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

[Anshuman Khandual]

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() ?

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.