Re: [PATCH] mm: fix NULL pointer dereference in alloc_pages_bulk_noprof

[Vlastimil Babka <vbabka@suse.cz>]

On 11/13/24 09:32, Jinjiang Tu wrote:
> We triggered a NULL pointer dereference for ac.preferred_zoneref->zone
> in alloc_pages_bulk_noprof() when the task is migrated between cpusets.
> 
> When cpuset is enabled, in prepare_alloc_pages(), ac->nodemask may be
> &current->mems_allowed. when first_zones_zonelist() is called to find
> preferred_zoneref, the ac->nodemask may be modified concurrently if the
> task is migrated between different cpusets. Assuming we have 2 NUMA Node,
> when traversing Node1 in ac->zonelist, the nodemask is 2, and when
> traversing Node2 in ac->zonelist, the nodemask is 1. As a result, the
> ac->preferred_zoneref points to NULL zone.
> 
> In alloc_pages_bulk_noprof(), for_each_zone_zonelist_nodemask() finds a
> allowable zone and calls zonelist_node_idx(ac.preferred_zoneref), leading
> to NULL pointer dereference.
> 
> __alloc_pages_noprof() fixes this issue by checking NULL pointer in commit
> ea57485af8f4 ("mm, page_alloc: fix check for NULL preferred_zone") and
> commit df76cee6bbeb ("mm, page_alloc: remove redundant checks from alloc
> fastpath").
> 
> To fix it, check NULL pointer for preferred_zoneref->zone.
> 
> Fixes: 387ba26fb1cb ("mm/page_alloc: add a bulk page allocator")
> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Thanks.

> ---
>  mm/page_alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c6c7bb3ea71b..4afe8bc06358 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4592,7 +4592,8 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
>  	gfp = alloc_gfp;
>  
>  	/* Find an allowed local zone that meets the low watermark. */
> -	for_each_zone_zonelist_nodemask(zone, z, ac.zonelist, ac.highest_zoneidx, ac.nodemask) {
> +	z = ac.preferred_zoneref;
> +	for_next_zone_zonelist_nodemask(zone, z, ac.highest_zoneidx, ac.nodemask) {
>  		unsigned long mark;
>  
>  		if (cpusets_enabled() && (alloc_flags & ALLOC_CPUSET) &&