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

Jianyong Wu
2021-12-13T05:24:43Z

Hi Catalin,

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@arm.com>
> Sent: Friday, December 10, 2021 7:22 PM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: will@kernel.org; Anshuman Khandual <Anshuman.Khandual@arm.com>;
> akpm@linux-foundation.org; ardb@kernel.org; linux-
> kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
> david@redhat.com; gshan@redhat.com; Justin He <Justin.He@arm.com>;
> nd <nd@arm.com>; Mark Rutland <Mark.Rutland@arm.com>
> Subject: Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create
> pud mapping
>
> 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.
>
For now, I only find the memory hotplug can conflict with it. I see that memory hotplug is enabled on arm64 in 4.20, thus what about changing "4.6" to "4.20".

Thanks
Jianyong
> --
> Catalin