Re: [PATCH v1] arm64/mm: avoid race condition of update page table when kernel init

[David Hildenbrand]

On 28.10.21 07:57, Anshuman Khandual wrote:
> 
> 
> 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]
>> -------------------------
>> virtio-mem can be initialized at [A] and spwan a workqueue to add
>> memory, therefore the race of update page table can happen inside [B].
>>
>> What's more, the race condition can happen even for ACPI based memory
>> hotplug, as it can burst into kernel boot period while page table is
>> updating inside mark_rodata_ro.
>>
>> That's why memory hotplug lock is needed to guard mark_rodata_ro to avoid
>> the race condition.
>>
>> It may only happen in arm64. As fixmap, which is the global resource, is
>> used in page table creating. So, the change is only for arm64.
>>
>> The error often occurs inside alloc_init_pud() in arch/arm64/mm/mmu.c
>>
>> the race condition flow is:
>>
>> *************** begin ************
>>
>> kerenl_init                                 virtio-mem workqueue
>> =========                                   ========
>> 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 *************
>>
>> I catch the related error when test virtio-mem (a new memory hotplug
>> driver) on arm64.
>>
>> How to reproduce:
>> (1) prepare a kernel with virtio-mem enabled on arm64
>> (2) start a VM using Cloud Hypervisor using the kernel above
>> (3) hotplug memory, 20G in my case, with virtio-mem
>> (4) reboot or start a new kernel using kexec
>>
>> Test for server times, you may find the error below:
>>
>> [    1.131039] Unable to handle kernel paging request at virtual address fffffbfffda3b140
>> [    1.134504] Mem abort info:
>> [    1.135722]   ESR = 0x96000007
>> [    1.136991]   EC = 0x25: DABT (current EL), IL = 32 bits
>> [    1.139189]   SET = 0, FnV = 0
>> [    1.140467]   EA = 0, S1PTW = 0
>> [    1.141755]   FSC = 0x07: level 3 translation fault
>> [    1.143787] Data abort info:
>> [    1.144976]   ISV = 0, ISS = 0x00000007
>> [    1.146554]   CM = 0, WnR = 0
>> [    1.147817] swapper pgtable: 4k pages, 48-bit VAs, pgdp=00000000426f2000
>> [    1.150551] [fffffbfffda3b140] pgd=0000000042ffd003, p4d=0000000042ffd003, pud=0000000042ffe003, pmd=0000000042fff003, pte=0000000000000000
>> [    1.155728] Internal error: Oops: 96000007 [#1] SMP
>> [    1.157724] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G         C        5.15.0-rc3+ #100
>> [    1.161002] Hardware name: linux,dummy-virt (DT)
>> [    1.162939] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> [    1.165825] pc : alloc_init_pud+0x38c/0x550
>> [    1.167610] lr : alloc_init_pud+0x394/0x550
>> [    1.169358] sp : ffff80001001bd10
>> ......
>> [    1.200527] Call trace:
>> [    1.201583]  alloc_init_pud+0x38c/0x550
>> [    1.203218]  __create_pgd_mapping+0x94/0xe0
>> [    1.204983]  update_mapping_prot+0x50/0xd8
>> [    1.206730]  mark_rodata_ro+0x50/0x58
>> [    1.208281]  kernel_init+0x3c/0x120
>> [    1.209760]  ret_from_fork+0x10/0x20
>> [    1.211298] Code: eb15003f 54000061 d5033a9f d5033fdf (f94000a1)
>> [    1.213856] ---[ end trace 59473413ffe3f52d ]---
>> [    1.215850] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
>>
>> 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@xxxxxxx>
>> ---
>>  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.
> 
> start_kernel(..)
> {
> 
> arch_call_rest_init(..)
> rest_init(..)
> 	kernel_thread(kernel_init, ...)
> 		kernel_init(..) {
> 			kernel_init_freeable(..)
> 				do_basic_setup(..)
> 					do_initcalls(..)
> 						........
> 						module_init(..) [A]
> 			................
> 			................
> 			mark_readonly(..)
> 				mark_rodata_ro(..)
> 					update_mapping_prot(..)
> 						__create_pgd_mapping(..) [B]
> 			}
> }
> 
> Are there no other non-hotplug call path after module_init(), which could
> land in __create_pgd_mapping() ?

I'd second that performing proper locking inside __create_pgd_mapping()
-- avoiding any memory hot(un)plug locks -- would be much cleaner and
future proof.

> 
> Please also note that this is not an existing problem on arm64 platform
> which needs to be fixed, as virtio-mem is yet to be enabled.
> 

As raised already, ACPI can hotplug memory that early as well IIRC.

-- 
Thanks,

David / dhildenb