Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping

[Catalin Marinas]

On Fri, Dec 10, 2021 at 05:54:32PM +0800, Jianyong Wu wrote:
> fixmap is a global resource and is used recursively in create pud mapping.
> It may lead to race condition when alloc_init_pud is called concurrently.
> 
> Fox example:
> alloc_init_pud is called when kernel_init. If memory hotplug
> thread, which will also call alloc_init_pud, happens during
> kernel_init, the race for fixmap occurs.
> 
> The race condition flow can be:
> 
> *************** begin **************
> 
> kerenl_init thread                          virtio-mem workqueue thread
> ==================                          ======== ==================
> alloc_init_pud(...)
>   pudp = pud_set_fixmap_offset(..)          alloc_init_pud(...)
> ...                                         ...
>     READ_ONCE(*pudp) //OK!                    pudp = pud_set_fixmap_offset(
> ...                                         ...
>   pud_clear_fixmap() //fixmap break
>                                               READ_ONCE(*pudp) //CRASH!
> 
> **************** end ***************
> 
> Hence, a spin lock is introduced to protect the fixmap during create pdg
> mapping.
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>

It looks fine to me:

Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>

Do we need a cc stable? Fixmap was introduced in 4.6, so usually:

Fixes: f4710445458c ("arm64: mm: use fixmap when creating page tables")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.6.x

but I haven't checked when memory hotplug was added to be able to
trigger the race. It may not need to go back that far.

-- 
Catalin