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SubjectRe: [PATCH v3 3/3] mm/mempolicy: add nodes_empty check in SYSC_migrate_pages
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Hi Vlastimil,

Thanks for your comment!
On 2017/11/28 1:25, Vlastimil Babka wrote:
> 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@huawei.com>
>> ---
>> 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.
hmm, maybe I do not get what you mean, the patch seems do not *escape* the
current cpuset? if CAP_SYS_NICE it also check current cpuset, right?

> 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?

So you mean:
1. we should remove the EPERM check above?
2. Not sure we should use subset check, or a non-empty-intersection for current cpuset?
(Please let me know, if have other points.)

For 1: I have checked the manpage of capabilities[1]:
CAP_SYS_NICE
[...]
*apply migrate_pages(2) to arbitrary processes* and allow
processes to be migrated to arbitrary nodes;

apply move_pages(2) to arbitrary processes;
[...]

Therefore, IMO, EPERM check should be something like:
if (currtent->mm != task->mm && !capable(CAP_SYS_NICE)) { // or if (currtent != task && !capable(CAP_SYS_NICE)) ?
err = -EPERM;
goto out_put;
}
And I kept it as unchanged to follow the original code's meaning.(For move_pages
also use the the logical to check EPERM). I also did not want to break the existing code. :)

For 2: we should follow the manpage of migrate_pages about EINVAL, as your listed in
the former discussion:
EINVAL... Or, _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.

So a non-empty-intersection check for current cpuset should be enough, right?
And Christoph seems do _not oppose_ this point. (I not sure whether he is *agree* or not).

[1] http://man7.org/linux/man-pages/man7/capabilities.7.html
>
> Note there's still a danger that we are breaking existing code so this
> will have to be reverted in any case...

I am not oppose if you want to revert this patch, but we should find a
correct way to fix the case above, right? Maybe anther version or a fix to fold?

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

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