Re: [PATCH] arm: pgtable: fix NULL pointer dereference issue

[Ezra Buehler <ezra@easyb.ch>]

Hi Qi,

Thanks for the fix. I will test it as well as I can.

On Wed, Feb 12, 2025 at 7:41 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>
> 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.
>
> Reported-by: Ezra Buehler <ezra@easyb.ch>

Perhaps a detail but, maybe better use "Ezra Buehler
<ezra.buehler@husqvarnagroup.com>" here.

Cheers,
Ezra.