Re: [PATCH] mm: mempolicy: Fix wrong use of maxnode in mempolicy API

[Michal Hocko <mhocko@suse.com>]

On Fri 08-11-19 23:49:29, Li Xinhai wrote:
> The maxnode used by mbind(), set_mempolicy() and migrate_pages() is not
> correctly handled in get_nodes(), where --maxnode cause the calculation
>   endmask = (1UL << (maxnode % BITS_PER_LONG)) - 1;
> ignores the highest node ID bit. Then, at
>   nodes_addr(*nodes)[nlongs-1] &= endmask;
> the highest node ID bit is cleared. Finally, cause mpol_new() think user
> does not pass in any node ID, and return EINVAL.
> 
> The results are:
> - Application receives EINVAL when only the highest node ID bit is set.
> - Application recevies no error when other bits set together with the
>   highest node ID bit, but that highest ID is ignored.

A similar patch has been posted [1] and rejected [2].

[1] http://lkml.kernel.org/r/1570882789-20579-1-git-send-email-zhangpan26@huawei.com
[2] http://lkml.kernel.org/r/20191014091243.GD317@dhcp22.suse.cz

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Hugh Dickins <hughd@google.com>
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> ---
>  mm/mempolicy.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4ae967b..2bdc365 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1328,7 +1328,6 @@ static int get_nodes(nodemask_t *nodes, const unsigned long __user *nmask,
>  	unsigned long nlongs;
>  	unsigned long endmask;
>  
> -	--maxnode;
>  	nodes_clear(*nodes);
>  	if (maxnode == 0 || !nmask)
>  		return 0;
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs