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

[Jianyong Wu]

Hi Catalin,

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@xxxxxxx>
> Sent: Friday, December 10, 2021 7:22 PM
> To: Jianyong Wu <Jianyong.Wu@xxxxxxx>
> Cc: will@xxxxxxxxxx; Anshuman Khandual <Anshuman.Khandual@xxxxxxx>;
> akpm@xxxxxxxxxxxxxxxxxxxx; ardb@xxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> david@xxxxxxxxxx; gshan@xxxxxxxxxx; Justin He <Justin.He@xxxxxxx>;
> nd <nd@xxxxxxx>; Mark Rutland <Mark.Rutland@xxxxxxx>
> 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@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.
>
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