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

[Anshuman Khandual]

On 09/04/2019 06:28 AM, 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
> 
> The keypoint is: we don't always have a hardware-managed access flag on
> arm64.
> 
> The root cause is in copy_one_pte, it will clear the PTE_AF for COW
> pages. Generally, when it is accessed by user, the COW pages will be set
> as accessed(PTE_AF bit on arm64) by hardware if hardware feature is
> supported. But on some arm64 platforms, the PTE_AF needs to be set by
> software.
> 
> 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 <Yibo.Cai@xxxxxxx>
> Signed-off-by: Jia He <justin.he@xxxxxxx>
> ---
>  mm/memory.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index e2bb51b6242e..b1f9ace2e943 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2140,7 +2140,8 @@ 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)
>  {
>  	debug_dma_assert_idle(src);
>  
> @@ -2152,20 +2153,30 @@ 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 *)(vmf->address & 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. If PTE_AF is cleared on arm64, it might
> +		 * cause double page fault here. so makes pte young here
>  		 */
> +		if (!pte_young(vmf->orig_pte)) {
> +			entry = pte_mkyoung(vmf->orig_pte);
> +			if (ptep_set_access_flags(vmf->vma, vmf->address,
> +				vmf->pte, entry, vmf->flags & FAULT_FLAG_WRITE))
> +				update_mmu_cache(vmf->vma, vmf->address,
> +						vmf->pte);
> +		}
> +
>  		if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))

Should not page fault be disabled when doing this ? Ideally it should
have also called access_ok() on the user address range first. The point
is that the caller of __copy_from_user_inatomic() must make sure that
there cannot be any page fault while doing the actual copy. But also it
should be done in generic way, something like in access_ok(). The current
proposal here seems very specific to arm64 case.