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

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Ezra,

On 2025/2/12 15:27, Ezra Buehler wrote:
> Hi Qi,
> 
> Thanks for the fix. I will test it as well as I can.

Thanks!

> 
> 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.

Got it. Will wait for your test results first.

Thanks,
Qi

> 
> Cheers,
> Ezra.