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

Catalin Marinas
2021-12-10T11:22:19Z

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@arm.com>

It looks fine to me:

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

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@vger.kernel.org> # 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