Messages in this thread | | | Date | Fri, 3 Dec 2021 17:44:11 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH v1] arm64/mm: avoid race condition of update page table when kernel init |
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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().
-- Catalin
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