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SubjectRe: [PATCH v1] arm64/mm: avoid race condition of update page table when kernel init
On Fri, Dec 03, 2021 at 07:13:31PM +0100, David Hildenbrand wrote:
> On 03.12.21 18:44, Catalin Marinas wrote:
> > On Thu, Oct 28, 2021 at 08:36:07AM +0100, Jianyong Wu wrote:
> >> From Anshuman Khandual <anshuman.khandual@arm.com>:
> >>> On 10/27/21 3:18 PM, Jianyong Wu wrote:
> >>>> Race condition of page table update can happen in kernel boot period
> >>>> as both of memory hotplug action when kernel init and the
> >>>> mark_rodata_ro can update page table. For virtio-mem, the function excute flow chart is:
> >>>>
> >>>> -------------------------
> >>>> kernel_init
> >>>> kernel_init_freeable
> >>>> ...
> >>>> do_initcall
> >>>> ...
> >>>> module_init [A]
> >>>>
> >>>> ...
> >>>> mark_readonly
> >>>> mark_rodata_ro [B]
> >>>> -------------------------
> > [...]
> >>>> We can see that the error derived from the l3 translation as the pte
> >>>> value is *0*. That is because the fixmap has been clear when access.
> >>>>
> >>>> Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> >>>> ---
> >>>> arch/arm64/mm/mmu.c | 2 ++
> >>>> 1 file changed, 2 insertions(+)
> >>>>
> >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index
> >>>> cfd9deb347c3..567dfba8f08a 100644
> >>>> --- a/arch/arm64/mm/mmu.c
> >>>> +++ b/arch/arm64/mm/mmu.c
> >>>> @@ -564,8 +564,10 @@ void mark_rodata_ro(void)
> >>>> * to cover NOTES and EXCEPTION_TABLE.
> >>>> */
> >>>> section_size = (unsigned long)__init_begin - (unsigned long)__start_rodata;
> >>>> + get_online_mems();
> >>>> update_mapping_prot(__pa_symbol(__start_rodata), (unsigned long)__start_rodata,
> >>>> section_size, PAGE_KERNEL_RO);
> >>>> + put_online_mems();
> >>>>
> >>>> debug_checkwx();
> >>>> }
> >>>
> >>> While this should solve the current problem i.e race between concurrent
> >>> memory hotplug operation and mark_rodata_ro(), but I am still wondering
> >>> whether this is the fix at the right place and granularity. Basically a hotplug
> >>> operation queued in an work queue at [A] can execute during [B] is the root
> >>> cause of this problem.
> >>
> >> Not exactly, this issue doesn't only happen at the the *pure* kernel
> >> boot. For example, hotplug memory through VM monitor when VM boot. We
> >> can't foresee when that happen. Thus, this issue can affect all kinds
> >> of memory hotplug mechanism, including ACPI based memory hotplug and
> >> virtio-mem. I'm not sure that fix it here is the best way. If the race
> >> only happens between kernel init and memory hotplug, I think it's fine
> >> to fix it here. IMO, this issue results from the race for "fixmap"
> >> resource. I wonder why this global resource is not protected by a
> >> lock. Maybe we can add one and fix it there.
> >
> > IIUC the race is caused by multiple attempts to use the fixmap at the
> > same time. We can add a fixmap_lock and hold it during
> > __create_pgd_mapping().
>
> IIRC that's something along the lines I suggested, so, yes :)

Yeah, I just echoed what you and Anshuman said ;).

--
Catalin

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