error as empty here */
if (unlikely(r < 0))
return false;
}
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
if (vq->avail_idx != vq->last_avail_idx) {
/* Since we have updated avail_idx, the following
* call to vhost_get_vq_desc() will read available
* ring entries. Make sure that read happens after
* the avail_idx read.
*/
smp_rmb();
return true;
}
return false;
return r;
}
EXPORT_SYMBOL_GPL(vhost_enable_notify);