Re: [PATCH untested] vhost: order avail ring reads after index updates

[Will Deacon]

On Wed, Mar 27, 2024 at 01:26:23PM -0400, Michael S. Tsirkin wrote:
> vhost_get_vq_desc (correctly) uses smp_rmb to order
> avail ring reads after index reads.
> However, over time we added two more places that read the
> index and do not bother with barriers.
> Since vhost_get_vq_desc when it was written assumed it is the
> only reader when it sees a new index value is cached
> it does not bother with a barrier either, as a result,
> on the nvidia-gracehopper platform (arm64) available ring
> entry reads have been observed bypassing ring reads, causing
> a ring corruption.
> 
> To fix, factor out the correct index access code from vhost_get_vq_desc.
> As a side benefit, we also validate the index on all paths now, which
> will hopefully help catch future errors earlier.
> 
> Note: current code is inconsistent in how it handles errors:
> some places treat it as an empty ring, others - non empty.
> This patch does not attempt to change the existing behaviour.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Gavin Shan <gshan@xxxxxxxxxx>
> Reported-by: Will Deacon <will@xxxxxxxxxx>
> Suggested-by: Will Deacon <will@xxxxxxxxxx>
> Fixes: 275bf960ac69 ("vhost: better detection of available buffers")
> Cc: "Jason Wang" <jasowang@xxxxxxxxxx>
> Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()")
> Cc: "Stefano Garzarella" <sgarzare@xxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> 
> I think it's better to bite the bullet and clean up the code.
> Note: this is still only built, not tested.
> Gavin could you help test please?
> Especially on the arm platform you have?
> 
> Will thanks so much for finding this race!

No problem, and I was also hoping that the smp_rmb() could be
consolidated into a single helper like you've done here.

One minor comment below:

>  drivers/vhost/vhost.c | 80 +++++++++++++++++++++++--------------------
>  1 file changed, 42 insertions(+), 38 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 045f666b4f12..26b70b1fd9ff 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1290,10 +1290,38 @@ 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;
> +	u16 avail_idx;
> +	int r = vhost_get_avail(vq, idx, &vq->avail->idx);
> +
> +	if (unlikely(r < 0)) {
> +		vq_err(vq, "Failed to access avail idx at %p: %d\n",
> +		       &vq->avail->idx, r);
> +		return -EFAULT;
> +	}
> +
> +	avail_idx = vhost16_to_cpu(vq, idx);
> +
> +	/* Check it isn't doing very strange things with descriptor numbers. */
> +	if (unlikely((u16)(avail_idx - vq->last_avail_idx) > vq->num)) {
> +		vq_err(vq, "Guest moved used index from %u to %u",
> +		       vq->last_avail_idx, vq->avail_idx);
> +		return -EFAULT;
> +	}
> +
> +	/* Nothing new? We are done. */
> +	if (avail_idx == vq->avail_idx)
> +		return 0;
> +
> +	vq->avail_idx = avail_idx;
> +
> +	/* 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();

I think you could use smp_acquire__after_ctrl_dep() if you're feeling
brave, but to be honest I'd prefer we went in the opposite direction
and used READ/WRITE_ONCE + smp_load_acquire()/smp_store_release() across
the board. It's just a thankless, error-prone task to get there :(

So, for the patch as-is:

Acked-by: Will Deacon <will@xxxxxxxxxx>

(I've not tested it either though, so definitely wait for Gavin on that!)

Cheers,

Will