Re: [PATCH] xen: remove size limit of privcmd-buf mapping interface

[Juergen Gross]

Ping?

Jan's remark regarding de-privileged qemu is no issue as the hypercall
node is being closed by the de-privilege library function.


Juergen

On 01/11/2018 13:33, Juergen Gross wrote:
> Currently the size of hypercall buffers allocated via
> /dev/xen/hypercall is limited to a default of 64 memory pages. For live
> migration of guests this might be too small as the page dirty bitmask
> needs to be sized according to the size of the guest. This means
> migrating a 8GB sized guest is already exhausting the default buffer
> size for the dirty bitmap.
> 
> There is no sensible way to set a sane limit, so just remove it
> completely. The device node's usage is limited to root anyway, so there
> is no additional DOS scenario added by allowing unlimited buffers.
> 
> While at it make the error path for the -ENOMEM case a little bit
> cleaner by setting n_pages to the number of successfully allocated
> pages instead of the target size.
> 
> Fixes: c51b3c639e01f2 ("xen: add new hypercall buffer mapping device")
> Cc: <stable@xxxxxxxxxxxxxxx> #4.18
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  drivers/xen/privcmd-buf.c | 22 ++++------------------
>  1 file changed, 4 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/xen/privcmd-buf.c b/drivers/xen/privcmd-buf.c
> index df1ed37c3269..de01a6d0059d 100644
> --- a/drivers/xen/privcmd-buf.c
> +++ b/drivers/xen/privcmd-buf.c
> @@ -21,15 +21,9 @@
>  
>  MODULE_LICENSE("GPL");
>  
> -static unsigned int limit = 64;
> -module_param(limit, uint, 0644);
> -MODULE_PARM_DESC(limit, "Maximum number of pages that may be allocated by "
> -			"the privcmd-buf device per open file");
> -
>  struct privcmd_buf_private {
>  	struct mutex lock;
>  	struct list_head list;
> -	unsigned int allocated;
>  };
>  
>  struct privcmd_buf_vma_private {
> @@ -60,13 +54,10 @@ static void privcmd_buf_vmapriv_free(struct privcmd_buf_vma_private *vma_priv)
>  {
>  	unsigned int i;
>  
> -	vma_priv->file_priv->allocated -= vma_priv->n_pages;
> -
>  	list_del(&vma_priv->list);
>  
>  	for (i = 0; i < vma_priv->n_pages; i++)
> -		if (vma_priv->pages[i])
> -			__free_page(vma_priv->pages[i]);
> +		__free_page(vma_priv->pages[i]);
>  
>  	kfree(vma_priv);
>  }
> @@ -146,8 +137,7 @@ static int privcmd_buf_mmap(struct file *file, struct vm_area_struct *vma)
>  	unsigned int i;
>  	int ret = 0;
>  
> -	if (!(vma->vm_flags & VM_SHARED) || count > limit ||
> -	    file_priv->allocated + count > limit)
> +	if (!(vma->vm_flags & VM_SHARED))
>  		return -EINVAL;
>  
>  	vma_priv = kzalloc(sizeof(*vma_priv) + count * sizeof(void *),
> @@ -155,19 +145,15 @@ static int privcmd_buf_mmap(struct file *file, struct vm_area_struct *vma)
>  	if (!vma_priv)
>  		return -ENOMEM;
>  
> -	vma_priv->n_pages = count;
> -	count = 0;
> -	for (i = 0; i < vma_priv->n_pages; i++) {
> +	for (i = 0; i < count; i++) {
>  		vma_priv->pages[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
>  		if (!vma_priv->pages[i])
>  			break;
> -		count++;
> +		vma_priv->n_pages++;
>  	}
>  
>  	mutex_lock(&file_priv->lock);
>  
> -	file_priv->allocated += count;
> -
>  	vma_priv->file_priv = file_priv;
>  	vma_priv->users = 1;
>  
>