Messages in this thread | | | Date | Fri, 3 Dec 2021 19:13:31 +0100 | Subject | Re: [PATCH v1] arm64/mm: avoid race condition of update page table when kernel init | From | David Hildenbrand <> |
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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 :)
-- Thanks,
David / dhildenb
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