Re: [PATCH v3 3/3] mm/mempolicy: add nodes_empty check in SYSC_migrate_pages

[Vlastimil Babka]

On 11/17/2017 02:37 AM, Yisheng Xie wrote:
> As manpage of migrate_pages, the errno should be set to EINVAL when
> none of the node IDs specified by new_nodes are on-line and allowed
> by the process's current cpuset context, or none of the specified
> nodes contain memory. However, when test by following case:
> 
> 	new_nodes = 0;
> 	old_nodes = 0xf;
> 	ret = migrate_pages(pid, old_nodes, new_nodes, MAX);
> 
> The ret will be 0 and no errno is set. As the new_nodes is empty,
> we should expect EINVAL as documented.
> 
> To fix the case like above, this patch check whether target nodes
> AND current task_nodes is empty, and then check whether AND
> node_states[N_MEMORY] is empty.
> 
> Signed-off-by: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
> ---
>  mm/mempolicy.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 65df28d..f604b22 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1433,10 +1433,14 @@ static int copy_nodes_to_user(unsigned long __user *mask, unsigned long maxnode,
>  		goto out_put;
>  	}

Let me add the whole preceding that ends on the lines above:

        task_nodes = cpuset_mems_allowed(task);
        /* Is the user allowed to access the target nodes? */
        if (!nodes_subset(*new, task_nodes) && !capable(CAP_SYS_NICE)) {
                err = -EPERM;
                goto out_put;
        }

>  
> -	if (!nodes_subset(*new, node_states[N_MEMORY])) {
> -		err = -EINVAL;
> +	task_nodes = cpuset_mems_allowed(current);
> +	nodes_and(*new, *new, task_nodes);
> +	if (nodes_empty(*new))
> +		goto out_put;

So if we have CAP_SYS_NICE, we pass (or rather skip) the EPERM check
above, but the current cpuset restriction still applies regardless. This
doesn't make sense to me? If I get Christoph right in the v2 discussion,
then CAP_SYS_NICE should not allow current cpuset escape. In that case,
we should remove the CAP_SYS_NICE check from the EPERM check? Also
should it be a subset check, or a non-empty-intersection check?

Note there's still a danger that we are breaking existing code so this
will have to be reverted in any case...

> +
> +	nodes_and(*new, *new, node_states[N_MEMORY]);
> +	if (nodes_empty(*new))
>  		goto out_put;
> -	}
>  
>  	err = security_task_movememory(task);
>  	if (err)
>