arm: pgtable: fix NULL pointer dereference issue

In the Linux kernel, the following vulnerability has been resolved:

arm: pgtable: fix NULL pointer dereference issue

When update_mmu_cache_range() is called by update_mmu_cache(), the vmf

parameter is NULL, which will cause a NULL pointer dereference issue in

adjust_pte():

Unable to handle kernel NULL pointer dereference at virtual address 00000030 when read

Hardware name: Atmel AT91SAM9

PC is at update_mmu_cache_range+0x1e0/0x278

LR is at pte_offset_map_rw_nolock+0x18/0x2c

Call trace:

update_mmu_cache_range from remove_migration_pte+0x29c/0x2ec

remove_migration_pte from rmap_walk_file+0xcc/0x130

rmap_walk_file from remove_migration_ptes+0x90/0xa4

remove_migration_ptes from migrate_pages_batch+0x6d4/0x858

migrate_pages_batch from migrate_pages+0x188/0x488

migrate_pages from compact_zone+0x56c/0x954

compact_zone from compact_node+0x90/0xf0

compact_node from kcompactd+0x1d4/0x204

kcompactd from kthread+0x120/0x12c

kthread from ret_from_fork+0x14/0x38

Exception stack(0xc0d8bfb0 to 0xc0d8bff8)

To fix it, do not rely on whether 'ptl' is equal to decide whether to hold

the pte lock, but decide it by whether CONFIG_SPLIT_PTE_PTLOCKS is

enabled. In addition, if two vmas map to the same PTE page, there is no

need to hold the pte lock again, otherwise a deadlock will occur. Just

add the need_lock parameter to let adjust_pte() know this information.