Subject: Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

From: Jia He <[email-masked]>
Date: Thu, 19 Sep 2019 10:16:34 +0800

Hi Kirill

[On behalf of [email-masked] because some mails are filted...]

On 2019/9/18 22:00, Kirill A. Shutemov wrote:
> On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote:
>> When we tested pmdk unit test [1] vmmalloc_fork TEST1 in arm64 guest, there
>> will be a double page fault in __copy_from_user_inatomic of cow_user_page.
>>
>> Below call trace is from arm64 do_page_fault for debugging purpose
>> [  110.016195] Call trace:
>> [  110.016826]  do_page_fault+0x5a4/0x690
>> [  110.017812]  do_mem_abort+0x50/0xb0
>> [  110.018726]  el1_da+0x20/0xc4
>> [  110.019492]  __arch_copy_from_user+0x180/0x280
>> [  110.020646]  do_wp_page+0xb0/0x860
>> [  110.021517]  __handle_mm_fault+0x994/0x1338
>> [  110.022606]  handle_mm_fault+0xe8/0x180
>> [  110.023584]  do_page_fault+0x240/0x690
>> [  110.024535]  do_mem_abort+0x50/0xb0
>> [  110.025423]  el0_da+0x20/0x24
>>
>> The pte info before __copy_from_user_inatomic is (PTE_AF is cleared):
>> [ffff9b007000] pgd=000000023d4f8003, pud=000000023da9b003, pmd=000000023d4b3003, pte=360000298607bd3
>>
>> As told by Catalin: "On arm64 without hardware Access Flag, copying from
>> user will fail because the pte is old and cannot be marked young. So we
>> always end up with zeroed page after fork() + CoW for pfn mappings. we
>> don't always have a hardware-managed access flag on arm64."
>>
>> This patch fix it by calling pte_mkyoung. Also, the parameter is
>> changed because vmf should be passed to cow_user_page()
>>
>> [1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork
>>
>> Reported-by: Yibo Cai <[email-masked]>
>> Signed-off-by: Jia He <[email-masked]>
>> ---
>>   mm/memory.c | 35 ++++++++++++++++++++++++++++++-----
>>   1 file changed, 30 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index e2bb51b6242e..d2c130a5883b 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
>>   					2;
>>   #endif
>>   
>> +#ifndef arch_faults_on_old_pte
>> +static inline bool arch_faults_on_old_pte(void)
>> +{
>> +	return false;
>> +}
>> +#endif
>> +
>>   static int __init disable_randmaps(char *s)
>>   {
>>   	randomize_va_space = 0;
>> @@ -2140,8 +2147,12 @@ static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
>>   	return same;
>>   }
>>   
>> -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
>> +static inline void cow_user_page(struct page *dst, struct page *src,
>> +				 struct vm_fault *vmf)
>>   {
>> +	struct vm_area_struct *vma = vmf->vma;
>> +	unsigned long addr = vmf->address;
>> +
>>   	debug_dma_assert_idle(src);
>>   
>>   	/*
>> @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
>>   	 */
>>   	if (unlikely(!src)) {
>>   		void *kaddr = kmap_atomic(dst);
>> -		void __user *uaddr = (void __user *)(va & PAGE_MASK);
>> +		void __user *uaddr = (void __user *)(addr & PAGE_MASK);
>> +		pte_t entry;
>>   
>>   		/*
>>   		 * This really shouldn't fail, because the page is there
>>   		 * in the page tables. But it might just be unreadable,
>>   		 * in which case we just give up and fill the result with
>> -		 * zeroes.
>> +		 * zeroes. On architectures with software "accessed" bits,
>> +		 * we would take a double page fault here, so mark it
>> +		 * accessed here.
>>   		 */
>> +		if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
>> +			spin_lock(vmf->ptl);
>> +			if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>> +				entry = pte_mkyoung(vmf->orig_pte);
>> +				if (ptep_set_access_flags(vma, addr,
>> +							  vmf->pte, entry, 0))
>> +					update_mmu_cache(vma, addr, vmf->pte);
>> +			}
> I don't follow.
>
> So if pte has changed under you, you don't set the accessed bit, but never
> the less copy from the user.
>
> What makes you think it will not trigger the same problem?
>
> I think we need to make cow_user_page() fail in this case and caller --
> wp_page_copy() -- return zero. If the fault was solved by other thread, we
> are fine. If not userspace would re-fault on the same address and we will
> handle the fault from the second attempt.

Thanks for the pointing. How about make cow_user_page() be returned

VM_FAULT_RETRY? Then in do_page_fault(), it can retry the page fault?

---
Cheers,
Justin (Jia He)

>
>> +			spin_unlock(vmf->ptl);
>> +		}
>> +
>>   		if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
>>   			clear_page(kaddr);
>>   		kunmap_atomic(kaddr);
>>   		flush_dcache_page(dst);
>>   	} else
>> -		copy_user_highpage(dst, src, va, vma);
>> +		copy_user_highpage(dst, src, addr, vma);
>>   }
>>   
>>   static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
>> @@ -2318,7 +2343,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>>   				vmf->address);
>>   		if (!new_page)
>>   			goto oom;
>> -		cow_user_page(new_page, old_page, vmf->address, vma);
>> +		cow_user_page(new_page, old_page, vmf);
>>   	}
>>   
>>   	if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false))
>> -- 
>> 2.17.1
>>
>>
-- 

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