mm: fix NULL pointer dereference in alloc_pages_bulk_noprof

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm: fix NULL pointer dereference in alloc_pages_bulk_noprof 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.

References

• https://git.kernel.org/stable/c/903d896448c2e50e8652aaba529a30d4d1eaa0e5
• https://git.kernel.org/stable/c/6addb2d9501ec866d7b3a3b4e665307c437e9be2
• https://git.kernel.org/stable/c/d0f16cec79774c3132df006cf771eddd89d08f58
• https://git.kernel.org/stable/c/31502374627ba9ec3e710dbd0bb00457cc6d2c19
• https://git.kernel.org/stable/c/8ce41b0f9d77cca074df25afd39b86e2ee3aa68e