[PATCH] arm64/mm: Fix use-after-free due to race between memory hotunplug and ptdump

[Ryan Roberts]

On 28/07/2025 11:31, Dev Jain wrote:
> Memory hotunplug is done under the hotplug lock and ptdump walk is done
> under the init_mm.mmap_lock. Therefore, ptdump and hotunplug can run
> simultaneously without any synchronization. During hotunplug,
> free_empty_tables() is ultimately called to free up the pagetables.
> The following race can happen, where x denotes the level of the pagetable:
> 
> CPU1					CPU2
> free_empty_pxd_table
> 					ptdump_walk_pgd()
> 					Get p(x+1)d table from pxd entry
> pxd_clear
> free_hotplug_pgtable_page(p(x+1)dp)
> 					Still using the p(x+1)d table
> 
> which leads to a user-after-free.

I'm not sure I understand this. ptdump_show() protects against this with
get_online_mems()/put_online_mems(), doesn't it? There are 2 paths that call
ptdump_walk_pgd(). This protects one of them. The other is ptdump_check_wx(); I
thought you (or Anshuman?) had a patch in flight to fix that with
[get|put]_online_mems() too?

Sorry if my memory is failing me here...