Re: [RFC] apparently broken error recovery in vhost_scsi_iov_to_sgl()

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Sep 24, 2017 at 11:36:33PM +0100, Al Viro wrote:
> Suppose vhost_scsi_iov_to_sgl() got a two-iovec array, mapped
> e.g. 20 pages from the first one just fine and failed on the
> second.
> 
> static int
> vhost_scsi_iov_to_sgl(struct vhost_scsi_cmd *cmd, bool write,
>                       struct iov_iter *iter,
>                       struct scatterlist *sg, int sg_count)
> {
>         size_t off = iter->iov_offset;
>         int i, ret;
> 
>         for (i = 0; i < iter->nr_segs; i++) {
>                 void __user *base = iter->iov[i].iov_base + off;
>                 size_t len = iter->iov[i].iov_len - off;
> 
>                 ret = vhost_scsi_map_to_sgl(cmd, base, len, sg, write);
>                 if (ret < 0) {
>                         for (i = 0; i < sg_count; i++) {
>                                 struct page *page = sg_page(&sg[i]);
>                                 if (page)
>                                         put_page(page);
>                         }
>                         return ret;
>                 }
>                 sg += ret;
>                 off = 0;
>         }
>         return 0;
> }
> 
> What are we trying to drop in the if (ret < 0) in there?  In the case
> above we step into it on the second pass through the loop.  The first
> 20 entries of sg had been filled... and sg had been increased by 20,
> so whatever we find and feed to put_page(), it won't be those 20 pages.
> Moreover, the caller will reset cmd->tvc_{prot_,}sgl_count to zero,
> so vhost_scsi_release_cmd() won't find them either.
>
> Am I missing something subtle here, or should that thing be doing
> something like

Looks right to me. I think Nicholas wrote this, CC him.

> 
> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index 046f6d280af5..e47c5bc3ddca 100644
> --- a/drivers/vhost/scsi.c
> +++ b/drivers/vhost/scsi.c
> @@ -688,6 +688,7 @@ vhost_scsi_iov_to_sgl(struct vhost_scsi_cmd *cmd, bool write,
>  		      struct scatterlist *sg, int sg_count)
>  {
>  	size_t off = iter->iov_offset;
> +	struct scatterlist *p = sg;
>  	int i, ret;
>  
>  	for (i = 0; i < iter->nr_segs; i++) {
> @@ -696,8 +697,8 @@ vhost_scsi_iov_to_sgl(struct vhost_scsi_cmd *cmd, bool write,
>  
>  		ret = vhost_scsi_map_to_sgl(cmd, base, len, sg, write);
>  		if (ret < 0) {
> -			for (i = 0; i < sg_count; i++) {
> -				struct page *page = sg_page(&sg[i]);
> +			while (p < sg) {
> +				struct page *page = sg_page(p++);
>  				if (page)
>  					put_page(page);
>  			}